Citation
Right turn acceleration lanes on urban/suburban arterial streets

Material Information

Title:
Right turn acceleration lanes on urban/suburban arterial streets
Creator:
Hodsdon, Jeffrey Charles
Publication Date:
Language:
English
Physical Description:
143 leaves : ; 28 cm

Subjects

Subjects / Keywords:
Traffic flow -- Colorado -- Denver Metropolitan Area ( lcsh )
Traffic flow -- Colorado -- Colorado Springs ( lcsh )
Traffic engineering -- Colorado -- Denver Metropolitan Area ( lcsh )
Traffic engineering -- Colorado -- Colorado Springs ( lcsh )
Traffic engineering ( fast )
Traffic flow ( fast )
Colorado -- Colorado Springs ( fast )
Colorado -- Denver Metropolitan Area ( fast )
Genre:
bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

Notes

Bibliography:
Includes bibliographical references (leaf 143).
General Note:
Department of Civil Engineering
Statement of Responsibility:
by Jeffrey Charles Hodsdon.

Record Information

Source Institution:
|University of Colorado Denver
Holding Location:
|Auraria Library
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
47108332 ( OCLC )
ocm47108332
Classification:
LD1190.E53 2000m .H62 ( lcc )

Full Text
RIGHT TURN ACCELERATION LANES ON
URBAN/SUBURBAN ARTERIAL STREETS
by
Jeffrey Charles Hodsdon
B.S., University of New Hampshire, 1989
A thesis submitted to the
University of Colorado at Denver
in partial fulfillment
of the requirements for the degree of
Master of Science
Civil Engineering
2000


2000 by Jeffrey Charles Hodsdon
All rights reserved.


This thesis for the Master of Science
degree by
Jeffrey Charles Hodsdon
has been approved
by
Date


Hodsdon, Jeffrey Charles (M.S., Civil Engineering)
Right Turn Acceleration lanes on Urban/Suburban Arterial Streets
Thesis directed by Professor Bruce N. Janson
ABSTRACT
This study presents an analysis of right turn acceleration lane$ on
urban/suburban arterial streets (RTALs) in the Colorado Springs and metro
Denver areas. The overall goal of the study is to determine optimal design
geometry for RTALs based on an analysis of vehicle characteristics and
driver behaviors. A large part of the study was new research of which site
specific geometric characteristics and other factors influence driver behavior
with respect to lane use decisions and lane operations.
Four separate analyses have been completed to provide basis for
recommendations relative to the geometric requirements for highly functional
acceleration lanes.
The first analysis included the development of an analytical model based on
the acceleration characteristics of vehicles and a 200 driver behavior model
of merging. This analysis attempted to determine appropriate lane length
and entry arc lengths necessary for vehicles to reach speeds of 45 mph.
The second analysis identifies the correlation between the site
characteristics and the drivers decision to use the acceleration lane (i.e.,
drivers decisions are indicators of the lanes safety, ease of use or comfort,
and lane effectiveness in executing the right turn movement).The third
analysis is similar to the second, but examined identifies the correlation
between the site characteristics and the use of a merging maneuver. The
final procedure involved an analysis of the lane operations relative to
disruption of traffic flow on the arterial.
Based on this analysis, lane lengths of 625 to 750 feet including transition
taper, arc lengths of at least 100 to 130 feet, which translate to curb radii of
about 65 to 85 feet, with lane widths of 13 to 14 feet provide conditions for
optimal performance of right turn acceleration lanes on modern access-
IV


controlled urban/suburban arterial streets with speed limits of 45 mph.
Although it will likely require more study of both safety and delay, and cost/
benefit analysis.
This abstract accurately represents the content of the candidatess thesis. I
recommend its publication.
Signed
Bruce U'f Janson
v


DEDICATION
I dedicate this thesis to my mother and father, for teaching me, among many
other things, a strong sense of perseverance.


ACKNOWLEDGMENT
My thanks to my advisor, Bruce Janson, for his support and wisdom. I also
wish to thank the staff of the Graduate School for their assistance and
guidance through the process. Thanks to Kevin Schulz for his help with the
data collection. Thanks to Melissa for her patience, support and
understanding.


CONTENTS
Figures ..................................................x
Tables ...................................................xii
CHAPTER
1. INTRODUCTION............................................ 1
2. PURPOSE AND SCOPE....................................... 3
Purpose.............................................. 3
Report Limitations................................... 4
3. BACKGROUND.............................................. 7
Speed Change Lanes/RTALs............................. 7
Types of Acceleration Lanes ................... 9
Existing policy on RTALs ........................... 12
State Department of Transportation Policies... 12
County Policies............................... 16
Municipal Policies............................ 16
Vehicle Characteristics............................. 18
Acceleration.................................. 18
Driver Behaviors.................................... 28
Lane Use ..................................... 28
Merging ...................................... 28
Acceleration Rates........................... 40
Acceleration Phase vs. Gap Acceptance......... 40
Other Lane Use Behavior....................... 42
Accident Rates ..................................... 43
RTAL Implementation Considerations and Costs........ 44


4. ANALYTICAL ESTIMATE OF APPROPRIATE RTAL LENGTH 46
Five mph Entry Model.............................. 47
Results...................................... 47
Twenty mph Entry Model............................ 52
Results...................................... 52
5. STUDY OF RIGHT TURN ACCELERATION USE................. 56
Methodology....................................... 57
Site Selection and Data Collection................ 58
Site Selection............................... 58
Data Collected .............................. 59
Sites Studied..................................... 66
Quantifying Lane Use, Operations and Performance. 71
Lane Use Analysis............................ 74
Right Turn Maneuver-type Analysis........... 102
Lane Operations............................. 121
6. SUMMARY AND CONCLUSIONS............................. 138
Summary of Results .............................. 138
Conclusions ..................................... 140
Recommendations for Further Study ............... 141
BIBLIOGRAPHY................................................. 143
ix


FIGURES
Figure
3.1 Speed Change Lanes ............................................ 8
3.2 Comparison of Right Turn Lane Types........................... 11
3.3 Speed/Distance/Acceleration Relationship...................... 21
3.4 Right Turn Entry Maneuver Types............................... 29
3.5 Driver Behavior Model of Merging Modified for RTALs......... 34
3.6: Predicted Ramp Vehicle Speed Profiles....................... 35
3.7 Observed Velocity Profiles ................................... 38
4.1 Acceleration Profile Starting at 5 mph, Average Driver...... 48
4.2 RTAL Merging Model 5 mph Entry Condition ................... 49
4.3 Acceleration Profile Starting at 5 mph, Aggressive Driver .. 50
4.4 Acceleration Profile Starting at 20 mph, Average Driver..... 53
4.5 RTAL Merging Model 20 mph Entry Condition .................. 54
4.6 Acceleration Profile Starting at 20 mph, Aggressive Driver 55
5.1 RTAL Measured Dimensions...................................... 60
5.2 Measurement of Entry Arc Length ............................. 62
5.3 Right Turn Channelization Types .............................. 63
5.4 Opposing Traffic Movements.................................... 65
5.5 Total Lane Length Vs. Lane Usage, All Right Turners .......... 85
5.6 Lane Length Vs. Lane Usage, All Right Turners................. 86
5.7 Total Lane Length Vs. Lane Usage, w/ Opposing Traffic Only... 87
5.8 Lane Length Vs. Lane Usage, wi Opposing Traffic Only.......... 88
5.9 Curb Arc Length Vs. Lane Usage, All Right Turners............ 89
5.10 Curb Arc Length Vs. Lane Usage, w/ Opposing Traffic Only .. 90
x


5.11 Lane Width Vs. Lane Usage, All Right Turners............. 91
5.12 Lane Width Vs. Lane Usage, w/ Opposing Traffic Only ..... 85
5.13 Total Lane Length Vs. Entry Type, w/ Opposing Traffic Only .... 109
5.14 Lane Length Vs. Entry Type, w/ Opposing Traffic Only.... 110
5.15 Curb Arc Vs. Entry Type, w/ Opposing Traffic Only ..... 111
5.16 Lane Width Vs. Entry Type, w/ Opposing Traffic Only ... 112


TABLES
Table
3.1 Minimum Acceleration Lane Length................................ 13
3.2 Speed Change Lane Criteria ..................................... 15
3.3 Acceleration Lane Criteria .................................... 17
3.4 Vehicle Average Maximum Acceleration Rates by Speed Range ... 23
3.5 Vehicle Distances Traveled .................................... 24
3.6 Acceleration Experiment Results................................ 26
3.7 Average Acceleration Rates for Speed Ranges.......... 27
5.1 Site Geometry................................................... 67
5.2 Traffic Volume Data ........................................... 68
5.3 Results Data All Vehicles ................................... 75
5.4 Results Data Vehicles with Opposing Traffic......... 76
5.5 Independent Variable (X) List.................................. 78
5.6 Dependent Variable List........................................ 80
5.7 Lane Usage by Site ........................................... 81
5.8 Lane Use by Site Characteristics.............................. 82
5.9 Simple Regression Analysis All Data ........................ 84
5.10 Stepwise Multiple Regression All Data ...................... 94
5.11 Stepwise Regression All Higher Opposing Data ................ 97
5.12 Stepwise Regression All Denver Sites....................... 98
5.13 Stepwise Regression Denver Sites with Higher Opposing Volume 99
5.14 Model Results................................................ 101
5.15 Dependent Variable (Y) List.................................. 104
5.16 Percent Merging by Site...................................... 105
xii


5.17 Percent Merging by Site Characteristics ..................... 107
5.18 Simple Regression Analysis All Data........................ 108
5.19 Stepwise Regression All Data .............................. 114
5.20 Stepwise Regression All Higher Opposing Data ................ 116
5.21 Stepwise Regression All Denver Sites....................... 117
5.22 Stepwise Regression Denver Sites with Higher Opposing Volume 118
5.23 Model Results................................................. 120
5.24 RTAL Incident Results....................................... 128
5.25 Stepwise Regression- All Denver Data ........................ 130
5.26 RTAL Incident Results Sites with Higher Volumes
and Zero Incidents........................................... 132
5.27 Application of Incidents Regression Model ................... 134
5.28 Right Turn Delay Comparison................................... 136
xiii


CHAPTER 1
INTRODUCTION
This study presents an analysis of right turn acceleration lanes on
urban/suburban arterial streets (RTALs) in the Colorado Springs and metro
Denver areas. The overall goal of the study is to determine optimal design
geometry for RTALs based on an analysis of vehicle characteristics and
driver behaviors. A large part of the study was new research of which site
specific geometric characteristics and other factors influence driver behavior
with respect to lane use decisions and lane operations. This information will
hopefully be used by engineers in the design of safe and effective lanes.
Also, background research was conducted on vehicle acceleration
characteristics, driver behavior while merging in acceleration lanes, and
existing design criteria for acceleration lanes. An analytical model of driver
behavior, assuming certain vehicle acceleration characteristics, was created
to determine required lane geometry.
Several RTALs have been constructed on urban/suburban arterial streets in
the Colorado Springs and Denver areas. Twenty-seven of these sites were
within the selected parameters and selected as appropriate for this study.
Research included identifying the physical characteristics and traffic
conditions/volumes at each site, videotaping traffic operations for selected
time periods, reviewing video tapes for the level of lane usage, and
determining driver behaviors and lane operations. Models were developed
using statistical techniques to determine which site conditions significantly
1


influence driver behavior and observed operational measures of
performance. A set of recommendations for the design of RTALs is
presented based on the studies contained within.
2


CHAPTER 2
PURPOSE AND SCOPE
Purpose
This study is intended to examine the driver behavior surrounding RTALs
and to determine how site geometric and traffic conditions may influence
driver behavior and lane performance. In doing this, research was
conducted to see what has been studied regarding these lanes. There is
little information available regarding this type of speed change lane. This
study is intended to begin to fill the gap in the research of these lanes.
This study also addresses differences between freeway acceleration lanes,
which are obviously very common (and on which there is published
research), and RTALs, which do not appear to be widely used outside of
Colorado (conjecture, based on the absence of requirements and published
standards for these types of lanes).
There is a belief among some traffic engineers in the Denver and Colorado
Springs areas that RTALs are not used at a level sufficient enough to
warrant their incorporation into the infrastructure or are too short to be
effective and may be present an unsafe method of entering an arterial traffic
stream. This study is primarily intended to identify the usage patterns, driver
behaviors, and how they may be related to certain geometric and/or traffic
conditions. A question that may be asked is: Are these RTALs a waste of
money and land? Perhaps many are not needed for capacity or safety, and
3


some may create more of a hazard than if there were no lane. Hopefully this
report will begin to provide some of the key components needed to answer
some of these questions. The key components likely include the
determination of effective geometric design of RTALs.
Report Limitations
This report does not include a comprehensive study of right turn movement
delay with and without the acceleration lane. It does not, in great depth,
address safety or the disruption of arterial traffic flow by right turning traffic
with and without an RTAL. No accident data were used in this study. Even if
an accident data set specifically related to RTALs was available, it would
likely be difficult to obtain any statistically significant results. A more
comprehensive study should probably be completed relating the relative
safety of intersections with a properly designed RTAL and without an RTAL.
This might address which is safer forcing motorists to wait for a gap in
through traffic and turning into the one of the through lanes, or providing the
RTAL and encouraging merging.
At Intersections without RTALs, motorists must select gaps in the through
traffic and turn into the through lane (or in the case of signalized
intersections, another option is to wait for a green signal indication).
Motorists at times select less than acceptable gaps, resulting in large speed
differences between the right turning vehicle (entering the arterial) and the
through vehicle(s). The presence of RTALs perhaps creates different safety
and operational problems, such as those inherent to merging maneuvers,
that might be avoided if cross street right turners were forced to wait for gaps
4


by not providing RTALs This study addresses safety of RTALs only to the
extent of viewing the videotapes and identifying maneuvers that are
potentially unsafe or disruptive to traffic flow.
There is limited published information available on RTALs. This report does
not include specific speed data for the through lanes on the arterial street
and speed/acceleration profiles for right turning vehicles using the RTAL.
Regarding speed data for the through traffic on arterial streets, no data were
used because speed data were not available from published documents for
arterial RTALs. Since the through traffic speeds on arterial streets at and
downstream of signalized intersections vary depending on the signal phase
at any given time, speed data might be difficult to relate to RTAL use. Most
arterial streets had speed limits of 45 mph. Regarding speed/acceleration
profiles for right turning vehicles using the RTAL, these would have been
helpful. However, these data were also not available from published
documents. Moreover, these data would have been difficult to collect.
Although it would have been helpful to have speed profiles specifically for
RTAL drivers, an attempt has been made to estimate these using
background research.
The effects of grades have not been specifically analyzed to determine if
they are a significant factor in RTAL use. A few of the RTALs studied in this
effort are on significant grades. As the through lanes adjacent to the RTALs
are also on the grade, this likely has an effect of the dynamics of merging
into the through traffic stream.
This study also did not specifically consider for every site, the possible
5


effects of downstream intersections. At some sites, the downstream
intersection may be close enough to affect usage of the RTAL. Motorists
wishing to turn left or U-turn at the intersection immediately downstream may
choose not to use the RTAL because the close intersection spacing makes it
impractical.
Some locations are signalized and some are stop-sign controlled, however,
this was accounted for in the opposing traffic identification in the data
analysis. Also the extent of vehicle platooning (i.e., the percent vehicles in
platoons or percent of time a platoon was present was not specifically
addressed). These arterial streets are generally characterized by platoon
traffic flow. The general volume of traffic was varied and most of the arterial
streets seemed to have similar levels of platooning. To consider signal
timing, phasing, offsets, and arrival types (arriving on beginning of red, end
of green etc.) in the analysis would greatly increase the complexity of the
study. Therefore, only the volume of opposing traffic was considered.
6


CHAPTER 3
BACKGROUND
Speed Change Lanes/RTALs
In the past two decades, the State of Colorado has placed greater emphasis
on the use of speed change lanes on rural highways and urban/suburban
arterial streets. Speed change lanes are right or left turn auxiliary lanes
adjacent to the through lane(s) that are used for acceleration, deceleration
or stacking of vehicles (deceleration lanes also provide storage/stacking
space for turning vehicles so they do not obstruct/impede through traffic).
These lanes are implemented so that the disruption of the through traffic
flow by those motorists entering or leaving a roadway is minimized.1 Speed
change lanes include right turn deceleration lanes, left turn deceleration
lanes, right turn acceleration lanes and the less common left turn
acceleration lanes. Figure 3.1 shows a graphic depiction of these lanes on
an arterial street. Speed change lanes can be constructed on freeways,
expressways, rural highways, and urban/suburban arterial streets, with the
intention of providing safer facilities with better traffic flow and higher
capacity.
The purpose of an acceleration lane is to provide a separate area from the
through lane(s) for vehicles to increase their speed from a stop (or slow
cornering speed) to a speed close to that of through traffic. By reducing the
Colorado State Highway Access Code, Code of Colorado Regulations 601-1, Vol. 2, p. 7.
7




speed differential between the through traffic and the vehicles entering the
roadway, a safer and less disruptive system is created. In contrast,
deceleration lanes allow vehicles to leave the through lane at a speed closer
to that of through vehicles and reduce speed to a stop or slower cornering
speed in a separate area from the through lane. Acceleration lanes can also
reduce delay at intersections, by allowing vehicles to execute right turns on
red or from a stop-sign controlled approach, with smaller gaps in the
oncoming, opposing through traffic thus reducing the delay for the right
turning movement.
Speed change lanes are essential and standard infrastructure on freeways
nationwide, and have been for decades. Right turn acceleration lanes are
being constructed on both rural highways and urban/suburban arterial
streets in Colorado and some other states. This report focuses on right turn
acceleration lanes on urban/suburban arterial streets. Urban/suburban
arterial streets are characterized by at-grade access points and
intersections, traffic signals, stop-sign-controlled cross-street approaches,
potentially heavy volumes, posted speeds of about 40-50 mph, and center
medians.
Types of Acceleration Lanes
There are two different types of acceleration lanes. One is the isolated and
separate acceleration lane that ends in a transition taper directing vehicles
back into the through lane. This study focuses only on this type of
acceleration lane. The second type is a continuous lane" that is a
combined acceleration lane and deceleration lane for a downstream
intersection. This would be a continuous right-turn lane connecting two
9


intersections. A continuous lane may connect several access points and
intersections. Figure 3.2 shows a comparison of the two types.
10


"Stand Alone" Right Turn Acceleration Lane
2^
Transition tapers
Right turn acceleration lane
Cross street/Mlnor street

Cross street/Mlnor street
Right turn deceleration lane
Continuous Right Turn Lane

Cross street/Minor street \ Cross street/mlnor street
Continuous right turn lane
Figure 3.2: Comparison of Right Turn Lane Types


Existing Policy on RTALs
The following is a sample of some of the state, county and municipal criteria
for right turn acceleration lanes on urban/suburban arterial streets. Most
states do not have specific standards for RTALs. Some of the state
standards for RTALs are included in access codes which regulate and
control private access and set requirements for intersections on the state
highways.
State Department of Transportation Policies
Oregon. The state of Oregon has an access code, however,
acceleration lanes are not specifically addressed. Deceleration lanes are
included2.
Washington. In the State Roadway Design Manual, acceleration
lanes are addressed, but states that they should only be considered for
rural, unsignalized intersections. Washington requires that a 12-foot
minimum width plus design shoulder with a 300-foot minimum taper that is
not sharper than 25 to1, The following Table 3.1 has been taken from the
State of Washington design manual. It identifies the required minimum
acceleration lane length given highway design speed and cornering speed.
2Oregon DOT Web-site.
12


Table 3.13 Minimum Acceleration Lane Length (not Including Taper) Washington State
Highway Design Speed (mph) Turning Roadway Design Speed (mph)
Stop 15 20
30 180
40 395 320 270
50 640 560 510
60 1115 1015 955
70 1495 1415 1350
Source: Washington State Design Manual, August 1997, pp. 910-29.
Colorado. The State of Colorado has an access code which regulates
all access to state highways, some of which are in urban and suburban
areas. The code identifies speed change lane requirements at access
points and intersections. Criteria vary depending on the highway
classification. The criteria vary from more relaxed criteria for arterial streets
in older or more densely developed urban areas, where intersections are
more closely spaced, to more restrictive criteria on newer or developing
suburban areas to strict standards for expressways. In newer/developing
major transportation corridors with arterial streets and expressways,
intersection spacing is greater between access points and cross streets.
There are stricter requirements for speed change lanes (i.e., they are
required at lower turning volume thresholds). The classification most
3Washington State Design Manual, August 1997, pp. 910-29.
13


relevant to the type of facility studied in this report are the Non-rural Arterial
(NR-A) and Expressway (EX) classifications. The Expressway classification
has. .
Typical spacing of intersecting streets, roads, and highways
shall be planned on intervals of one mile... This category is
appropriate for use on highways that have the capacity for high
speed and relatively high traffic volumes in an efficient and safe
manner. They provide for interstate, interregional, intra regional
and intercity travel needs and to a lesser degree, some intra city
travel needs. Direct access service to abutting land is
subordinate to providing service to through traffic movements.*
The NR-A classification has ...
The desirable standard for the spacing of all intersection public
ways and other accesses that will be full movement or have the
potential for signalization is one-half mile intervals... This
Category is appropriate for use on non-rural highways that have
the capacity for medium to high speeds and provide for medium
to high traffic volumes over medium and long distances in an
efficient and same manner. They provide for interregional, intra
regional, intercity and intra city travel needs in suburban and
urban areas as well as serving as important major arterial streets
in smaller cities and towns. Direct access service to abutting
land is subordinate to providing service to through traffic
movements.4 5
For NR-A class roads, acceleration lanes are required when the right turning
volume (from the cross street to the arterial) is greater than 50 vehicles per
hour (some conditions apply). As shown in Table 3.2, the Colorado
4Colorado State Highway Access Code, p. 35.
5Ibid., pp.40-41.
14


Table 3*2: Speed Change Lane Criteria
Components of Speed Chango Lana Length
. Access Category Left turn deceleration lane ' Right turn deceleration lane Acceleration lane
F-W Design must meet federal interstate standards, and no less then E-X
E-X taper + decel Jength+storage taper+decel. length acceLlength + taper
R-A * decel. length + storage * decel. length * accel. length
R-B * decel. length + storage * decel. length * accel. length
NR-A * decel. length + storage * decel. length * accel. length
NR-B taper + storage taper+storage * accel. length
NR-B>40mph * decel. length decel. length * accel. length
NR-C taper + storage taper+storage * accel. length
NR-C>40mph * decel. length * decel. length * accel. length
Design Criteria for Acceleration and Deceleration Lanes
Posted Speed Limit in MPH 25 30 35 40 45 60 55: 60 65 70
Deceleration Length In feet 180 250 310 370 435 500 600 700 800 900
Acceleration Length In feet N/A 190 270 380 550 760 960 1170 1380 1590
Transition Taper Ratio 7.&1 fcl 1fc1 I2rl 1&&1 1&1 1&£rl 2S1 25.-1 2&1
Source: 1998 Colorado State Highway Accosa Coda, p. 35.


Department of Transportation requires a 550 foot long lane including a 160-
foot long transition taper (13.5 x 12 feet = 160 feet) for a 45 mph posted
speed limit on an N-RA-class road. Expressways require 550 feet plus the
160-foot taper.6
County Policies
El Paso County, Colorado. This county, of which the City of Colorado
Springs is the seat, has adopted portions of the Colorado State Access
Code as its access code for determination of speed change lanes on County
arterial and expressway roads7.
Municipal Policies
The City of Colorado Springs. The City of Colorado Springs generally
requires right turn acceleration lanes for major arterial streets. No right
turning volume thresholds are given. As shown in Table 3.3, for 45 mph
facilities, the acceleration length required is 300 feet assuming a stop
condition for the right turning vehicle and 250 feet assuming that vehicles
turn the corner from the cross street into the RTAL at 15 mph. In addition, a
180-foot long transition taper is also required.8
6Ibid., pp. 53-56.
7El Paso County Interim Access Code, 1999, pp. 53-56.
BCity of Colorado Springs Subdivision Policy Manual -Traffic Engineering Section, p. 8.
16


Table 3.3: Acceleration Lane Criteria
AGCELEttAfrtPH LAKE AKD gAPfcft_Ll
LAtfE LfeHGTH

S
Slt>P CONPlTlOtt tRPM 15 MtH* TAPERIaE
30 - id' 125* 120*
3d i?5* 1501 150*
40 250' 200* ISO*
45 '300* 250* 180*
Assumes vehicles start at id Milas par hour
Source: City of Colorado Springs Subdivision Policy Manual
Traffic Engineering Section, p. 8.


Vehicle Characteristics
Acceleration
An understanding of the acceleration characteristics of vehicles is important
in the determination of well-functioning acceleration lane design. Vehicle
characteristics relative to the time or distance required for accelerating to
speeds of 45 or 50 mph or higher found on urban and suburban arterial
streets are necessary components.
Physical Relationships. Typical vehicle acceleration rates vary by
speed (velocity), for example the acceleration rate from zero to 30 mph is
different from that for 30 to 60. Generally, acceleration rates decrease as
speed increases. This fact about vehicle acceleration is significant when
determining required acceleration distances for a street posted at 35 mph
versus an arterial posted at 45 mph or a freeway posted at 65 or 75 mph.
The following relationships between speed, time, distance traveled and
constant acceleration can be used to estimate a variable acceleration rate
function over a speed range by using different constant average
acceleration rates for smaller speed ranges.
v = v0 + at (3.1)
s = s0 + v0t + Vz at2 (3.2)
v2 = v02 + 2a(s-s0) (3.3)
18


where v = final velocity (speed)
V0 = initial velocity (speed)
a = average acceleration rate
t = time in seconds
s = distance at time t and
s0 = starting distance distance v0
If a vehicle requires 10 seconds to accelerate from zero to 45 mph ideal
conditions, the average rate of acceleration (or a constant rate) over the 10
seconds can be determined, as can be the distance traveled.
v = v0 + at
so 45 = 0 + a(10)
and a = 4.5 mph/sec or 6.6 ft/sec2
to calculate distance traveled:
s = s0 + v0t + Viz at2
s= 0 + 0 (8.5) + 14 (6.6 ft/sec2)(10 sec)2 s = 330 feet
Again, this assumes a constant rate of acceleration or one that doesnt vary
as a function of time. This is the average rate of acceleration from a stop to
45 mph.
As the continuous function which expresses the acceleration rate as a
19


function of velocity for a typical or design vehicle is unavailable, one has
been estimated applying the equations for constant acceleration using a
series of progressively different acceleration rates (varying by speed range)
instead of using one average acceleration rate for zero to 45. This would
estimate the acceleration profile from zero to 45 mph by breaking the
function into smaller speed ranges, each with their own constant
acceleration rate. This creates an estimate of the true variable acceleration
rate function which can be used to estimate distances and times.
So using the equations that assume a set of constant acceleration rates a1(
a2, a3 af for the ranges of v from v0 to v., initial velocity to final velocity.
The equations above can be rewritten as:
for time:
ti^vlaJsfy-VoJ/a, (3.4)
and for distance:
s1=f(v|a)= (v.)2 v02 )/2at
(3.5)
The same equations can be rewritten for successive ranges of velocity and
acceleration. This is illustrated in Figure 3.3.
By breaking the function into velocity ranges, using different acceleration
20


r
Distance (s)
>
Figure 3.3: Speed/Distance/Acceleration Relationship


rates for each velocity range, estimates of cumulative times and distances
traveled can be determined for any point between zero and 45 mph. Also,
this procedure can work in reverse order to estimate acceleration rates using
time and distance data.
Published Data on Acceleration Rates. The following maximum
acceleration rates were published by the Highway Research Board. Table
3.4 Shows typical maximum acceleration rates for different types of vehicles
on level roads circa 1965-19709
9 NCHRP Report 111, Washington DC: Highway Research Board, 1971.
22


Table 3.4 Vehicle Average Maximum Acceleration Rates by Speed Range10 (mph/sec)
Vehicle Type to 30 mph 30-40 mph 40-50 mph 50-60 mph
Large 7 5 4 3
Intermediate 5 5 4 3
Compact 5 4 3 2.2
Small 4 2 1.2 0.7
Composite 5 4.7 3.8 2.8
Pickup 5 2 1.8 1.5
2 axle SU Truck 1 1 0.6 .2
Multi-Unit 1 0.8 0.4
Source: NCHRP Report 111, Washington D.C., Highway Research Board, 1971
Assuming the acceleration rates in Table 3.4 for the speed ranges as shown,
Equation 2 can be used to calculate the distance traveled within the speed
ranges and as a cumulative total. These distances are shown in Table 3.5.
10ibid.
23


Table 3.5 Vehicle Distances Traveled (Feet) (assuming maximum acceleration rates in Table 3.4)
Vehicle Type to 30 mph 30*40 mph/ (cumulative) 40-50 mph/ (cumulative) 50-60mph/ (cumulative)
Large 94 102 (196) 165 (361) 268 (629)
Intermediate 132 102(234) 165 (399) 268 (667)
Compact 132 128 (260) 220(480) 366 (846)
Small 165 256 (421) 550 (971) 1152 (2123)
Composite 132 109 (241) 173 (414) 288 (702)
Pickup 132 256 (388) 366(754) 537 (1291)
2 axle SU Truck 660 513(1173) 1100 (2273) 4033 (6306)
Multi-Unit 660 641 (1301) 1650 (2951)
Based on this information, the distance required for a vehicle to accelerate
from zero to 50 mph is 480 feet for a compact car and 414 feet for the 1965-
1970 composite car. These represent distances calculated using maximum
acceleration rates.
Acceleration Experiment. To estimate typical acceleration
characteristics of a common modern vehicle, a simple field test was
completed using a 1995 Nissan Altima a mid-size sedan with a manual
transmission and a four-cylinder, 2.4 liter engine. On level terrain, markers
were placed at a start point, 200 feet, 300 feet, 400 feet and 500 feet.
Several acceleration trials were completed, during which the approximate
24


speed was recorded upon passing each of the markers following an
acceleration from a stop condition at the start marker. The goal of this test
was to determine average acceleration rates for different velocity ranges.
Table 3.6 below, shows the results of the experiment. The experiment was
also conducted with a 20-mph speed crossing the start marker.
25


Table 3.6 Acceleration Experiment Results
Start from Stop
Speed at Start Speed at 200 ft Speed at 300 ft Speed at 400 ft Speed at 500 ft
0 30 35 42 42
0 32 37 42 44
0 35 37 42 46
0 35 35 42 45
0 30 35 40 46
0 46
0 40
0 43
Average 32.4 35.8 41.6 44.0
Start From 20 mph
20 38 40 45 48
20 42 45 48
20 50
Average 38 41 45 48.7
26


Using the incremental speed increases from zero to 200 feet, 200 to 300
feet, 300 to 400 and 400 to 500, the average acceleration rates within
convenient ranges were calculated. The speed range limits, which are not
round numbers, have been taken from Table 3.6. Table 3.7 below shows
the speed ranges and acceleration rates calculated over those speed
ranges.
Table 3.7 Average Acceleration Rates for Speed Ranges (from the Vehicle Acceleration Experiment) (in mph/sec for velocity ranges)
speeds 32.4 35.8 41.6 44
Zero to 3.85 3.13 3.17 2.84
32.4 to 1.7 2.50 2.17
35.8 to 2.39 2.40
41.6 to 1.50
The table shows a general decrease in average acceleration rates from the
lower speed ranges to the higher speed ranges. There are exceptions as in
the 1.7 mph/second rate for the second speed range. This could be due to
the fact that a manual transmission was used and measurement error.
These measurements are useful, as they provide a set of acceleration rates
from a 1990's vehicle in a test specifically tailored for the needs of this
study. These data are used in the determination of rates used in the
following section.
27


Driver Behavior
Lane Use
The geometry of acceleration lanes on arterial streets varies as do the driver
behaviors associated with these lanes. Some motorists making a right turn
onto the arterial street, instead of using the RTAL, wait at the intersection for
an acceptable gap in the traffic stream and then make the right turn
movement directly into the through lane and then accelerate. Other
motorists turn into the acceleration lane, accelerate to a speed closer to that
of through traffic, and complete a merging maneuver into the through lane.
These two types of maneuvers are illustrated in Figure 3.4. There are other
combinations of maneuvers that fall between these two cases. With the
case of a deceleration lane, it is uncommon to see the speed change lane
not being utilized.
Merging
To gain a better understanding of merging behavior of motorists using
acceleration lanes, several professional journals were reviewed. One study
that focuses on driver behavior is entitled Driver Behavior Mode/ of
Merging11. This study focused on merging using freeway acceleration
lanes, which are obviously characterized by ramp-type entries with higher
11Richard M. Michaels and Joseph Fazio, Driver Behavior Model of Merging," Transportation
Research Record 1213, pp. 4-10.
28




design speeds than RTAL entries at intersections. This study presents a
theory and develops an analytical model of the freeway-acceleration-lane
driver decision process when selecting a gap in freeway traffic acceptable
for merging. The study noted that accepted acceleration lane design and
criteria have resulted from technical data on vehicle characteristics, notably
acceleration capabilities, not as much from empirical observations of driver
behavior when merging. A speed difference between merging vehicles and
vehicles on the mainline is generally less than 5 mph. This is reflected in
current design methods and standards. The required freeway acceleration
lane length depends on the speed of mainline traffic, grade of the ramp and
the radius of the curved entry to the lane.12
The Merging Process. The Michaels and Fazio study presents a
hypothesis that the act of merging consists of a series of tasks that the driver
must complete in order to perform the merge. The competent driver
performs these tasks in sequence with continuity and fluidity.
The following are the merging tasks:
1. Ramp curve tracking
2. Steering transition from the ramp to the acceleration
lane
3. Acceleration
4. Gap search
nlbid., p. 4.
30


5. Steering transition from the acceleration lane to the
freeway or Abort13
The first two tasks occur on the entry ramp to the acceleration lane. The
motorist must guide the vehicle around the curve or to the acceleration lane
before being able to begin to search for gaps in the freeway traffic stream.14
This is one difference between the freeway acceleration lane studied in the
Michaels and Fazio report and the arterial RTAL studied in this report.
Curved entry ramps to freeway acceleration lanes generally have a
significantly longer radius with a higher design speed than the intersection
corners which are the entry curves to arterial RTALs. In addition, freeway
entry curves are more separated from the mainline, perhaps making it more
difficult to search for gaps. The RTAL sites do not have ramp-type entries -
only conventional intersection corners or a channelized right turn lanes.
Some of these have some fairly large curve radii by conventional
intersection standards, but certainly different in character from freeway ramp
entries. At most of the RTAL entry corners, the motorist is able to make an
initial determination of gap distribution in the upstream traffic stream and
begin a gap selection process prior to entry to the RTAL. This would differ
from that which is hypothesized in the Michaels and Fazio report of merging
conditions for freeway on-ramps. The model in that report assumes that gap
selection does not occur during ramp curve tracking or the transition from
the curved entry to the acceleration lane.
13 Ibid.
uibid.
31


The level of gap selection prior to lane entry (versus more gap selection in
the lane parallel to the mainline) may depend on the size of the radius or the
arc of the entry to the RTAL. This may also depend on if there are lanes
adjacent with queued vehicles that could block visibility to the left. With a
larger arc, there is likely less occurrence of gap selection prior to lane entry.
Possible reasons for less attention to early gap search with a higher speed
ramp entry are: because the entry into the lane at higher speed requires
more attention to the guiding of the vehicle; the entry is less abrupt creating
the feeling that it is safer to enter the lane because the vehicle position is
more parallel to the mainline as opposed to perpendicular with a tight
corner. That is the case at some intersections. The feeling of being hit
broadside is perhaps lessened.
The model contained in the Michaels/Fazio report assumes that gap
selection and the viewing of upstream traffic occurs once the vehicle is in the
acceleration lane parallel to the freeway as long as the lane is of sufficient
length. The model described in the report is based on a theory of gap
selection by evaluating of the angular velocity between the drivers merging
vehicle and the vehicles approaching on the mainline. Once in the
acceleration lane, a driver will evaluate the rate of change of the angle
between the lines of sight connecting his vehicle and approaching vehicles
(one vehicle leads the gap, another vehicle lags behind the gap). When the
angular velocity between these lines is less than the drivers threshold for
acceptable merging, then the decision to begin steering into the through lane
is made. The model suggests that if the angular velocity threshold is not met
and no sufficient gap is available, that the driver would reject the gap(s) and
enter what is referred to as an acceleration phase where the merging
32


vehicle accelerates to achieve a speed closer to that of the freeway vehicles.
After this acceleration phase, another gap search phase will occur followed
perhaps by another acceleration phase and gap search phase until the lane
ends or an acceptable gap is found.15
It is reasonable to assume that although the arterial RTAL user leaving an
intersection is often able to evaluate the traffic stream for gaps prior to
entering the lane, once in the lane, the process may be similar to that of the
freeway merging with iterative gap search and acceleration phases (provided
the lane is of sufficient length). This is illustrated in Figure 3.5.
Arguably, the best method of merging into a congested roadway is to
accelerate continuously until a speed close to that of the mainline is reached.
However, this is difficult for the merging vehicle, given the varying speeds,
grades, acceleration capability of vehicle, varying lane lengths and other
criteria to determine with confidence whether the lane is long enough to
reach an acceptable speed within the length of the lane. Therefore, drivers
resort to the gap search/acceleration/gap search process as it is easier and
yields more confidence to judge, in an iterative manner, the lane length and
traffic speed conditions with multiple snapshots. Given the iterative
process, the speed profile of the merging vehicle should resemble the graph
shown in Figure 3.6.
Factors Influencing Merging Behavior, Aside from the length of the
acceleration lane parallel to the mainline itself, merging behavior is
15Ibid., p.5.
33






Initial Gap Search Phase prior to entering the intersection
Guiding vehicle into RTAL
Start of the RTAL
Acceleration Phase
Gap Search Phase
Steering Maneuver into through lane (or)
Acceleration Phase
Gap Search Phase
Steering Maneuver into through lane (or)
Abort
Figure 3.5: Driver Behavior Model of Merging-Modified for RTALs


DISTANCE
Figure 3.6: Predicted Ramp Vehicle Speed Profiles
i
Sou rce: Driver Behavior Model of Merging,
Michaels and Fazio, p. 6.
35


constrained by two factors. One is the controlling level of ramp curvature.
This sets the maximum speed of the vehicle at the entry point of the
acceleration lane- the section parallel to the mainline. In loop ramps, this is
determined by the design speed of the curve.16 This entry speed principle
may also hold true also for the RTAL on urban/suburban streets as the entry
speed may be about 5 mph at an intersection with a tight corner or 15-20+
mph for corners with wider radiuses/channelized entries. The only exception
to this is that the driver may not fully utilize the maximum allowable speed as
there may be gap search activity during the curve negotiation. This may
dictate that motorists negotiate the curve at slightly lower speed as his
attention is divided between guiding the vehicle and searching for a gap.
The second factor identified as influencing merging behavior is the gap
distribution of the freeway traffic stream which is said to be largely dictated
by volume.17 In the case of arterial RTALs, this gap distribution may be
significantly different from with freeways, as the presence of traffic signals
can create platoons of vehicles rather than a random-type flow associated
with freeways. At a signalized intersection, once the mainline arterial signal
turns green, there may be a long, dense platoon conflicting with the right
turning (RTAL) traffic from the cross street looking to enter the arterial.
Often between platoons, especially following a mainline red signal indication,
there may be large gaps or a period of sporadic/light traffic flow. Thus, in a
matter of a few moments, the gap distribution may change from a period of
very tight gaps followed by a period of numerous acceptable/large gaps.
16ibid., p. 6.
11 Ibid.
36


gaps. This is why (along with the entry intersection geometry), RTAL
motorists, unlike in the case of freeways, may complete a gap search prior
to entering the lane and may opt to wait for a series of larger gaps as they
understand the platoon flow characteristics of arterial streets versus the
continuous random flow characteristics of freeways. With freeway
acceleration lanes, most motorists realize that the distribution of gaps is
more random, so it is usually more prudent to simply continue into the
acceleration lane without stopping or slowing significantly.
Acceleration Lane Speed Profile. The actual documented speed
profile based on the results of that test for the use of a freeway acceleration
lane is represented in Figure 3.7. The results represented in the figure show
that during the gap search phases of the merging process that there was a
speed reduction, or a period of negative acceleration (deceleration), while
the motorists searched for an acceptable gap. During this period, drivers
were attending to the gap search process and not to maintaining vehicle
speed. The study showed that with more successive trials or iterations, the
probability of merging increased. Also, the probability of merging increased
as the speed difference between the merging vehicle and the mainline
vehicles decreases. The probability of merging also increases with more
time and distance given for mainline and merging motorist interactive
maneuvering (i.e., the motorists on the mainline adjust speed and position to
create gaps for the merging motorist). Drivers in the through lanes generally
respond to vehicles in the acceleration lane by either speeding up or slowing
down to help create an acceptable gap to the front or rear for the merging
vehicle. Under certain situations, motorists elect to move from the right
37


Figure 3.7: Observed Velocity Profiles
Source: Driver Behavior Model of Merging,
Michaels and Fazio, p. 7.


through-lane to the left, to create a gap in the right through-lane.18 These
interactive maneuvers would seem especially important for arterial RTALs,
as the closely spaced gaps within platoons make gap selection even more
difficult for the RTAL driver who may attempt to enter the traffic stream
during the presence of a large platoon.
Aggressive vs. Average Drivers. It is perhaps obvious that
acceleration lane driver behaviors vary. Drivers on the more aggressive or
well-practiced end of the spectrum utilizing maximum acceleration from the
beginning of an acceleration lane of reasonable length will enable them to
reach a speed close to mainline traffic more quickly. This will greatly
enhance their ability to find an acceptable gap and merge. The speed
profile of these drivers should be more continuous without a break in
positive acceleration rather than discrete as shown in Figure 3.7
representing the more average driver.19
Other Differences. Other differences between freeway acceleration
lanes and arterial RTALs are: Motorists are generally not as familiar with
arterial RTALs as with freeway on-ramps/speed change lanes. There are
many more freeway acceleration lanes as it is national standard for freeways
to have acceleration lanes/ramps. This is not the case at urban/suburban
arterial intersections. Also, the edges of the transition tapers of RTALs are
often concrete-curb, whereas many freeway ramps end with a striped taper
that is followed by a paved shoulder. This shoulder can provide some
l*lbid p. 8.
l9lbid.
39


measure of refuge, if an abort operation is necessary. Concrete-curb
tapered RTALs do not have that refuge. Vehicles on the mainline arterial
may realize this and make more of these adjustments to accommodate the
RTAL merging vehicles. These tapers may cause more hesitation by RTAL
motorists.
Acceleration Rates
This report assumes that the particular process of acceleration/ gap search
holds true for arterial RTALs once in the lane. This report the driver behavior
model for merging contains a graph showing distance traveled versus
speed. Acceleration rates for vehicles in the acceleration lane were
determined from that particular study. These acceleration rates, calculated
from the change in velocity over distance traveled, have been used in part in
Section IV, to estimate the most typical acceleration rates for vehicles using
urban RTALs. The following are acceleration rates based on the change in
vehicular velocity over distance for both for the acceleration phase of the
merge and the negative acceleration or deceleration phase during the gap
search phase. Based on the graph, the acceleration rate between a speed
of 33 mph and 40mph was 3.6 mph/sec, and between 38 and 43mph, was
2.9 mph/sec. These are acceleration rates during the acceleration phase.
During the gap search process, the rate was a deceleration of about -2.75
mph/sec.
Acceleration Phase vs. Gao Acceptance
A study published by the ASCE Journal of Transportation Engineering about
40


the vehicle flow characteristics of acceleration lanes also concluded that
driver behavior in acceleration lanes is not only an acceleration component
but also that gap acceptance procedures are incorporated in the merging
process on to freeway facilities. It was described that only slight
acceleration is accomplished in this lane, where the initial speeds are about
35 to 40 mph. This particular article perhaps places even more importance
on the gap-search phases of freeway acceleration lane use (i.e., the key
elements of merging are those related to gap acceptance with acceleration
being ancillary).20 While gap acceptance maneuvers may be more important
in the freeway merging process (with entries being higher speed, and less
acceleration needed) in order for RTALs to be effective, acceleration may be
more important than with freeway acceleration lanes, due to slower entry
speeds. With most arterial RTALs, more substantial acceleration needs to
occur for vehicles to reach free flow speeds close to those of the mainline of
the arterial. Perhaps for RTALs, more of the gap search can be
accomplished at the intersection or there is needs to be more combined
acceleration/gap search procedures (i.e., accelerating while searching for
gaps). The report also addresses that freeway traffic flow and gap
distributions are important considerations.21 These distributions are
important for arterial streets as well, but distributions can be very different.
The RTAL driver has a similar challenge of gap-acceptance, but also a more
substantial component of acceleration. As much as this acceleration
component can be eliminated with a faster entry, the more the lane length
20Abishai Polus and Moshe Livneh, Members, ASCE and Jorge Factor,ASCE, Journal of
Transportation Engineering, Vol III, No. 6, Nov. 1985, American Society of Civil Engineers,
p. 603.
21lbid.
41


can be used for gap searching. This would reduce the driver workload of
the motorist using the RTAL.
Other Lane Use Behavior
There are motorists who enter the through lane at the first available
opportunity and others who stay in the acceleration lane until the transition
taper guides them into the through lane. The motorist using all of the lane
even though a gap of the right size becomes available prior to reaching the
end of the lane is explained in this report which sites three reasons:
(1) They want to increase their speed by accelerating at a
comfortable rate beyond the merging end (2) they feel more secure
on the extra lane, being apart from the through traffic or (3) They feel
that proper driving behavior requires performing the merging
maneuver as close to the end of the acceleration lane as possible,
and at a very flat angle relative to the through lane, allowing
oncoming vehicles on the highway to change lanes if necessary...22
A major report on freeway acceleration lanes in Indiana states that
to obtain maximum efficiency and safety in the operation of
acceleration ... lanes, and to maintain efficiency on the main facility, it
is necessary to relate the design of such lanes to traffic behavior, as
indicated by the requirements and desires of drivers..The optimum
condition of operation for an acceleration lane is to have acceleration
lane traffic accelerate on the acceleration lane and merge into the
through lane traffic at approximately the same direction of travel and
22Ibid., p.603.
42


at the same speed as the through lane traffic.23
Motorists turning onto a highway or arterial need to accelerate up to a speed
close to that of through traffic. It is hazardous and causes disruption to the
flow of traffic in the through lanes when acceleration by a vehicle entering
the facility takes place in the through lane.24 The report says that it is
optimal for motorists entering the through lane to use the acceleration lane
for the vehicles complete acceleration phase, as opposed to entering the
through lane and finishing acceleration in the through lane. Therefore, they
should be designed so that the motorist can enter the through lane without a
speed differential requiring significant continued acceleration.25
Accident Rates
A report entitled The Relationship of Accidents to Length of Speed Change
Lanes and Weaving Areas on Interstate Highways showed that accident
rates surrounding the use of acceleration lanes increased as the relative
percentage of merging vehicles increased. The study indicated that this
occurred regardless of the acceleration lane length. The study also showed
that with shorter length acceleration lanes that the accident rate increased.
This occurred regardless of the ratio of traffic merging onto the freeway to
mainline traffic volume. When the percentage of traffic merging relative to
the mainline through traffic is less than 6 percent, the length of the
acceleration lane does not result in a large increase in accident rates. The
^Neddy C. Jouzy and Harold L. Michael, Use and Design of Acceleration and Deceleration
Lanes in Indiana," Highway Research Record, Highway Research Board, No. 9, p. 25.
24 Ibid.
25lbid.
43


report says the savings in accidents for the additional length of speed
change lanes probably would not offset the cost of the additional length.26
Additional length results in a significant savings in accidents in the cases
where the merging to through traffic percentage exceeds 6 percent. In
addition, increased length on acceleration lanes should improve the
operation of the facility if properly used by vehicle drivers.27" Although these
findings are for freeway acceleration lanes, the principles may be
transferable in some manner. However, with RTALs, the gap distribution
and level of platooning of traffic also would need to be factored in.
RTAL Implementation Considerations and Costs
Regarding the implementation of RTALs, the following situations may apply:
RTALs may be (1) incorporated into new street design and
construction/major street widening projects, or (2) they may be added to an
existing street with the addition of a new access or intersection or as part of
intersection improvements.
It is generally less expensive to simply incorporate the acceleration lane
design into new street construction or major widening. Only an extra 6-14
feet of additional pavement would be needed and perhaps that much extra
right-of-way as paved shoulders may be reduced and sidewalks, which may
be detached from the edge of traveled way, could be attached adjacent to
26JA Cirillo, The Relationship of Accidents to Length of Speed Change Lanes and Weaving
Areas on Interstate Highways," Highway Research Record, Highway Research Board, No.
312, p. 25.
27Ibid., p 24-25.
44


RTALs as the RTAL serves as an additional buffer to the through lanes.
Cost of adding an RTAL to an existing street can often involve the removal
of existing curbs, reconstructing existing sidewalk and replanting
landscaping, removal of mature trees and other features that may be close
to the existing curb or roadside, relocation of existing utility infrastructure
along the edge of the street, acquisition of additional right-of-way, disruption
of traffic during construction. Consideration should also be given to
economic costs of reducing the size of a parcel of land for adding extra width
to the street cross section, as new developments often have setback
requirements, landscaping and parking requirements, and drainage features.
45


CHAPTER 4
ANALYTICAL ESTIMATE OF
APPROPRIATE RTAL LENGTH
The following is an analytical model used to mathematically estimate the
length required for arterial RTALs. This model has been developed based
on the driver behavior model of Merging identified in the previous section.
Based on the speed profiles contained in the driver behavior model of
merging, the experiment conducted in the previous section and the recorded
maximum acceleration rates from Table 3.4, the following set of estimated
acceleration rates was selected for use in the lane length determination
model.
0-15 mph
15-30 mph
30-40 mph
40-50 mph
Gap Search Phase
4.0 mph/second
3.75 mph/second
3.5 mph/second
2.5 mph/second
-2.5 mph/sec
This particular model, although similar in concept to the freeway model,
varies in that lane entry speeds assumed are significantly lower and
estimates of acceleration rates at lower speeds were used to estimate
speed/distance profiles at the lower speeds.
46


Five MPH Entry Model
This assumes use of the RTAL on an arterial under congested conditions,
where the vehicle cannot enter the lane without accelerating and merging.
The first set of calculations assumes an entry speed (at the start of the lane)
of 5 mph based on a fairly tight corner and the fact that a vehicle accelerates
to 5 mph over a fairly short distance between the stop condition and the start
of the lane at the PCR. For this model, it is assumed that the gap search
begins prior to lane entry, and that the first phase is an acceleration phase
for 100 feet followed by a one second gap search phase. It is assumed that
this acceleration/gap search process continues for three iterations (i.e.,
acceleration, gap search, acceleration, gap search etc.
The graph in Figure 4.1 shows the speed/distance diagram for an average
driver who might adhere to the developed model(average drivers). This
slope of the lines represents the acceleration values shown above. Figure
4.2 illustrates the phases shown in Figure 4.1. The graph in Figure 4.3
represents a more aggressive driver who accelerates through the gap
search process rather than suspending acceleration during this phase.
Results
To achieve a speed of 45 mph at the end of the lane, given this RTAL entry
speed of 5 mph, an average driver would need about 660 feet of lane length.
For the aggressive driver who accelerated through the acceleration and gap
47


Acceleration Profile
Distance vs. Speed Starting at 5 mph
0 100 200 300 400 500 600 700
Distance Traveled (ft)
___________________ Average Driver___________________
Figure 4.1: Acceleration Profile Starting at 5 mph, Average Driver



1 O mph Initial Gap Search Phase prior to entering the intersection
2 Acceleration from O mph to 5 mph
3 Start of the RTAL Speed 5 mph
4 Acceleration Phase
5 Gap Search Phase one second duration
6 Final Acceleration Phase
Figure 4.2: RTAL Merging Model 5 mph Entry Condition


Figure 4.3: Acceleration Profile Starting at 5 mph, Aggressive Driver


search phases starting at 5 mph the lane length would be 445 feet.
51


Twenty MPH Entry Model
The next set assumes an entry speed of 20 mph that results from a larger
curb radius on the corner of the intersection Figure 4.4 shows the profiles for
average drivers and Figure 4.5 illustrates the phases. The profile for more
aggressive drivers for this 20-mph entry is shown in Figure 4.6. As in the 5
mph model, it is assumed that the gap search begins prior to lane entry, and
that the first phase is an acceleration phase for 100 feet followed by a one
second gap search phase. The acceleration/gap search process continues
for three iterations (i.e., acceleration, gap search, acceleration, gap search
etc).
Results
To achieve a speed of 45 mph at the end of the lane, given this RTAL entry
speed of 20 mph, an average driver would need about 620 feet of lane
length. For the aggressive driver, who might continue accelerating through
the gap search phases the lane length would need to be 370 feet.
52


Acceleration Profile
Distance vs. Speed -Starting at 20 mph
0 100 200 300 400 500 600 700
Distance Traveled (ft)
________________________________ Average Driver________________
Figure 4.4: Acceleration Profile Starting at 20 mph, Average Driver


Figure 4.5: RTAL Merging Model 20 mph Entry Condition


Acceleration Profile
Distance vs. Speed -Starting at 20 mph
0 100 200 300 400 500 600 700
Distance Traveled (ft)
- Aggressive Driver
Figure 4.6: Acceleration Profile Starting at 20 mph, Aggressive Driver


CHAPTER 5
STUDY OF RTAL USE
This section presents a study of right turn acceleration lanes on
urban/suburban arterial streets in the Colorado Springs and metro Denver
areas. The overall goal of the study has been to identify which geometric
characteristics and traffic conditions influence driver behavior with respect to
lane use and lane operations.
Several sites on urban/suburban arterial streets exist in the Colorado
Springs and Denver areas that have right turn acceleration lanes. Twenty-
seven of these sites were selected to be included in this study as they meet
study criteria of lane type (stand-alone RTAL), and arterial speed (40-50
mph). Research included identifying the physical characteristics of each
site, videotaping the traffic operations for each site, reviews of video tapes to
determine the level of lane usage, other driver behavior. Regression models
have been developed to represent the driver behavior based on data
collected. Models were developed so that statistical techniques could be
used to determine which site geometric characteristics and traffic conditions
significantly influence driver behavior and measures of effectiveness. A set
of recommendations based on the case study for geometry of RTALs is
presented.
56


Methodology
The following steps were completed in the study of RTALs in Colorados
urban/suburban areas:
11 Search the Denver and Colorado Springs Metropolitan areas for
acceleration lanes fitting the criteria. Criteria includes Stand alone
acceleration lanes on urban/suburban arterial streets with posted speed
limits of 40 to 50 mph.
2) Conduct a field survey of each site. This included field measurements of
the lane geometry and traffic control at each site and a videotape of traffic
operations. During videotaping, initial general assessments of operations
and notations of driver behaviors for each right turning motorist were made.
Behavior noted was whether or not the lane was used, type of entry into the
through traffic, and how much of the lane was used.
3) Replay of videotape: This allowed further data collection including the
determination of traffic volumes (right-turning and opposing), the presence of
opposing traffic, the delay per vehicle using a stopwatch, length of lane
used, signal timing/phasing, and general observations of how the lane was
used and the safety/effectiveness of lane operations.
4) Compilation of results: Following the review of the videotape, a database
was created with results organized and sorted.
57


5) Data analysis: This process involved the completion of simple and
multiple regression models and use of the statistical information from the
regression.
6) Develop and present findings: Using results of the analysis, the
determination of whether lane length, entry arc, opposing traffic, and other
elements were identified as significantly influencing driver behaviors.
7) Recommendations: This section presents modeled levels of performance
given geometric elements.
Site Selection
Site Selection and Data Collection
Sites selected for this study are located in the Denver, Colorado area and
Colorado Springs, Colorado. RTAL sites were selected for study if they met
the criteria. Lanes studied are "stand-alone acceleration lanes ending in a
taper (no continuous acceleration/deceleration lanes). The sites studied
have posted speed limits of 40-50 mph, and two or three through lanes in
each direction. The lanes selected varied in length, width, curb radius,
channelizing island type (some have no channelizing island), traffic volume,
and traffic control (some locations were signalized and some were stop-sign
controlled).
Site videotaping was conducted during or just prior to the morning or
afternoon peak travel periods. Most morning peak hour analyses were
completed in the middle of the AM peak period (7-8 A.M.). Many of the
58


afternoon analyses were taken early in the PM peak period (4-5 PM) The
Denver area sites selected are on Wadsworth Boulevard, Kipling Parkway,
104th Avenue, and Sheridan Boulevard in the Denver Metropolitan area. The
Colorado Springs sites are on Powers Boulevard, Academy Boulevard,
Union Boulevard and Research Parkway. Cross streets are generally lower
volume.
Data Collected
The following specific data have been collected to characterize each site.
Both geometric elements and traffic conditions were recorded.
Geometric Conditions. Measurements of the geometric
elements/features of each site were taken- these included: RTAL lane
length, widths, taper length, number of through lanes, type (if any) of
channelization for right turning vehicles, inside curb arc length at the
intersection, median types, shoulder types, shoulder widths, and traffic
control. The list below identifies the specifics. Figure 5.1 shows the
geometric conditions for the sites studied.
Lane Length has been measured from the curb PT (point of tangency) to
the beginning of the taper. This is shown in Figure 5.1.
Lane Width has been measured from the left edge of the lane to the edge of
asphalt (in the case of a curb and gutter street) or the inside edge of
shoulder, (in the case of streets without curb and gutter). In some cases, the
lane width varies over the length of the lane. The width was measured in the
59




first section of the lane. This is also shown in Figure 5.1. Lane Taper has
been measured from the end of the acceleration lane to the end of the taper
or the point where the striping matches the original right edge of the right
through lane. The lane taper is illustrated in Figure 5.1.
The Entry arc is a measurement intended to estimate the tightnessof
curvature of the corner between the cross street and the arterial and entry
into the acceleration lane. As shown in Figure 5.2, the measurement was
taken along the inside face-of-curb (for curb and gutter streets) and along
the right-edge lane striping or edge of the lane for streets without curb and
gutter. The measurement is of the curved portion, from the PC to the PT.
The reason this measurement was taken is that a larger curb can translate
to a faster cornering design speed and easier transition and entry into the
lane. Use of this particular measurement when comparing sites is only valid
for 90-degree, right angle intersections. With intersections greater than 90
degrees less arc length is needed to achieve the same curvature or curb
radius. Two sites have intersection angles at greater than 90 degrees. For
these intersections, the measured arc was adjusted mathematically to obtain
an equivalent arc length if the intersection were at 90 degrees. The
correction accounts for the appropriate sharpness of curvature, however,
there may be other elements inherent to a skewed intersection such as
perceptions of oblique entry vs. ninety-degree entries which have not been
accounted for. The entry arc was used in leu of corner curb radiuses, as
this would be difficult to measure in the field without survey equipment or as-
built design drawings. Moreover, the radius of curvature can vary through
the curve.
61


Figure 5.2: Measurement of Entry Arc Length
Channelization Type: This identifier describes the type of channelizing
island( if any) for entry into the RTAL. The island types are shown in Figure
5.3. Some sites do not have channelizing islands. Many of those that do
have striped islands or raised islands outlined with concrete curb and gutter.
Generally, the larger the island, the larger the curb entry arc. For this
reason the island size was not measured. Raised islands can provide a
measure of protection or a level of comfort for vehicles entering the RTAL -
protection from through and opposing left turning traffic.
Median Type: This indicates the type of center median for both the arterial
and the cross street. All site major (arterial) streets have some type of
center median. Most have raised center medians. This is typical of newer
62



Arterial Street
| [ RTAL'
Cross Street/Minor Street
No Channelizing Island
Arterial Street
Cross Street/Minor Street
Striped Channelizing Island
Arterial Street
RTAL
Cross Street/Minor Street
Raised Channelizing Island
Figure 5.3: Right Turn Channelization Types
63


suburban arterial streets in Colorado.
Posted Speed: the posted speed limit on the arterial street for each site.
The posted speed limit on most arterial streets was 45 miles per hour.
Traffic Control: Indicates the type of intersection control (traffic signal or
Stop-sign) for the minor street approach to the arterial.
Traffic Conditions. The weather and time of day of the videotape and
accompanying field records were noted. All lanes were clear of excess
sand/gravel at the time of data collection, and there were no accidents.
Traffic signal timing/phasing might have changed for different time periods,
however, this was not specifically recorded. Elements listed below are
illustrated in Figure 5.4.
Opposing Through Traffic: This is the volume of through traffic on the
arterial in the same direction as the RTAL- Data includes the total volume
counted, the time period counted, the calculated hourly volume per lane
(Total hourly volume divided by the number of through lanes).
The Right Turn volume- The volume of traffic turning from the cross street
onto the arterial street in the direction of the RTAL some motorists used the
RTALs and some did not. This is identified in the results section. This
volume identifies the number of potential users of the RTAL.
Opposing left turn volume: The volume of traffic turning left from the cross
64


r
v
(§) Right turning vehicle under study
Opposing Through traffic
(2) Opposing left turning traffic
(3) Opposing U-tuming traffic (included in opposing left turning traffic)
Figure 5.4: Opposing Traffic Movements


street leg opposite the RTAL onto the arterial street to travel in the same
direction as the RTAL. In some instances this left turn volume conflicts with
the right turn volume such as where this opposing left turn has a protected
phase. Also with a raised channelizing island, left turning vehicles may
execute left turns even with right turning vehicles as it acts as a separator.
Number of Through Lanes: This identifies the number of through lanes in
the direction of the arterial in the same direction of the RTAL.
Sites Studied
The specific geometric characteristics and traffic volumes at each site are
listed in Tables 5.1 and 5.2 respectively. The following describes sites with
some unique situations that require additional brief explanations.
Site: Northbound Wadsworth at Ken Karvl
This site is at the intersection of an arterial (Ken Karyl) and an
expressway/parkway (Wadsworth Boulevard). Northbound Wadsworth is
on an upgrade at the intersection (the northbound RTAL is on an upgrade).
The intersection is skewed the north/south legs (the orientation of
Wadsworth is slightly northwest to southeast and Ken Karyl is east/west).
This configuration favors easier entry into the acceleration lanes because
less than a ninety-degree turn is required. Most motorists appeared
comfortable entering the lane with vehicles approaching in the adjacent
through-lane even with no channelizing island. There are few intersections
in this data set that are skewed, and it is a significant difference. The skew
also affects the eastbound and westbound left turns. It appears that
66


Table 5-1: Site Geometry
Site Identification Geometry
Number Intersection Location RTAL Lenath Taper Lenath Length Plus Taoer RTAL Width "Arc" Lenath Raised Channelizing Island? Island Identifier Signalized Intersection? Duration of Count Number | of Through Lanes
(feet) (feet) (feet) (feet) (feet) 1 "ves" 1 *"yes" (minutes)

1 NB Academy a Sams Access 425 150 575 12 19 none 0 0 60 3
2 NB Powers <3>. Galley 300 200 500 12 175 ves-ralsed 1 1 30 3
3 Union& Research 250 180 430 12 45 no 0 1 . 30 3
4 Union & Acacia 220 225 445 12 45 no 0 0 60 3
5 Platte/PhllllDS 66 Access 250 250 500 23 46 no 0 0 60 2
6 T-Gao and Union 100 80 180 10.5 45 raised islnd. 1 1 45 3
7 SB Wadsworth a.94th 175 250 425 12.5 30 none 0 1 60 2
8 SB Wadsworth a104th 485 200 685 13.5 109 raised Islnd. 1 1 60 2
9 NB Wadsworth Stanford 250 225 475 11 48 none 0 1 60 3
10 SB Wadsworth a Indeoend. 325 130 455 14 81 none 0 1 60 2
11 WB 104th Brvant 350 170 520 13 35 none 0 0 60 2
12 EB 104th a Lowell 140 130 270 10 39 none 0 1 60 2
13 EB 104th Wolff 185 180 365 10 47 none 0 0 60 2
14 NB Wadsworth a 108th 155 475 630 13.5 75 none 0 1 60 2
15 NB Wadsworth 104th 190 290 480 11.5 73 none 0 1 60 2
16 EB Ken Karvl a Simms 360 350 710 11.25 90 none 0 1 60 2
17 SB Federal a 114th 770 50 820 12 35 none 0 0 60 2
18 NB Klolina a Quincv 550 180 730 12 130 yes-raised 1 1 60 2
19 NB Wadsworth a Accessno Bellvlew 400 180 580 12 150 ves-ralsed 1 0 60 3
20 SB Klolina a Lakehurst 550 185 735 12 67 no 0 1 60 2
21 SB KlDllna a Asburv 170 200 370 11.6 43 none 0 1 45 2
22 SB Sheridan a 100th 190 150 340 12 72 none 0 0 60 2
23 SB Klolina aRoxburv 575 185 760 12 56 none 0 1 60 2
24 NB KlDllna a Coal mine 425 270 695 12 118 ves-ralsed 1 1 30 2
25 SB Wadsworth a 108th 180 400 580 12 78 none 0 1 60 2
26 NB Wadsworth a Ken Karvl 430 200 630 15 71 none 0 1 30 2
27 SB Wadsworth a Ken Karyl 170 185 355 16 45 none 0 1 60 i I


o>
00
Table 5.2: Traffic Volume Data
Site Identification Ooooslno Volumes RTAL Right-turning Volume (vph) Number ofThrougt Lanes Oppr elng Volumes per lane I
Number Intersection Location Opposing Through Volume (vph) Opposing Right-turning Volume (vph) Opposing Lsft-tuming Volume (vph) Opposing Volume perLane- Throughs (vphpl) Opposing Volume per Lane- Lefts (vphpl) Opposing Volume I per Lane I Total of Thrus 4LTs (VDhOl)

|
1 1 NB Academy (SI Sams aco. 1723 n/a 0 55 3 574 0 574 l
I 2 NB Powers (St Gallev 936 n/a 6 99 3 312 0 312
I 3 Unlon& Research ,662 n/a 13 7 i 187 13 200
4 Union & Acacia 2023 117 0 146 3 674 0 674
I 5 Platte/Phllllos 66 Access 150$ 16 0 M 2 765 0 755
II 6 T-Gbd and Union 1870 n/a 0 U 3 623 0 323 I
7 SB Wadsworth (3194th 1120 n/a 20 121 2 660 20 580
I 8 SB Wadsworth (3) 104th 635 n/a fj 256 2 268 74 342
9 NB Wadsworth (81 Stanford 1943 n/a 19 163 3 648 19 667
y 10 SB Wadsworth ffl Indeoendenoe 693 n/a 0 66 2 447 0 447
11 WB 104th a Bryant 1056 n/a 0 11 2 528 0 528
I 12 EB 104th ffl Lowell 708 n/a 0 109 2 354 0 354
13 EB 104th ffl Wolff 721 n/a 0 51 2 361 0 381
14 NB WadsworthO 108th 1367 n/a 166 23 2 684 168 850
15 NB Wadsworth ffl 104th 1376 n/a 75 69 2 688 75 763
16 EB Ken Karvl ffl Simms 363 n/a 118 5i 2 182 118 300
17 SB Federal 114th n/a o. 28 2 376 0 376
18 NB Klollno ffl Quincy 1331 n/a 248. _ 3T $ 666 248 914 I
19 NB Wadsworth ffl Access $243 n/a 0 ?1 3 748 0 748 I
20 SB Klollno ffl Lakehurst 1592 n/a 110 69 2 781 110 891 I
21 SB Klollno ffl Asburv 1627 n/a 28 80 $ 813 28 841 I
22 SB Sheridan ffl 100th 1$86 n/a 9 28 2 693 0 693 I
23 SB Klollno fflRoxburv 868 n/a 24 2 434 0 434 I
24 NB Klollno ffl Coal mine 609 n/a 76 . 04 2 305 78 381
25 SB Wadsworth ffl 1Q8th 446 n/a 66 46 2 33 66 289
28 NB Wadsworth ffl Ken Karvl 757 n/a i4i 125 2 379 242 621
27 SB Wadsworth ffl Ken Karvl 755 n/a 44 61 2 378 44 422


less than a ninety-degree turn is required. Most motorists appeared
comfortable entering the lane with vehicles approaching in the adjacent
through-lane even with no channelizing island. There are few intersections
in this data set that are skewed, and it is a significant difference. The skew
also affects the eastbound and westbound left turns. It appears that
EASTBOUND/WB right turners realize this and use the acceleration lane to
merge with these left turners because they realize they turn into the right
through-lane as they need more room to turn. The grade may also have an
effect on the use of the lane. There was some significant platooning of
vehicles NB and some large gaps following those platoons.
Site: Southbound Wadsworth at Ken Karvl
This site is at the intersection of an arterial (Ken Karyl) and an
expressway/parkway (Wadsworth Boulevard). Northbound Wadsworth is
on a grade at the intersection (the northbound RTAL is on an upgrade).
The intersection is skewed the north/south legs (the orientation of
Wadsworth is slightly northwest to southeast and Ken Karyl is east/west).
This configuration favors easier entry into the acceleration lanes because
less than a ninety-degree turn is required. Most motorists appeared
comfortable entering the lane with vehicles approaching in the adjacent
through-lane even with no channelizing island. There are few intersections
in this data set that are skewed, and it is a significant difference. The skew
also affects the eastbound and westbound left turns, as they have a more
than 90 degree angle to turn in. It appears that eastbound/westbound right
turners realize this and use the acceleration lane to merge with these left
turners because they realize they turn into the right through-lane as they
69


need more room to turn. The grade may also have an effect on the use of
the lane. The eastbound to southbound right turn volume is fairly light and
so is the WB left turn volume. Generally, if a right turning motorist arrived
and was ready to turn right on red when a SB platoon was passing, the right
turner would not enter the RTAL to merge, but would wait for a gap, then use
the lane. The lane is wide but short. Three of the turning vehicles surveyed
were large multi unit trucks that could not possibly turn the corner into the
RTAL.
Site: Westbound Platte at Phillips 66 access
Platte Avenue is the arterial street with the RTAL at a gas station/fast food
access. This access is a right-in/right-out access just west of a major
intersection of Murray/Platte. There is a deceleration lane approaching the
access. The acceleration lane is unique, as there is extra width between the
curb and the edge of traveled way. The striping is confusing. The through
traffic was fairly heavily platooned with some intermediate gaps as the
access is just downstream from the signal. Some right turning motorists,
who waited for gaps, waited a significant amount of time. There is a
guardrail adjacent to the curb return.
Although the curb entry arc is only 27 feet, there is about 23 feet between
the right edge of the through lane and the curb. Many drivers used it for
merging, as motorists appeared comfortable entering the lane. It seemed to
be due to the extra width as a buffer to oncoming traffic. Once in the lane,
some motorists appeared comfortable waiting in the lane until there was a
gap to merge into.
70


typical of RTALs at intersections along Powers. There is a very large WB
right turn channelizing island. The inside radius is large and
unimproved/confusing. The vast majority of vehicles did not stop prior to
entering the lane. Some vehicles had to slow down in the lane to merge due
to its short length.
Quantifying Lane Use. Operations and Performance
Following the documentation of independent site conditions such as lane
widths, traffic controls and traffic volumes and resulting lane
usage/performance was quantified and recorded, both under real time in the
field and during the playback of the videotape.
The following are part of this analysis: Lane use determination, the
identification of the presence of opposing traffic, right turn entry method, and
an incident analysis.
The Lane Use determination: A vehicle was identified as having used the
lanefyes or no) if the motorist guided his vehicle into the lane and was (at
least initially) in the lane facing the new direction of travel. With this vehicle
path, at least about 25 or 50 feet of the lane length was used. If a vehicle
turned into the through lane immediately or cut across the first 25 or 50 feet
of the right turn lane to short cut to the through lane, the vehicle was
identified as not using the lane.
Another component was the determination of the presence of opposing
traffic when the motorist had arrived at the intersection and was ready to
make a right-turn. The presence of opposing traffic indicator was not at the
71


time the right turn was executed, but at the time the motorist reached the
stop bar and was in a position ready to make a right-turn. The presence of
opposing volume was identified as both upstream through traffic and left-
turning traffic from the opposite direction of travel turning to travel the same
direction as the right turner under study. As shown previously in Figure 5.4,
many sites are at signalized intersections where the determination of the
presence of opposing traffic was a judgement call dependent in part on the
phase of the signal (i.e., if the right turner under study had a green signal,
this would result in a right turn with no opposing traffic, in most cases). It
was recorded as no opposing traffic. If he had a red signal, with a green
phase for the arterial street but no vehicles coming or vehicles far enough
upstream so a right turn could be made as if there were no acceleration lane
and minimal disruption of the oncoming vehicle(s). Opposing volume is
basically anything discouraging or preventing a vehicle from otherwise
making an immediate turn into a through lane if, hypothetically, no right turn
lane existed due to the geometric conditions at some sites, such as those
with channelizing islands, motorists were able to easily execute right turns
with opposing volume without stopping.
The next element of the lane use performance data set is the right turn
entry method /merging determination. The identifier differentiates motorists
entering the traffic stream using a merging maneuver or generally stopping
and waiting for an acceptable gap in the through traffic and then pulling into
the through lane as shown in Figure 3.4. In the majority of instances, the
execution of a merging maneuver corresponded with the identifier for use of
the lane, and the completion of a non-merging or gap-acceptance
maneuver corresponded with the motorist not using the lane. The attempt is
72


to correlate the drivers decision whether to use the lane and use it to
attempt a merging-type maneuver or not use the lane to attempt a merging
maneuver. This could either be a driver not using the lane or a case of a
driver using the lane but not using a merging maneuver is a motorist
entering the lane to use it as refuge to wait for an acceptable gap, rather
than accelerating to merge between oncoming vehicles.
Incident Analysis: The lane performance data set also includes an identifier
to quantify lane operations relative to the disruption of traffic flow on the
arterial. Lane operations relative to disruption of traffic flow on the arterial
were the focus of a separate analysis. The videotapes were reviewed to
identify lane operational incidents by type and frequency. The recorded
incidents are defined as near misses, disruption of vehicles on the mainline,
sharp or abrupt maneuvers that required short reaction times, or unusually
difficult or risky maneuvers for the driver entering the through lane. These
are types of maneuvers that have the potential to result in crashes or driver
frustration. The goal was to attempt to identify the lane geometry or site
conditions that may be correlated with higher incident occurrence levels.
The goal of the modeling and analysis of the data is to determine which
characteristics of each field site, have a significant effect on a motorists
decision to use the lanes and the other aspects of the lane operations.
These driver behaviors are expressed in terms of response variables in an
attempt to correlate these behaviors to site characteristics. The goal is not
only to determine which characteristics have an effect, but which have the
most significant effect on the decision to use the lanes and how the lanes
are used, and which characteristics result in the smoothest lane operations.
73


Lane Use Analysis
This presents the site analysis focusing on the driver decision to use the
RTAL lane or to not use the RTAL. A motorist is said to have used the
lane if the vehicle turns completely into the lane, and parallel to the through
lane. Use or non use does not consider how the vehicle enters the traffic
stream (this is addressed in the next subsection), only whether the lane was
used or not used. Results are shown in Table 5.3 for all right turning
motorists and Table 5.4 for right turning motorists with opposing volume
only. The statistical analysis of these data uses variables to represent site
characteristics such as lane length, width and corner arc length. These
analyses attempt to determine which characteristics are sufficiently
correlated to lane use to be statistically significant. For example, given sites
of varying lane lengths, is there sufficient evidence in the data to show that
lane length has a statistically significant effect on a drivers decision to use
an RTAL? Regression techniques have been used, as the sites selected
have a variety of different characteristics and differing levels of lane usage.
Variables have been assigned and simple linear regression analyses were
performed looking at each characteristic separately. Then the multiple
regression was completed to account for any combined or interactive effects
of the variables and to determine the relative importance of each
independent variable to the others.
Variables. Variables have been assigned to represent both site
characteristics (x independent or predictor variables) and driver behaviors
(y dependent or response variables)
74


Table 5.3: Results Data All Vehicles
Site identification RTAL Data for All Vehicles
Number Intersection Location Right #not Turning Using # using Percent # Using # Using Percent Using gap merging Using Merging Technique Technique RTAL Technique
Volume Lane lane

1 NB Academv (82 Sams acc. 55 38 17 . 51 4 31% 7%
2 NB Powers (82 Gallev 69 14 85 15 84 86% 85%
3 Unlon& Research 70 37 33 47 23 47% 33%
4 Union & Acacia 146 138 18 135 11 12% 8%
5 Platte/PhllllDs 68 Access 41 15 26 25 16 63% 39%
6 T-Gap and Union 74 : . 54 20 59 15 27% 20%
7 SB Wadsworth (8294th 121 77 44 82 39 36% 32%
8 SB Wadsworth (82104th 256 18 238 25 231 93% 60%
9 NB Wadsworth <82 Stanford 163 88 75 97 66 46% 40%
10 SB Wadsworth (82 Independ. 68 26 42 32 36 62% 53%
11 WB 104th (82 Brvant 11 4 7 5 6 64% 55%
12 EB 104th (82 Lowell 109 96 13 97 12 12% 11%
13 EB 104th <82 Wolff 51 38 13 41 10 25% 20%
14 NB Wadsworth (82108th 23 10 13 . 14 9 57% 39%
15 NB Wadsworth (82104th 69 11 58 11 58 84% 84%
18 EB Ken Karyl (82 Simms 31 27 4 27 4 13% 13%
17 SB Federal <82114th 28 12 16 14 14 57% 50%
18 NB Klpllno (82 Quincy 224 102 122 110 114 54% 51%
19 NB Wadsworth (82 Accessno bellvlew 31 5 26 4 27 84% 87%
20 SB KlPlino <82 Lakehurst 69 28 41 32 37 59% 54%
21 8B KlDllna (82 Asburv 60 55 25 56 24 31% 30%
22 SB Sheridan (82100th . 28 12 16 12 18 57% 57%
23 SB Klpllno <82Roxburv 24 18 6 18 6 25% 25%
24 NB Klpllno (82 Coal mine 124 20 104 24 100 84% 81%
25 SB Wadsworth (82108th - 46 17 29 18 28 63% 61%
26 NB Wadsworth (82 Ken Karyl 125 35 90 39 86 72% 69%
27 SB*Wadsworth (82 Ken Karyl 61 28 33 29 .32 54% 52%


Table 5.4: Results Data Vehicles with Opposing Traffic
Site Identification RTAL Data for Vehicles w th Opposing Traffic Volume Only
Number Intersection Location Right turning Volume with # not Opposing Using # using Traffic RTAL RTAL Percent # using # using Percent Using merging gap Using Merging Technique Technique RTAL Technique


1 NB Academy <81 Sams acc. 41 36 5 4 37 12% 10%
2 NB Powers <81 Galley 48 4 44 45 3 92% 04%
3 Union& Research 23 10 13 7 16 57% 30%
4 Union & Acacia 74 61 13 7 67 18% 9%
5 Platte/Phllllps 66 Access 32 12 20 13 19 63% 41%
6 T-GaD and Union 44 26 18 14 30 41% 32%
7 SB Wadsworth 8 SB Wadsworth 9 NB Wadsworth <31 Stanford 128 60 68 59 69 53% 46%
10 SB Wadsworth <81 Independ. 43 14 29 24 19 67% 56%
11 WB 104th <81 Bryant 10 3 7 6 4 70% 60%
12 EB 104th 13 EB 104th <31 Wolff 39 26 13 10 29 33% 26%
14 NB Wadsworth <31108th 11 2 9 6 5 82% 55%
15 NB Wadsworth <32104th 52 8 44 45 7 85% 87%
18 EB Ken Karyl <32 Simms 8 8 . 0 0 8 0% 0%
17 SB Federal <82114th 23 11 12 12 11 52% 52%
18 NB Kipling <& Quincy 137 37 100 95 42 73% 69%
19 NB Wadsworth <32 Accessno bellview 27 5 22 23 4 81% 85%
20 SB Kipling @2 Lakehurst 48 12 36 33 15 75% 69%
21 SB Kipling ffi Asbury 45 26 19 18 27 42% 40%
22 SB Sheridan <82 100th 23 11 12 12 11 52% 52%
23 SB Kipling <82Roxburv 13 10 3 3 10 23% 23%
24 NB Kipling <82 Coal mine 67 4 63 63 4 94% 94%
25 SB Wadsworth (82108th 17 4 13 12 5 76% 71%
26 NB Wadsworth (32 Ken Karyl 76 10 66 62 14 87% 82%
27 SB Wadsworth <32 Ken Karyl 13 3 10 8 5 77% 62%


regression was completed to account for any combined or interactive effects
of the variables and to determine the relative importance of each
independent variable to the others.
Variables. Variables have been assigned to represent both site
characteristics (x independent or predictor variables) and driver behaviors
(y dependent or response variables)
Independent X or Predictor Variables. Table 5.5 below presents a list of
variables used to quantify site characteristics both geometric(fixed) and
changeable conditions. These variables represent the site traffic operations
environment presented to the right turning motorist. Provided with each is
a description of what the variable represents. The site selection and data
collection section provides a detailed description of the key geometric
elements and traffic volumes.
77


Table 5.5 Independent Variable (X) List
Variable Name Description
LANE_LENGTH The length of the acceleration lane not including the transition taper
LANE_TAPER The length of the acceleration lane including the transition taper
La+TapA2 LANE_TAPER value squared
Ln_L_T The natural log of LANE_TAPER
ARC The length of the inside edge of the comer between the cross street and the arterial street RTAL (see Figure 5.2)
ARCA2 The value of ARC squared
LN(ARC) The natural log of the value of ARC
WIDTH The width of the acceleration lane
WIDTH A2 The value of WIDTH squared
INV10WID The inverse of WIDTH (multiplied by 1,000 for compatibility with stats package)
EXP(WID) e raised to the WIDTH power; the exponential function evaluated for the value WIDTH
SIGNAL an indicator variable identifying if the intersection is signalized or unsignalized (1 or 0)
DENVER An indicator variable identifying if the site was located in metro Denver (1 or 0)
TOTAL_CONF The volume per lane of conflicting through traffic on the arterial plus opposing left turning traffic
THRU_CONF The volume per lane of conflicting through traffic on the arterial
LEFT_CONF The volume per lane of conflicting left turning traffic from the cross street
78


Independent variable Multicolinearitv. The appendix contains
the covariance matrix for the independent variables. A covariance matrix
has been developed to determine if any pairs of these variables are closely
correlated. Values close to 1 have a high level of correlation and values
close to zero have very little correlation. The table also lists a p-value for
each pair that identifies the statistical probability that there is no correlation
between the two variables. Based on a review of the matrix, some of the
related variables such as TOTAL_CONF and THRU_CONF are obviously
closely correlated as the through conflicting traffic is such a large proportion
of the total conflicting traffic. Such is the case with LANE_LEN and
LANE_TAPE(R). Thus only the TOTAL_CONF and LANE_TAPE(r) variables
were used in the regression analysis described in the following section.
Another pair of independent variables that are statistically correlated and in
fact have a high level of correlation are ARC and ISLAND. This pair has a
correlation coefficient of 0.74. This is not surprising, as raised concrete
channelizing islands cannot be accommodated on corners with a tight
turning radius ( and corresponding smaller arc). At intersections where a
channelizing island has been constructed, the result is a larger radius. In
most cases, as the size of the island increases, so does the radius/arc.
Given this relationship, only one of these variables was used in the analysis
at a time.
Dependent (Y) or Response Variables. Table 5.6 below contains a
list of dependent or response variables used to represent and quantify driver
behaviors relative to lane use decisions. The following are descriptions of
what each variable represents.
79


Table 5.6 Dependent Variable (Y) List
Variable Name Description
ALL_USE Percent of all right turning vehicles using the acceleration lane
OPPJJSE Percent of only right turning vehicles with opposing volume using the acceleration lane
Analysis. Lane use by site. The values of these variables are
percentages determined from viewing the videotapes. The following Table
5.7 shows the lane performance for each site in terms of percent lane use
sorted from the highest to lowest percentage or decreasing level of usage.
Lane use by individual site characteristics. Table 5.8 shows the data
sorted first by site characteristics then by the resulting lane use variable
values.
Sinale/Simole Regression. To determine how each of the site
characteristics are correlated with the lane use, simple linear regression
analyses were completed. The goal was to determine the best line/curve for
the variables LANE_TAPER, ARC AND WIDTH. Five different functions
were used to fit the data: line, quadratic, inverse, natural log and
exponential. The Appendix contains the complete tables of the results.
Excluded Sites. Based on initial regression runs, sites west-
bound Coalmine at Wadsworth and eastbound Ken Karyl at Simms have
80


Table 5.7 1 Lane Usage by Site Percent of | Vehicles I Site Location Using RTALI
SB Wadsworth @ 104th 93% I
NB Powers @ Galley 86%
NB Wadsworth @ 104th 84%
NB Kipling @ Coal mine 84%
NB Wadsworth Accessno bellview 84%
NB Wadsworth Ken Karyl 72%
WB104th < Brvant 64%
Platte/Phillips 66 Access 63%
SB Wadsworth @ 108th 63%
SB Wadsworth @ Independ. 62%
SB Kipling @ Lakehurst 59%
SB Federal > 114th 57%
SB Sheridan @ 100th 57%
NB Wadsworth @> 108th 57%
NB Kipling SB Wadsworth Ken Karyl 54%
Union& Research 47%
NB Wadsworth <& Stanford 46%
SB Wadsworth (94th 36%
WB Coalmine @>Wads. 33%
SB Kipling 0) Asbury 31%
NB Academy @> Sams acc. 31%
T-Gap and Union 27%
EB 104th m Wolff 25%
SB Kipling Roxbury 25%
EB Ken Karyl 0>. Simms 13%
Union & Acacia 12%
EB 104th m. Lowell 12%


Table 5.8: Lane Use by Site Characteristics
Lane Use by Lane Length (plus taper)
Lane Length Percent
Plus Taper Lane
Site Location (feet) Usage
SB Federal 0.114th 820 57%
WB Coalmine aWads. 765 33%
SB Kiplinq aRoxburv 760 25%
SB Kiplinq & Lakehuist 735 59%
NB Kiplinq 6! Quincv 730 54%
EB Ken Karvl G Simms 710 13%
NB Kiplinq G Coal mine 6S5 84%
SB Wadsworth G. 104th 685 93%
NB Wadsworth G. 108th 630 57%
NB Wadsworth 6 Ken Karvt 630 72%
NB Wadsworth G n/o Bellview 580 84%
SB Wadsworth G. 108th 580 63%
NB Academy G Sams acc. 575 31%
WB 104th G Bryant 520 64%
Piatte/Phillips 66 Access 500 63%
NB Powers G Galley 500 86%
NB Wadsworth G 104th 480 84%
NB Wadsworth G Stanford 475 46%
SB Wadsworth & Independ. 455 62%
Union & Acacia 445 12%
Unioni Research 430 47%
SB Wadsworth 94th 425 36%
SB Kiplinq & Asburv 370 31%
EB 104th e Wolff 365 25%
SB Wadsworth G. Ken Karvl 355 54%
SB Sheridan G 100th 340 57%
EB 104th 6 Lowell 270 12%
T-Gap and Union 180 27%
Lane Use by Lane Width
Lane Percent
Width Lane
Site Location (feet) Usage
Piatte/Phillips 66 Access 23 63%
NB Wadsworth G. Ken Karvl 15 72%
SB Wadsworth a Ken Karvl 15 54%
SB Wadsworth a Independ. 14 62%
SB Wadsworth a 104th 13.5 93%
NB Wadsworth a 108th 13.5 57%
WB 104th a Bryant 13 64%
SB Wadsworth a94th 12.5 36%
Unions Research 12 47%
NB Academy G Sams acc. 12 31%
Union & Acacia 12 12%
NB Powers G Galley 12 86%
NB Kiplinq G Coal mine 12 84%
SB Wadsworth G 108th 12 63%
SB Kiplinq aRoxburv 12 25%
SB Sheridan G. 100th 12 57%
SB Federal & 114th 12 57%
NB Kiplinq G Quincv 12 54%
NB Wadsworth a. n/o Bellview 12 84%
SB Kiplinq G Lakehurst 12 59%
SB Kiplinq G. Asburv 11.5 31%
NB Wadsworth a 104th 11.5 84%
WB Coalmine aWads. 11.5 33%
EB Ken Karvl G. Simms 11.25 13%
NB Wadsworth a Stanford 11 46%
T-Gap and Union 10.5 27%
EB 104th a Lowell 10 12%
EB 104th a Wolff 10 25%
Lane Use by Entry Arc Length
Entry Arc Percent
Length Lane
Site Location (feet) Usage
NB Powers a Galley 175 86%
NB Wadsworth G n/o Bellview 150 84%
NB Kiplinq G Quincv 130 54%
NB Kiplinq &. Coal mine 118 84%
SB Wadsworth G 104th 109 93%
EB Ken Karvl G. Simms 90 13%
WB Coalmine aWads. 85 33%
SB Wadsworth G. Independ. 81 62%
SB Wadsworth G 108th 78 63%
NB Wadsworth G 108th 75 57%
NB Wadsworth a 104th 73 84%
SB Sheridan a 100th 72 57%
NB Wadsworth G Ken Karvl 71 72%
SB Kiplinq a Lakehurst 57 59%
SB Kiplina aRoxburv 55 25%
NB Wadsworth G Stanford 48 46%
EB 104th a Wolff 47 25%
Piatte/Phillips 66 Access 46 63%
T-Gap and Union 45 27%
SB Wadsworth G Ken Karvl 45 54%
Unioni Research 45 47%
Union & Acacia 45 12%
SB Kiplino G Asburv 43 31%
EB 104th a Lowell 39 12%
SB Federal a 114th 35 57%
WB 104th a Bryant 35 64%
SB Wadsworth B94th 30 36%
NB Academy Gt Sams acc. 19 31%
Lane Use by Type of Channelization
Raised Percent
Channelizing Lane
Site Location Island? Usage
SB Wadsworth G. 104th ves-raised 93%
NB Powers G Galley yes-raised 86%
NB Wadsworth G n/o Bellview ves-raised 84%
NB Kiplinq G Coal mine yes-raised 84%
NB Kiplinq a. Quincv yes-raised 54% S
T-Gao and Union ves-raised 27%
NB Wadsworth G 104th none 84% !
NB Wadsworth G. Ken Karvl none 72% I
WB 104th a Bryant none 64%
SB Wadsworth G. 108th none 63%
Piatte/Phillips 66 Access none 63%
SB Wadsworth G Independ. none 62%
SB Kiplinq G. Lakehurst none 59%
SB Sheridan G. 100th none 57%
SB Federal a 114th none 57%
NB Wadsworth G 108th none 57%
SB Wadsworth & Ken Karvl none 54%
Union& Research none 47%
NB Wadsworth G Stanford none 46%
SB Wadsworth aS4th none 36%
WB Coalmine aWads. none 33%
SB Kiplinq G Asburv none 31%
NB Academy G Sams acc. none 31%
EB 104th a Wolff none 25%
SB Kiplino aRoxburv none 25%
EB Ken Karvl & Simms none 13%
Union & Acacia none 12%
EB 104th a Lowell none 12%
82


been eliminated as uncharacteristic sites, These data points have been
identified as outliers. After reviewing the data from these sites, it has been
determined that the data collected from these two sites are not consistent
with the other RTAL sites. The site with the RTAL on Coalmine and the
cross street being Wadsworth is not representative because Wadsworth is
more of a major street and the majority of the signal greet time is assigned
to Wadsworth and the use of the RTAL was low. A similar situation was the
case with eastbound Ken Karyl at Simms site which showed a low incidence
of lane usage. Both also have a fairly heavily used downstream intersection
on the opposite side of the street from the RTAL that is relatively close to the
main intersection. A significant percentage of right turning motorists would
cut across the through lanes to turn left at these access points. Table 5.9
shows the functions with the highest R squared values and F statistic
values. Linear regressions were also completed for indicator independent
variables DENVER, SIGNAL, AND ISLAND. The purpose of these
regressions was to identify general correlation between each independent
variable and the response variables. These analyses have also been used
to select the variables for use as input into the multiple linear regression.
For example, for variable ARC, not only the value of ARC itself was
selected, but also the natural log of ARC because the model with the
natural log of arc vs. the dependent variables also had a relatively high R-
squared value. The graphs of the data points used in the regression
analysis are shown in Figures 5.5 to 5.12.
83


Table 5.9 Simple Regression Analysis All Data Except for Sites #16 and #15
Independent Variable Dependent Variable Regression Type R squared Value Regression F- value P- value
ARC ALLJJSE quadratic .52 11.93 .0003
ARC ALLJJSE natural log .49 22.19 .0001
ARC OPP_USE quadratic .54 13.10 .0002
ARC OPP__USE natural log .53 25.61 .0000
WIDTH ALL_USE quadratic .28 4.254 .0274
WIDTH ALL_USE exponential .25 7.55 .0114
WIDTH OPPJJSE quadratic .31 4.85 .0180
WIDTH OPP_USE inverse .30 10.015 .0043
l_ANE_TAPER ALLJJSE quadratic .29 4.51 .0228
LANE_TAPER ALL_USE natural log .23 6.94 .0148
LANE_TAPER OPPJJSE quadratic .32 4.85 .0180
84


Total Lane Length Vs. Lane Usage All Right Turners a nno/
1 UVJ /o 0 80%- O) CO cn o/v Ba )0
a
3 DU /o -41 c CD AHO/. -
i. 0 CL o noA
AU /o no/.
U /o c ^ 1 i | 1 | 1 i | 1 i i 1 i 1 1 1 100 200 300 400 500 600 700 800 9C Lane Length plus Taper (ft)
Figure 5.5: Total Lane Length Vs. Lane Usage, All Right Turners


Lane Length Vs. Lane Usage All Right Turners
1 uu /o )0
3 DU /o M c jj ^rU /O u ZU /o - m
U /o ( 1 i ^ 1 i i i 1 i i i i i i i i 100 200 300 400 500 600 700 800 9( Lane Length not IncludingTaper (ft)
Figure 5.6: Lane Length Vs. Lane Usage, All Right Turners


Total Lane Length Vs. Lane Usage W/ Opposing Traffic Only
1 uu /o /i) ono/ 1 )0
Si ou /o U) CO *
3 DU /o m AC\0L
g 4U /o 0) n ono/.
no/
U/o ( 1 | | i | | |- | ) 100 200 300 400 500 600 700 800 9( Lane Length plus Taper (ft)
Figure 5.7: Total Lane Length Vs. Lane Usage, w/ Opposing Traffic Only