ANALYSIS OF PROTECTED-PERMITTED LEFT TURNS
A Project
Presented to the faculty of the Department of Civil Engineering
California State University, Sacramento
Submitted in partial satisfaction of
the requirements for the degree of
MASTER OF SCIENCE
in
Civil Engineering
(Transportation Engineering)
by
Jason Quiñones, P.E.
SPRING
2014
ANALYSIS OF PROTECTED-PERMITTED LEFT TURNS
A Project
by
Jason Quiñones, P.E.
Approved by:
__________________________________, Committee Chair
Dr. Kevan Shafizadeh, P.E., PTP, PTOE
__________________________________, Second Reader
Dr. Ghazan Khan
____________________________
Date
ii
Student: Jason Quiñones
I certify that this student has met the requirements for format contained in the University
format manual, and that this project is suitable for shelving in the Library and credit is to
be awarded for the project.
__________________________, Graduate Coordinator
Dr. Matthew Salveson, P.E.
Department of Civil Engineering
iii
___________________
Date
Abstract
of
ANALYSIS OF PROTECTED-PERMITTED LEFT TURNS
by
Jason Quiñones, P.E.
This project focuses on protected-permitted left-turn phasing, including the different
types of signal arrangements and indications, their benefits, and their drawbacks. Several
previous studies were reviewed, and a number of agencies were contacted for information
related to the operations of intersections with protected-permitted phasing. Twelve
intersections in Sacramento County, California were analyzed using Synchro traffic
analysis software to compare protected-only left-turn phasing with typical protectedpermitted phasing. The analysis showed significant improvements in delay, especially
with the left-turn movements benefitting from the protected-permitted phasing. Based on
the available information, it can be shown that protected-permitted phasing increases
traffic operational efficiency, decreases delay, reduces fuel consumption, and decreases
emissions. Overall, the implementation of protected-permitted phasing can be beneficial
when done properly by experienced traffic engineers.
The twelve Sacramento County intersections that were analyzed as part of this project
show that there is promise in the use of PPLT in the Sacramento area. Multiple
intersections are estimated to experience significant operational improvements with
PPLT based on the predictions of the traffic analysis software. Considering these results
iv
and the research conducted by sources cited in this report, the Sacramento County
Department of Transportation should proceed with the implementation of PPLT.
_______________________, Committee Chair
Dr. Kevan Shafizadeh, P.E., PTP, PTOE
_______________________
Date
v
ACKNOWLEDGEMENTS
I would like to thank my advisor Dr. Kevan Shafizadeh for his guidance and patience
over the years. I am also grateful to Dr. Ghazan Khan for serving as the second reader
and Dr. Matthew Salveson for serving as the graduate coordinator. I would also like to
thank the many professors and staff of CSU Sacramento who assisted me throughout the
time I have been associated with the university.
Much appreciation goes to Doug Maas of the Sacramento County Department of
Transportation for providing the data for the twelve intersections that were analyzed. I
am also appreciative of Mark Miller for taking time out for a phone interview. Finally,
thank you to Erwin Ching, Paula Corlett, Yves d’Anjou, Doug Fong, Mark Luszcz, Andy
McGovern, Rick Perez, and others for providing information for my report.
Special thanks to my best friend of many years, Brandon Black, who has always been
there for me in good times and bad. Because of him, I strive to do my best.
Most of all, I would like to thank my mother, Michele Huesties, for demonstrating a
strong work ethic and setting a good example. She has always been there for her family,
friends, and students, often doing much more than could ever be expected.
Thank you all.
vi
TABLE OF CONTENTS
Page
Acknowledgements ..................................................................................................... vi
List of Tables .............................................................................................................. ix
List of Figures ............................................................................................................... x
Chapter
1. INTRODUCTION .................................................................................................. 1
Purpose.............................................................................................................. 2
Objectives ......................................................................................................... 3
Project Report Organization ............................................................................. 4
2. LEFT-TURN PHASE TYPES ................................................................................ 5
Protected Left Turns ......................................................................................... 5
Permitted Left Turns ......................................................................................... 6
Protected-Permitted Left-Turn (PPLT) Phasing ............................................... 7
Cluster (Doghouse) Display.............................................................................. 8
Dallas Phasing ................................................................................................... 9
Flashing Red Arrow Arrangement .................................................................. 11
Flashing Red Ball Arrangement...................................................................... 12
Flashing Yellow Arrow Arrangement ............................................................ 14
Flashing Yellow Ball Arrangement ................................................................ 16
3. SAFETY AND OTHER CONSIDERATIONS .................................................... 17
Yellow Trap .................................................................................................... 17
vii
Intersection Geometry and Orientation ........................................................... 20
Pedestrians, Bicyclists, and Traffic Composition ........................................... 21
Gap Dependency ............................................................................................. 23
4. DISCUSSION AND ANALYSIS ........................................................................ 24
Research Findings ........................................................................................... 24
Crash Rates ..................................................................................................... 25
Use of Protected-Permitted Left Turns ........................................................... 26
Estimation of Operational Improvements from PPLT .................................... 29
Change of Left-Turn Delay from PPLT.......................................................... 32
Change of Cross Traffic Delay from PPLT .................................................... 38
Change of Intersection Delay from PPLT ...................................................... 42
Additional Measures of Effectiveness ............................................................ 45
5. CONCLUSION AND RECOMMENDATIONS ................................................. 47
Appropriateness of Protected-Permitted Left Turns ....................................... 47
Crash Data Limitations ................................................................................... 47
Summary of Intersection Analysis .................................................................. 48
Future Study and Implementation ................................................................... 48
Appendix A: Crash Data from Washington County, Oregon .................................... 50
Appendix B: Cupertino Protected-Permitted Signal Phasing Removal ..................... 54
Appendix C: Introducing the New Flashing Yellow Arrow Left-Turn Signal .......... 59
Appendix D: Intersection Analysis Reports .............................................................. 62
References ................................................................................................................. 207
viii
LIST OF TABLES
Tables
Page
1. Signal Indication Abbreviations ............................................................................. 5
2. Protected-Permitted Signal Arrangement Types .................................................... 8
3. Agencies Contacted and PPLT Type(s) ................................................................ 27
4. List of Analyzed Sacramento County Intersections.............................................. 29
5. Signalized Intersection Level of Service (LOS) ................................................... 32
6. Left-Turn Morning (AM) PPLT Improvements ................................................... 33
7. Left-Turn Noon PPLT Improvements .................................................................. 34
8. Left-Turn Afternoon (PM) PPLT Improvements ................................................. 35
9. Left-Turn Average PPLT Improvements .............................................................. 37
10. Cross Traffic Morning (AM) PPLT Improvements .............................................. 39
11. Cross Traffic Noon PPLT Improvements ............................................................. 40
12. Cross Traffic Afternoon (PM) PPLT Improvements ............................................ 41
13. Cross Traffic Average PPLT Improvements ........................................................ 42
14. Intersection Morning (AM) PPLT Improvements ................................................ 43
15. Intersection Noon PPLT Improvements ............................................................... 43
16. Intersection Afternoon (PM) PPLT Improvements .............................................. 44
17. Intersection Average PPLT Improvements ........................................................... 45
18. Additional Measures of Effectiveness .................................................................. 46
ix
LIST OF FIGURES
Figures
Page
1. Protected Left-Turn Phase ...................................................................................... 6
2. Permitted Left-Turn Phase ...................................................................................... 7
3. Cluster (Doghouse) Display Graphic and Photo ..................................................... 9
4. Dallas Display Graphic ......................................................................................... 10
5. Flashing Red Arrow Arrangement Graphic .......................................................... 12
6. Flashing Red Ball Arrangement Graphic .............................................................. 13
7. Flashing Yellow Arrow Arrangement Operations ................................................ 15
8. Flashing Yellow Ball Arrangement Graphic ........................................................ 16
9. Doghouse Display Yellow Trap............................................................................ 19
10. Flashing Yellow Arrow Safety Guidance ............................................................. 22
11. Reduction in Delay and Fuel Consumption Due to PPLT .................................... 25
12. Map of Analyzed Sacramento County Intersections ............................................ 30
13. Left-Turn Delay Improvement .............................................................................. 36
14. Intersection Delay Improvement........................................................................... 44
x
1
Chapter 1
INTRODUCTION
Automobiles were used for 88 percent of commuting and over 90 percent of travel overall
in the United States in 2000 (O’Sullivan, 2007). As the population continues to increase,
and the transportation infrastructure becomes more encumbered, traffic engineers will
need to utilize all tools that are at their disposal. Protected-permitted left-turn (PPLT)
phasing allows more vehicles to pass through an intersection in a given time, without
expensive upgrades to the intersection’s geometry, such as additional lanes. The focus on
efficiency has become a widespread movement. Federal transportation policy has
evolved over the years, shifting away from capacity and toward mobility with the passage
of the Intermodal Surface Transportation Efficiency Act (ISTEA) in 1991 and the
Transportation Equity Act for the 21st Century (TEA-21) in 1998 (Fulton and Shigley,
2005). ISTEA also linked transportation planning and air quality planning, requiring
regions that do not meet federal air quality standards to submit a transportation plan that
will address the issue within a certain timeframe (Hanson and Giuliano, 2004). By
improving the efficiency of intersections, emissions are decreased, resulting in a positive
effect on air quality.
There are varieties of traffic signal phases and indications that are available to maximize
the efficiency of traffic operations, as described in the Manual on Uniform Traffic
Control Devices (Federal Highway Administration, 2009). Several styles of protected-
2
permitted left-turn phasing are acceptable, and various arrangements are used by different
agencies across the country. Researchers continue to analyze these varying phase types
to clarify their strengths and weaknesses.
Purpose
The problem of traffic congestion may be mitigated somewhat by the implementation of
protected-permitted left turns. This research seeks to investigate protected-permitted left
turns and summarize the advantages and disadvantages. This includes determining the
variety of protected-permitted left-turn phases that are currently in use throughout the
United States by contacting industry professionals and reviewing prior research. The
operational improvement of twelve intersections after implementing protected-permitted
left turns is estimated with traffic analysis software (Synchro by Trafficware). The
tradeoff between increased efficiency and the potential for higher crash rates is also
explored.
Regions that experience significant growth may be interested in protected-permitted leftturn phasing to improve the throughput of their existing transportation infrastructure. In
the Sacramento metro area, for example, protected-permitted left-turns are rare, yet the
region has increased tremendously in size and population. From 2000 to 2010,
approximately 90 square miles of rural land in the Sacramento region experienced urban
development (Reese, 2011). Therefore, the Sacramento area could be a good location for
the implementation of new protected-permitted left-turn phasing. According to Doug
3
Maas (Senior Transportation Engineer at the Sacramento County Department of
Transportation), Sacramento County has expressed an interest in implementing the
flashing yellow arrow variant of PPLT (personal communication, February 26, 2014).
Possibly the most attractive quality of protected-permitted left turns are their relatively
low installation cost. For example, in Washington County, Oregon, 372 traffic signals
were converted to protected-permitted left-turn control at an average cost of less than
$1,500 per approach (Bessman et al., 2011). Agencies experiencing budget constraints
along with traffic congestion may consider cost-effective measures such as the
implementation of protected-permitted left-turn phasing. Some types of protectedpermitted signal indications require minimal changes to existing displays. This project
may be of use to agencies that are considering protected-permitted operations.
Objectives
The analysis of the twelve Sacramento County intersections will approximate the
quantitative improvements of several measures of effectiveness by utilizing protectedpermitted left turns. These measures include delay, fuel usage, vehicle stops, and several
types of emissions. The delay improvement is considered for both the overall
intersections and some of the individual approaches. Based on the results of the analysis,
recommendations are made for the use of protected-permitted left-turn phasing and
potential future studies.
4
Project Report Organization
Subsequent chapters address the objectives of this project, the findings of the research,
and the results of the analysis. Chapter 2 describes the different types of left-turn phases,
and the variety of signal indicators that are utilized for protected-permitted left turns.
Safety concerns related to PPLT are discussed in Chapter 3, such as the “yellow trap.”
Chapter 3 also includes considerations of intersection characteristics, traffic composition,
and a description of gap dependency. Discussion of the research findings and results of
the analysis are in Chapter 4. Finally, Chapter 5 includes the conclusion and
recommendations based on the research and analysis of this project.
5
Chapter 2
LEFT-TURN PHASE TYPES
The three types of left-turn phases are protected, permitted (or permissive), and
protected-permitted (or protected-permissive). These types are defined based on whether
a left-turning vehicle must yield to oncoming through traffic. Signal indications vary and
are often identified with abbreviations (Table 1). For example, the flashing yellow arrow
indication is often abbreviated as FYA.
Table 1: Signal Indication Abbreviations
Abbreviation
Type
Meaning
F
Illumination
Flashing
S
Illumination
Steady
G
Color
Green
Y
Color
Yellow
R
Color
Red
A
Shape
Arrow
B
Shape
Ball (Circular)
Protected Left Turns
The most common left-turn phase in most metro areas is the protected phase. During the
steady green arrow indication, vehicles turning left should not encounter conflicting
traffic entering the intersection if all drivers obey their signals. The protected phase is
appropriate for heavier traffic volumes, and for intersections with multiple left-turn lanes
6
(Ozmen, Tian, and Gibby, 2009). A graphic of the protected left-turn phase is shown in
Figure 1.
Figure 1: Protected Left-Turn Phase
(Source: Federal Highway Administration, 2009)
Permitted Left Turns
A permitted left-turn signal allows vehicles to enter the intersection and turn left only if
there is no oncoming through traffic, as opposing through traffic has the right-of-way.
This left-turn phase type is not appropriate for intersections with heavy volumes, as
vehicles wanting to complete a left turn would be forced to wait for long periods,
resulting in a large queue. This type of signal is common on smaller roads, especially
7
when there is only one traffic lane. Lower traffic volumes enable opposing directions to
have a permitted signal simultaneously, resulting in the through traffic in each direction
having the right-of-way, with the left-turning vehicles from both directions waiting for
gaps in order to complete their turns. A graphic of the most common steady green ball
version of the permitted left-turn phase is shown in Figure 2.
Figure 2: Permitted Left-Turn Phase
(Source: Federal Highway Administration, 2009)
Protected-Permitted Left-Turn (PPLT) Phasing
The combination of the protected phase and the permitted phase results in protectedpermitted phasing. This type of signal operation provides greater flexibility to traffic
engineers when working to achieve maximum efficiency in traffic operations. There are
close to 110,000 signals in the United States that have at least one approach with a
protected-permitted phase, representing approximately 29% of all traffic signals
(Brehmer et al., 2003). Table 2 lists the different protected-permitted signals that are in
use throughout the country.
8
Table 2: Protected-Permitted Signal Arrangement Types
Signal Arrangement Type
Permitted Phase Indication
Cluster (Doghouse)
Steady Green Ball
Dallas Phasing
Steady Green Ball (Visibility Limited)
FRA
Flashing Red Arrow
FRB
Flashing Red Ball
FYA
Flashing Yellow Arrow
FYB
Flashing Yellow Ball
Cluster (Doghouse) Display
One common type of protected-permitted left-turn signal is the cluster display, often
referred to as the doghouse display because of how the five indicators are arranged. The
doghouse display typically has a steady red ball on top, a steady yellow arrow adjacent to
a steady yellow ball on the mid-level, and a steady green arrow adjacent to a steady green
ball on the bottom. During the protected phase, the steady green arrow is illuminated.
The steady yellow arrow indicates that the protected phase is ending. The steady green
ball is illuminated during the permitted phase, while the steady yellow ball indicates that
the permitted phase is ending. When the red ball is illuminated, traffic must remain
stopped and may not enter the intersection. A graphical representation and a picture of a
doghouse display are shown in Figure 3.
9
Figure 3: Cluster (Doghouse) Display Graphic and Photo
(Source: Federal Highway Administration, 2009)
A limitation of the doghouse display is that the protected phase cannot follow the
permitted phase (lagging protected phase), because oncoming left-turning traffic can
become trapped in the intersection when their permitted phase transitions to a red
indication in order to stop oncoming through traffic and to allow the permitted phase to
begin. This situation is called the yellow trap, which is explained in further detail in
Chapter 3.
Dallas Phasing
The Dallas Phasing (also known as the Dallas Display) signal arrangement is very similar
to the doghouse display. The Dallas Display may be arranged in a cluster like the
doghouse display, or it may be found as a five-section horizontal or vertical arrangement.
10
The critical difference is that the steady green and yellow balls meant for the left-turn
lane are visually obscured from the adjacent through traffic by using louvered signal
lenses. This solution enables an approach to have a permitted phase for left-turning
vehicles while simultaneously showing a red indication for the through traffic in that
same direction. Because through traffic cannot see the steady green ball indicating the
permitted phase for the left-turning traffic, the lagging protected-phase limitation that
results in a yellow trap for the doghouse display is eliminated. The steady red, yellow,
and green balls and arrows of the Dallas Display serve the same functions for the leftturning traffic as those of the doghouse display. A graphical representation of the Dallas
Display is shown in Figure 4.
Figure 4: Dallas Display Graphic
(Source: Brehmer et al., 2003)
11
Flashing Red Arrow Arrangement
The flashing red arrow arrangement is a protected-permitted signal type that utilizes a
flashing red arrow for the permitted phase. This signal type typically uses a steady red
ball for the stop indication and a steady green arrow for the protected phase. Depending
on the agency, a steady yellow ball or arrow may be used to indicate that the protected
phase is ending (the indications used for this signal type vary). According to Mark
Luszcz of the Delaware Department of Transportation, this protected-permitted signal
variant is currently used in Delaware (personal communication, November 22, 2013).
This type was also formerly used in Cupertino, California (Erwin Ching of the City of
Cupertino Department of Public Works, personal communication, September 25, 2013).
A graphic of the flashing red arrow arrangement is shown in Figure 5 on the following
page.
12
Figure 5: Flashing Red Arrow Arrangement Graphic
(Source: Noyce and Kacir, 2001)
Flashing Red Ball Arrangement
The flashing red ball arrangement is a protected-permitted signal type that utilizes a
flashing red ball for the permitted phase. The same red ball remains steady to indicate
that left turns are not permitted. A steady green arrow indicates the protected phase, and
a steady yellow arrow indicates a warning that the protected phase is ending. This left-
13
turn phase indication is currently used in the State of Michigan, but is being phased out
(Paula Corlett of the Michigan Department of Transportation, personal communication,
November 22, 2013). A graphic of the flashing red ball arrangement is shown in
Figure 6.
Figure 6: Flashing Red Ball Arrangement Graphic
(Source: Noyce and Kacir, 2001)
14
Flashing Yellow Arrow Arrangement
The protected-permitted flashing yellow arrow is becoming more widely accepted since it
was recently added to the Manual on Uniform Traffic Control Devices (Federal Highway
Administration, 2009). A graphical representation of the operation of the flashing yellow
arrow arrangement is shown in Figure 7 on the next page.
The flashing yellow arrow arrangement can be found in several variations. The most
common is the four-section vertical setup shown in Figure 7. There is also a threesection signal where the solid green arrow and flashing yellow arrow are in the same
location, enabled by the use of a bimodal lens. Another three-section signal uses the
same center yellow arrow as both the flashing permitted indication and the steady
indication warning that the left-turn phase is ending. Implementing one of the two types
of three-section signals would require fewer hardware alterations and may be completed
primarily by reprogramming the traffic control system.
15
Figure 7: Flashing Yellow Arrow Arrangement Operations
(Source: Brehmer et al., 2003)
16
Flashing Yellow Ball Arrangement
The flashing yellow ball arrangement is a protected-permitted signal type that utilizes a
flashing yellow ball for the permitted phase. A separate steady yellow ball warns that the
permitted phase is ending. A steady green arrow indicates the protected phase, while a
steady yellow arrow indicates that the protected phase is ending. The green and yellow
arrows share the same location on the signal head using a bi-modal lens. This protectedpermitted signal variant has been used in Seattle, Washington (Noyce and Kacir, 2001).
A steady red ball indicates that left turns are not permitted. A graphic of the flashing
yellow ball arrangement is shown in Figure 8.
Figure 8: Flashing Yellow Ball Arrangement Graphic
(Source: Noyce and Kacir, 2001)
17
Chapter 3
SAFETY AND OTHER CONSIDERATIONS
The most recognized disadvantage of protected-permitted left turns is the potential for
increased crash rates. This issue arises from the fact that additional conflict points are
introduced during the permitted phase. If all drivers obey their signals, intersections with
protected left turns do not create a situation where a left-turning vehicle crosses the path
of oncoming through traffic. During a permitted phase, there is always the possibility
that a driver may have a misjudgment of speed and/or distance, potentially resulting in a
collision.
Yellow Trap
The yellow trap is a safety consideration, and signal operations should be managed in
such a way that a yellow trap is avoided. A yellow trap occurs when a left-turning
vehicle in a permitted phase is waiting for a gap in oncoming traffic so the turn may be
completed, when the permitted phase begins to end (yellow indication). Assuming that
the oncoming through traffic’s green indication is transitioning to red as well, the driver
of the left-turning vehicle sneaks into the intersection, with the intention of completing
the turn after the oncoming through traffic comes to a stop. However, the left-turning
vehicle’s permitted phase was ending only because the through traffic in the same
direction was being stopped (red indication) to allow the opposing left-turn traffic to
receive a lagging-protected phase (green arrow). A lagging-protected phase is used when
18
the oncoming through traffic phase occurs before oncoming left-turn traffic is given their
protected phase (green arrow). Because some signal arrangements cannot simultaneously
give a permitted phase to left-turning vehicles and a non-permitted phase (red indication)
to the through traffic coming from the same direction, all of the indications for the
approach transition to a steady red signal. The vehicle that proceeded into the
intersection because the driver incorrectly predicted an opportunity to complete the left
turn then becomes stranded in the intersection while the left-turn signal transitions to
steady red, while the opposing through traffic continues to receive a steady green
indication. This situation, referred to as the yellow trap, can result in a collision if the
driver attempts to complete the left turn while trapped (Brehmer et al., 2003).
The doghouse display is the most common protected-permitted left-turn arrangement that
is limited by the yellow trap (see Figure 9 on the next page). As a result, a laggingprotected phase should not be used at intersections that utilize the doghouse display. The
Dallas Display addresses this problem by using visibility limitations with louvered signal
lenses, as discussed in Chapter 2.
19
Figure 9: Doghouse Display Yellow Trap
(Source: Brehmer et al., 2003)
20
Intersection Geometry and Orientation
The geometry of an intersection is also important when evaluating the suitability of
protected-permitted phasing. The number of lanes of an approach can place limitations
on the flexibility of signal operations. If an approach only has one lane (or allows both
through and left-turn movements from a lane), the only options are protected split
phasing (opposite sides take turns) or permissive phasing. If an approach has three lanes,
careful consideration must be given that the traffic volume at that intersection may be too
high to accommodate protected-permitted phasing (Ozmen et al., 2009). Additional
considerations include approach grades, sight distances, speed limits, and whether the
two roads at the intersection meet orthogonally. Grades can affect breaking distances,
requiring additional time to react to potentially conflicting vehicles. Intersections with
limited sight distances would be inappropriate for protected-permitted phasing because
drivers would have difficulty observing left-turning vehicles crossing their path, and vice
versa. Intersections at right angles are preferred for protected-permitted phasing because
skewed intersections can cause scanning for other vehicles to be awkward.
Orientation of an intersection can also be important because of the potential for glare.
Roads that run east-west can be susceptible to considerable glare during sunrise and
sunset (Steyn et al., 2013). Vehicles travelling through an intersection during a permitted
phase must watch for vehicles crossing their path in addition to the signal and other
potential hazards. Due to the increased risk, intersections with protected-permitted
phasing should be operated in protected mode during periods of significant glare.
21
Pedestrians, Bicyclists, and Traffic Composition
Drivers must watch for many obstacles and potential hazards, especially when travelling
through intersections with multiple conflict points. Although other vehicles are a
concern, especially during the permitted phase of protected-permitted left-turn
operations, pedestrians and bicyclists must be considered as well. During the permitted
phase, pedestrians or bicyclists could cross a left-turning vehicle’s path. See Figure 10,
which provides safety guidance related to flashing yellow arrow protected-permitted
phasing, as presented by Washington County, Oregon.
22
Figure 10: Flashing Yellow Arrow Safety Guidance
(Source: Washington County, Oregon, 2013)
23
Traffic composition includes both the types of vehicles and drivers that frequent an
intersection. A high percentage of large trucks could obstruct views, while the presence
of too many unskilled drivers could be a problem as well. When considering the use of
protected-permitted phasing, all of these factors should be considered. If trucks tend to
congregate during a certain time of day, or traffic composition is altered because of a
nearby school, an intersection’s protected-permitted phasing could be set to protected
mode during that time.
Gap Dependency
Protected-permitted phasing can utilize gap dependency, which limits the use of the
permitted phase based on traffic density. If there are no significant gaps in the oncoming
through traffic, this mode of operation will prevent the permitted phase. Gap dependency
dynamically activates the permitted phase based on gaps in conflicting oncoming through
traffic as measured by vehicle detection systems (Steyn et al., 2013). Traffic waiting to
turn left is enabled to do so with a protected phase in situations where gap dependency
does not allow a permitted phase.
24
Chapter 4
DISCUSSION AND ANALYSIS
Research Findings
Qualitatively, sources consistently state that protected-permitted phasing results in higher
efficiency of traffic operations, less delay, fewer emissions, and less fuel usage (Bessman
et al., 2011; Brehmer et al., 2003). Identifying reliable sources with quantitative
information related to those results is difficult. Kenneth R. Agent, a transportation
research engineer with the University of Kentucky Transportation Center conducted a
study of four trial intersections determined that protected-permitted phasing reduces leftturn delay by 50% and total delay by 24% when compared to protected phasing (Agent,
1979). A study conducted by Kittelson & Associates, Inc., estimated reductions in delay
(average slightly above 40%) and fuel consumption (average slightly below 20%) by
implementing protected-permitted left-turn (PPLT) control as shown in Figure 11
(Bessman et al., 2011).
25
Figure 11: Reduction in Delay and Fuel Consumption Due to PPLT
(Source: Bessman et al., 2011)
Crash Rates
The influence of protected-permitted phasing on crash rates is debatable. Some agencies
have not experienced a noticeable increase in collisions (Perez, 2014); while other
research has concluded that the introduction of additional conflict points during the leftturn permissive phase results in higher crash rates (Brehmer et al., 2003). There are some
measures that can be implemented to mitigate a potential increase in collisions, such as
gap dependency. Appendix A shows crash data for a set of intersections in Washington
County, Oregon with protected-permitted phasing before and after the implementation of
gap dependency (discussed in Chapter 3). The set of crash data shows how sensitive
crash rates are based on the number of crashes. Since crash rates are very low in general,
26
the inclusion of an additional crash has a significant impact on the rate. This sensitivity
of crash rates is important to consider when deciding whether to attribute a crash to a
specific traffic operation. Traffic safety specialists estimate that improper driving causes
90 percent of all accidents (Wright, 1996). In addition, a significant number of accidents
involve drivers with suspended licenses or no license (Mark Miller of the City of
Fullerton, California Public Works Department, personal communication, September 17,
2013), all of which makes it difficult to determine if a particular collision occurred due to
issues with the transportation infrastructure, or if driver behavior was the primary
collision factor. Even worse, many accident reports do not state which signal indication
was active during the crash (Brehmer et al., 2003).
Use of Protected-Permitted Left Turns
As part of this research, engineers at agencies that utilize protected-permitted phasing
were contacted via email in September through November of 2013. These agencies were
identified through previous research and by references from other agencies. Agencies
with PPLT were selected based on the type of permitted phase indication that was used,
with a goal of including representatives of the different types in this report. According to
Yves d’Anjou of the Broward County, Florida Traffic Engineering Division, many
agencies do not attempt to track crash rates or efficiency improvements related to
protected-permitted phasing (personal communication, November 22, 2013). Table 3
lists the agencies that were contacted and the types of protected-permitted phasing they
use.
27
Table 3: Agencies Contacted and PPLT Type(s)
Agency
PPLT Type(s)
Cities
Beaverton, OR
FYA
Carson City, NV
FYA
Cupertino, CA
FRA (discontinued)
Fullerton, CA
FYA
Reno, NV
FYA
Santa Clarita, CA
FYA
Sparks, NV
FYA
Tucson, AZ
FYA
Woodburn, OR
FYA
Counties
Broward County, FL
FYA
Jackson County, OR
FYA (3-section)
Montgomery County, MD
Oakland County, MI
Washington County, OR
FYA
FRB / FYA
FYA (bimodal lens)
States
Delaware
FRA / Doghouse
Michigan
FRB / FYA
Several agencies changed their protected-permitted phasing after the studies cited in this
report were completed, and some agencies are in the process of transitioning. Carson
City, Nevada is transitioning all of their protected-permitted left turns to the flashing
28
yellow arrow type. As of the date of this report, the city is over 80% finished, with the
remaining intersections to be transitioned by the end of 2014 (Doug Fong of the Carson
City, Nevada Public Works, personal communication, September 18, 2013). Cupertino,
California was identified through research as utilizing the flashing red arrow display
(Noyce and Kacir, 2001), but the city council decided to discontinue the use of that
protected-permitted phase type per the recommendation of their Department of Public
Works (Erwin Ching of the City of Cupertino Department of Public Works, personal
communication, September 25, 2013). This recommendation, included in Appendix B,
was based on the flashing red arrow display being non-standard and a concern for safety
and driver confusion. The city of Tucson, Arizona implemented a few flashing yellow
arrow protected-permitted displays after their inclusion in the Manual on Uniform Traffic
Control Devices, and according to Andy McGovern of the City of Tucson, Arizona
Department of Transportation, the results have been positive (personal communication,
November 21, 2013). The Delaware Department of Transportation continues to use the
flashing red arrow display, and utilizes the doghouse display at many intersections as
well (Mark Luszcz of the Delaware Department of Transportation, personal
communication, November 22, 2013). The state of Michigan is in the process of
converting their protected-permitted phasing from flashing red ball to flashing yellow
arrow (Paula Corlett of the Michigan Department of Transportation, personal
communication, November 22, 2013). Appendix C contains information from the
Michigan Department of Transportation regarding their new flashing yellow arrow
displays.
29
Estimation of Operational Improvements from PPLT
To estimate how implementation of PPLT may improve the operational effectiveness of
existing intersections, Doug Maas of the Sacramento County Department of
Transportation identified twelve intersections for analysis. The twelve intersections are
listed in Table 4 below and shown in Figure 12 on the following page. Table 4 includes
which left-turn approach directions were analyzed with PPLT: eastbound (E),
westbound (W), northbound (N), and southbound (S). Two intersections, Garfield and
Marconi Avenues, and Walnut and Cypress Avenues, were analyzed with PPLT
operating on all four left-turn movements.
Table 4: List of Analyzed Sacramento County Intersections
East / West
Direction
Madison Avenue
PPLT
Analyzed
E, W
2
North / South
Direction
Fair Oaks
Promenade
Franklin Boulevard
Meadowgate Drive
N, S
Area of Sacramento
County
Fair Oaks /
Orangevale
Florin
3
Fulton Avenue
Northrop Avenue
N, S
Arden-Arcade
4
Garfield Avenue
Marconi Avenue
E, W, N, S
Carmichael
5
Manzanita Avenue
Lincoln Avenue
N, S
Carmichael
6
Power Inn Road
Auberry Drive
N, S
Florin
7
Tallyho Drive
Kiefer Boulevard
E, W
Rosemont
8
Walnut Avenue
Cypress Avenue
E, W, N, S
Carmichael
9
Walnut Avenue
Engle Road
N, S
Carmichael
10
Walnut Avenue
North Avenue
N, S
Carmichael
11
Wright Street
Marconi Avenue
E, W
Arden-Arcade
12
Zinfandel Drive
Douglas Road
E, W
Mather
1
30
Figure 12: Map of Analyzed Sacramento County Intersections
(Google Maps; data provided by the Sacramento County Department of Transportation)
The data from these intersections were entered into traffic analysis software (Synchro by
Trafficware), and an operational analysis was conducted to study the changes of various
measures of effectiveness. Data included the phase timing, geometry, and traffic volume
counts for the morning (AM), noon, and afternoon (PM). Each intersection was analyzed
in both the current (baseline) condition and the PPLT-implemented condition for each
time period. This report focuses on the estimated decrease in delay and improvement of
31
the level of service (LOS); however, the Sacramento County Department of
Transportation has also expressed an interest in the potential decrease of emissions and
the number of vehicles that must stop at the intersection (Doug Maas of the Sacramento
County Department of Transportation, personal communication, February 26, 2014). The
reports summarizing the results from the traffic analysis software are included in
Appendix D. These reports also include information about the intersection
characteristics, such as the lane configurations for each approach and traffic control (turn
type).
Delay is measured by the average number of seconds a vehicle must wait before
proceeding through the intersection. Delay may be used to determine the level of service
of a turning movement, through movement, approach, or intersection (Transportation
Research Board, 2010). Table 5 summarizes how delay corresponds to the different
levels of service for signalized intersections, based on the Highway Capacity Manual.
32
Table 5: Signalized Intersection Level of Service (LOS)
Level of Service
Average Delay – Signalized Intersection
A
≤ 10 seconds
B
10 - 20 seconds
C
20 - 35 seconds
D
35 - 55 seconds
E
55 - 80 seconds
F
> 80 seconds
(Source: Transportation Research Board, 2010)
Change of Left-Turn Delay from PPLT
The estimated change in level of service and delay for the left-turn movements after
implementation of PPLT are shown in Tables 6, 7, and 8. The westbound left-turn
movement at Fair Oaks Promenade and Madison Avenue is omitted from the tables
because there were no left-turning vehicles shown in the traffic volume counts. (Delay is
determined by the average time spent waiting per vehicle, so zero vehicles would be the
equivalent of dividing by zero, and therefore is undefined.)
33
Table 6: Left-Turn Morning (AM) PPLT Improvements
Intersection
Fair Oaks Promenade & Madison
Franklin & Meadowgate
Franklin & Meadowgate
Fulton & Northrop
Fulton & Northrop
Garfield & Marconi
Garfield & Marconi
Garfield & Marconi
Garfield & Marconi
Manzanita & Lincoln
Manzanita & Lincoln
Power Inn & Auberry
Power Inn & Auberry
Tallyho & Kiefer
Tallyho & Kiefer
Walnut & Cypress
Walnut & Cypress
Walnut & Cypress
Walnut & Cypress
Walnut & Engle
Walnut & Engle
Walnut & North
Walnut & North
Wright & Marconi
Wright & Marconi
Zinfandel & Douglas
Zinfandel & Douglas
Left Turn
Baseline
PPLT Added
LOS
Eastbound
C
Northbound
C
Southbound
C
Northbound
D
Southbound
D
Eastbound
C
Westbound
C
Northbound
C
Southbound
C
Northbound
C
Southbound
C
Northbound
C
Southbound
D
Eastbound
C
Westbound
C
Eastbound
D
Westbound
D
Northbound
F
Southbound
D
Northbound
D
Southbound
D
Northbound
C
Southbound
C
Eastbound
C
Westbound
C
Eastbound
C
Westbound
C
PPLT
LOS
A
A
A
B
B
B
B
B
B
A
A
A
A
A
B
B
C
D
B
B
B
A
B
A
A
B
A
Delay
Decrease
80%
66%
72%
73%
74%
61%
61%
62%
62%
78%
74%
71%
76%
66%
62%
72%
31%
38%
63%
63%
63%
72%
68%
75%
69%
60%
73%
34
Table 7: Left-Turn Noon PPLT Improvements
Intersection
Fair Oaks Promenade & Madison
Franklin & Meadowgate
Franklin & Meadowgate
Fulton & Northrop
Fulton & Northrop
Garfield & Marconi
Garfield & Marconi
Garfield & Marconi
Garfield & Marconi
Manzanita & Lincoln
Manzanita & Lincoln
Power Inn & Auberry
Power Inn & Auberry
Tallyho & Kiefer
Tallyho & Kiefer
Walnut & Cypress
Walnut & Cypress
Walnut & Cypress
Walnut & Cypress
Walnut & Engle
Walnut & Engle
Walnut & North
Walnut & North
Wright & Marconi
Wright & Marconi
Zinfandel & Douglas
Zinfandel & Douglas
Left Turn
PPLT Added
Eastbound
Northbound
Southbound
Northbound
Southbound
Eastbound
Westbound
Northbound
Southbound
Northbound
Southbound
Northbound
Southbound
Eastbound
Westbound
Eastbound
Westbound
Northbound
Southbound
Northbound
Southbound
Northbound
Southbound
Eastbound
Westbound
Eastbound
Westbound
Baseline
LOS
C
D
C
D
D
C
C
C
C
D
D
B
B
C
C
C
C
D
D
C
C
C
C
C
C
B
B
PPLT
LOS
A
B
B
B
B
A
A
B
B
A
A
A
A
B
B
A
B
B
B
B
B
A
A
A
A
A
A
Delay
Decrease
72%
65%
64%
56%
65%
65%
67%
57%
55%
82%
74%
68%
68%
63%
60%
68%
57%
58%
63%
65%
66%
68%
64%
71%
68%
52%
53%
35
Table 8: Left-Turn Afternoon (PM) PPLT Improvements
Intersection
Fair Oaks Promenade & Madison
Franklin & Meadowgate
Franklin & Meadowgate
Fulton & Northrop
Fulton & Northrop
Garfield & Marconi
Garfield & Marconi
Garfield & Marconi
Garfield & Marconi
Manzanita & Lincoln
Manzanita & Lincoln
Power Inn & Auberry
Power Inn & Auberry
Tallyho & Kiefer
Tallyho & Kiefer
Walnut & Cypress
Walnut & Cypress
Walnut & Cypress
Walnut & Cypress
Walnut & Engle
Walnut & Engle
Walnut & North
Walnut & North
Wright & Marconi
Wright & Marconi
Zinfandel & Douglas
Zinfandel & Douglas
Left Turn
PPLT Added
Eastbound
Northbound
Southbound
Northbound
Southbound
Eastbound
Westbound
Northbound
Southbound
Northbound
Southbound
Northbound
Southbound
Eastbound
Westbound
Eastbound
Westbound
Northbound
Southbound
Northbound
Southbound
Northbound
Southbound
Eastbound
Westbound
Eastbound
Westbound
Baseline
LOS
C
E
D
F
D
C
C
C
C
D
E
C
B
C
C
D
D
F
F
D
D
C
D
C
C
C
C
PPLT
LOS
A
C
A
C
B
B
B
B
B
A
B
A
A
B
B
A
C
C
C
B
B
A
A
A
A
B
B
Delay
Decrease
75%
37%
76%
78%
60%
59%
62%
61%
59%
85%
76%
76%
72%
67%
54%
76%
41%
57%
62%
68%
65%
76%
76%
71%
63%
57%
58%
36
Figure 13 is a graphical representation of the decrease in left-turn approach delay
compared to the volume of the oncoming through traffic.
Figure 13: Left-Turn Delay Improvement
Based on the analysis of the twelve Sacramento County intersections, there does not
appear to be a significant relationship between oncoming traffic volume and the delay
improvement of the left-turn movements. It would be reasonable to expect improvements
in delay to diminish when opposing through traffic volume reaches very high levels, but
the through traffic of these intersections during these time periods are well below
saturation rates (around 1,700 vehicles per hour per lane, according to the traffic analysis
software). The highest through traffic volume, at just over 1,200 vehicles per hour with
two lanes, is around 600 vehicles per hour per lane. These results suggest that there are
37
enough sufficient gaps in oncoming through traffic for the permitted phase of the PPLT
to be effective. The variance in delay improvements within similar volume ranges could
reasonably be attributed to differences in intersection characteristics, geometry, traffic
composition, and pedestrian volumes.
Table 9 shows the average percentage decrease in left-turn approach delay for the
morning (AM), noon, and afternoon (PM) time periods, along with the overall average.
These are averages of the intersections, and do not represent per-vehicle results, as the
intersections are weighted equally and are not adjusted for traffic volumes.
Table 9: Left-Turn Average PPLT Improvements
Left-Turn Approach Averages
Average Morning (AM)
Average Noon
Average Afternoon (PM)
Overall Average
Delay Decrease
66%
64%
65%
65%
The average estimated decrease in the left-turn approach delay after implementation of
PPLT is over 60%. This seems reasonable, as left-turning vehicles would be able to
complete their turn when there is little or no oncoming through traffic. The left-turn
approaches with the greatest improvement are at intersections where left-turning vehicles
wait the longest for oncoming through traffic while gaps sufficient to enable a left-turn
movement are present.
38
Change of Cross Traffic Delay from PPLT
The estimated change in level of service and delay for the cross traffic approaches after
implementation of PPLT are shown in Tables 10, 11, and 12. The cross traffic is
composed of all vehicles approaching the intersection from a direction perpendicular to
the PPLT lanes, including through, left, and right-turning traffic. The northbound
approach at Fair Oaks Promenade and Madison Avenue is omitted from the morning
(AM) table because the volume was zero for that direction during that hour.
39
Table 10: Cross Traffic Morning (AM) PPLT Improvements
Intersection
Fair Oaks Promenade & Madison
Franklin & Meadowgate
Franklin & Meadowgate
Fulton & Northrop
Fulton & Northrop
Garfield & Marconi
Garfield & Marconi
Garfield & Marconi
Garfield & Marconi
Manzanita & Lincoln
Manzanita & Lincoln
Power Inn & Auberry
Power Inn & Auberry
Tallyho & Kiefer
Tallyho & Kiefer
Walnut & Cypress
Walnut & Cypress
Walnut & Cypress
Walnut & Cypress
Walnut & Engle
Walnut & Engle
Walnut & North
Walnut & North
Wright & Marconi
Wright & Marconi
Zinfandel & Douglas
Zinfandel & Douglas
Cross
Traffic
Southbound
Eastbound
Westbound
Eastbound
Westbound
Eastbound
Westbound
Northbound
Southbound
Eastbound
Westbound
Eastbound
Westbound
Northbound
Southbound
Eastbound
Westbound
Northbound
Southbound
Eastbound
Westbound
Eastbound
Westbound
Northbound
Southbound
Northbound
Southbound
Baseline
LOS
B
C
B
B
C
B
B
B
C
C
B
B
B
B
C
D
C
D
D
C
D
C
B
B
B
C
C
PPLT
LOS
B
C
B
B
C
B
B
B
B
C
B
B
A
B
C
C
C
C
C
C
D
C
B
B
B
C
B
Delay
Decrease
2%
3%
-1%
14%
15%
3%
12%
8%
17%
0%
0%
3%
3%
6%
6%
27%
4%
23%
26%
0%
0%
2%
2%
2%
3%
12%
11%
40
Table 11: Cross Traffic Noon PPLT Improvements
Intersection
Cross Traffic
Fair Oaks Promenade & Madison
Fair Oaks Promenade & Madison
Franklin & Meadowgate
Franklin & Meadowgate
Fulton & Northrop
Fulton & Northrop
Garfield & Marconi
Garfield & Marconi
Garfield & Marconi
Garfield & Marconi
Manzanita & Lincoln
Manzanita & Lincoln
Power Inn & Auberry
Power Inn & Auberry
Tallyho & Kiefer
Tallyho & Kiefer
Walnut & Cypress
Walnut & Cypress
Walnut & Cypress
Walnut & Cypress
Walnut & Engle
Walnut & Engle
Walnut & North
Walnut & North
Wright & Marconi
Wright & Marconi
Zinfandel & Douglas
Zinfandel & Douglas
Northbound
Southbound
Eastbound
Westbound
Eastbound
Westbound
Eastbound
Westbound
Northbound
Southbound
Eastbound
Westbound
Eastbound
Westbound
Northbound
Southbound
Eastbound
Westbound
Northbound
Southbound
Eastbound
Westbound
Eastbound
Westbound
Northbound
Southbound
Northbound
Southbound
Baseline
LOS
C
B
B
C
C
C
B
B
B
B
C
C
B
B
B
C
C
B
C
C
C
C
C
B
B
C
B
B
PPLT
LOS
C
B
B
C
C
C
B
B
B
B
C
C
B
A
B
C
C
B
B
B
C
C
C
B
B
B
B
B
Delay
Decrease
5%
6%
3%
2%
11%
11%
11%
16%
19%
22%
2%
3%
4%
4%
7%
8%
25%
27%
30%
27%
1%
1%
1%
1%
5%
6%
6%
6%
41
Table 12: Cross Traffic Afternoon (PM) PPLT Improvements
Intersection
Cross Traffic
Fair Oaks Promenade & Madison
Fair Oaks Promenade & Madison
Franklin & Meadowgate
Franklin & Meadowgate
Fulton & Northrop
Fulton & Northrop
Garfield & Marconi
Garfield & Marconi
Garfield & Marconi
Garfield & Marconi
Manzanita & Lincoln
Manzanita & Lincoln
Power Inn & Auberry
Power Inn & Auberry
Tallyho & Kiefer
Tallyho & Kiefer
Walnut & Cypress
Walnut & Cypress
Walnut & Cypress
Walnut & Cypress
Walnut & Engle
Walnut & Engle
Walnut & North
Walnut & North
Wright & Marconi
Wright & Marconi
Zinfandel & Douglas
Zinfandel & Douglas
Northbound
Southbound
Eastbound
Westbound
Eastbound
Westbound
Eastbound
Westbound
Northbound
Southbound
Eastbound
Westbound
Eastbound
Westbound
Northbound
Southbound
Eastbound
Westbound
Northbound
Southbound
Eastbound
Westbound
Eastbound
Westbound
Northbound
Southbound
Northbound
Southbound
Baseline
LOS
C
C
C
C
C
C
C
C
C
C
C
C
A
B
C
C
E
C
D
D
C
C
C
B
B
C
B
B
PPLT
LOS
C
C
C
C
C
C
B
B
C
B
C
C
A
B
C
C
D
C
C
C
C
C
C
B
B
B
B
B
Delay
Decrease
7%
6%
2%
2%
-16%
-16%
17%
16%
18%
29%
0%
1%
1%
1%
8%
8%
36%
13%
24%
20%
4%
4%
2%
4%
5%
6%
9%
9%
42
Table 13 shows the average percentage decrease in cross traffic delay for the morning
(AM), noon, and afternoon (PM) time periods, along with the overall average. Like the
left-turn approach averages, these are averages of the intersections, and are not adjusted
for differences in traffic volumes among the intersections.
Table 13: Cross Traffic Average PPLT Improvements
Cross Traffic Averages
Average Morning (AM)
Average Noon
Average Afternoon (PM)
Overall Average
Delay Decrease
8%
10%
8%
8%
The average estimated decrease in cross traffic delay after the implementation of PPLT is
approximately 8%. This shows that PPLT can benefit cross traffic by clearing leftturning vehicles in less time, decreasing the wait for the other approaches to receive their
next green phase.
Change of Intersection Delay from PPLT
The estimated change in intersection level of service and delay after implementation of
PPLT are shown in Tables 14, 15, and 16. This is a measure of how PPLT may affect an
intersection overall, as it considers the average delay of all vehicles utilizing the
intersection.
43
Table 14: Intersection Morning (AM) PPLT Improvements
Intersection
Fair Oaks Promenade & Madison
Franklin & Meadowgate
Fulton & Northrop
Garfield & Marconi
Manzanita & Lincoln
Power Inn & Auberry
Tallyho & Kiefer
Walnut & Cypress
Walnut & Engle
Walnut & North
Wright & Marconi
Zinfandel & Douglas
Baseline
LOS
A
B
B
B
A
B
B
D
C
B
A
B
PPLT
LOS
A
B
B
B
A
B
B
C
C
B
A
B
Intersection
Delay Decrease
3%
2%
9%
11%
-1%
9%
7%
21%
3%
5%
1%
20%
Table 15: Intersection Noon PPLT Improvements
Intersection
Fair Oaks Promenade & Madison
Franklin & Meadowgate
Fulton & Northrop
Garfield & Marconi
Manzanita & Lincoln
Power Inn & Auberry
Tallyho & Kiefer
Walnut & Cypress
Walnut & Engle
Walnut & North
Wright & Marconi
Zinfandel & Douglas
Baseline
LOS
A
B
C
B
B
A
B
C
B
B
B
B
PPLT
LOS
A
B
C
B
B
A
B
B
B
B
B
B
Intersection
Delay Decrease
9%
17%
6%
16%
10%
13%
8%
27%
4%
6%
10%
13%
44
Table 16: Intersection Afternoon (PM) PPLT Improvements
Intersection
Fair Oaks Promenade & Madison
Franklin & Meadowgate
Fulton & Northrop
Garfield & Marconi
Manzanita & Lincoln
Power Inn & Auberry
Tallyho & Kiefer
Walnut & Cypress
Walnut & Engle
Walnut & North
Wright & Marconi
Zinfandel & Douglas
Baseline
LOS
B
C
C
C
B
A
B
D
C
B
B
B
PPLT
LOS
A
B
C
B
B
A
B
C
C
B
B
B
Intersection
Delay Decrease
11%
12%
19%
18%
14%
9%
9%
26%
9%
10%
10%
13%
Figure 14 is a graphical representation of the decrease in intersection delay compared to
the intersection traffic volume.
Figure 14: Intersection Delay Improvement
45
A correlation between delay improvements from PPLT and intersection traffic volume is
not apparent from these results. It is likely that differences among the intersections have
a greater impact on delay improvements than traffic volumes do.
Table 17 shows the average percentage decrease in intersection delay for the morning
(AM), noon, and afternoon (PM) time periods, along with the overall average. These are
averages of the intersections only, and are not adjusted for the different traffic volumes
among the intersections.
Table 17: Intersection Average PPLT Improvements
Intersection Averages
Average Morning (AM)
Average Noon
Average Afternoon (PM)
Overall Average
Delay
Decrease
7%
12%
13%
11%
Based on this analysis, overall intersection delay decreases an average of 11% when
PPLT is implemented. Considering the volume of traffic utilizing these intersections, the
aggregate decrease in delay is considerable.
Additional Measures of Effectiveness
The traffic analysis software (Synchro) estimates the changes in other measures of
effectiveness in addition to delay and level of service. These additional measures include
the number of stops (vehicles per hour), the amount of fuel used (gallons per hour), and
46
three types of emissions (CO, NOx, and VOC, in grams per hour). This information is
summarized in Table 18 below.
Table 18: Additional Measures of Effectiveness
Estimated Percentage Change After PPLT Implementation
Measure
AM
Noon
PM
Overall
Stops
-1.9%
-3.2%
-2.2%
-2.4%
Fuel Usage
-4.2%
-2.8%
-3.7%
-3.6%
CO Emissions
-2.8%
-3.5%
-4.6%
-3.7%
NOx Emissions
-2.9%
-3.5%
-4.5%
-3.7%
VOC Emissions
-2.9%
-3.5%
-4.6%
-3.7%
Based on this analysis, improvements in vehicle stops, fuel usage, and emissions are
predicted for all time periods. Overall, the number of vehicle stops is estimated to drop
by 2.4%, fuel usage is estimated to decrease by 3.6%, and emissions are predicted to
decrease by 3.7%, all of which suggests that the use of PPLT can result in other benefits
in addition to improvements in delay.
47
Chapter 5
CONCLUSION AND RECOMMENDATIONS
Appropriateness of Protected-Permitted Left Turns
Protected-permitted left turns can improve efficiency of traffic operations, decrease
delay, lower emissions, and save fuel. However, protected-permitted left turns have the
potential to increase crash rates because of the additional conflict created during the
permitted phase. Many factors should be taken into account when deciding whether to
implement protected-permitted phasing at a particular intersection. The geometry and
orientation of the intersection should be considered, along with the density of pedestrians
and bicyclists in the area, and the traffic composition, both vehicles and drivers. Yellow
traps must be avoided. Operational efficiency and safety may be improved through the
usage of gap dependency, limiting the permitted phase to appropriate times. Protectedpermitted phasing is a valuable tool that traffic engineers have at their disposal. When
implemented properly, the benefits of protected-permitted phasing can be experienced
and appreciated by the motorists who use the improved intersections.
Crash Data Limitations
The effect that protected-permitted phasing has on crash rates was not determined
through this research effort. Accident reports are often insufficient in terms of assigning
a contributing influence to a particular signal display. One recommendation that can be
made from this project is an improved effort to record relevant information pertaining to
48
crashes so trends can be determined. Several studies mentioned the increased crash risk
of protected-permitted phasing, but the information that is available remains scarce.
Summary of Intersection Analysis
The analysis of twelve intersections in Sacramento County predicted that the
implementation of PPLT decreases the delay of left-turn movements by an average of
65%, cross traffic delay by an average of 8%, and intersection delay by an average of
11%. Both the approach and intersection level of service improved or stayed the same in
every case. The positive effect of PPLT on delay is diminished at intersections that have
approaches with very low traffic volume, as demonstrated by the morning (AM)
intersection delay results at Manzanita and Lincoln Avenues, where a significant majority
of the traffic volume is north-south through traffic. The analysis also predicts
improvements in vehicle stops (2.4%), fuel usage (3.6%), and emissions (3.7%). These
results should be useful to the Sacramento County Department of Transportation, as
several intersections show potential for significant improvement from PPLT, such as
Fulton and Northrop Avenues, Garfield and Marconi Avenues, and Walnut and Cypress
Avenues.
Future Study and Implementation
Based on the research completed by previous studies, the traffic operations industry
would benefit from a new study conducted with direct field measurements. A region
currently lacking in protected-permitted phasing could be identified, along with a number
49
of intersections that might benefit from inclusion in the study. The efficiency and crash
rates of the chosen intersections could then be evaluated for a period before
implementation of protected-permitted phasing, and again for a period after
implementation. Such a study would benefit both the community it took place in, and the
understanding of traffic operations as a whole.
The twelve Sacramento County intersections that were analyzed as part of this project
show that there is promise in the use of PPLT in the Sacramento area. Multiple
intersections are estimated to experience significant operational improvements with
PPLT based on the predictions of the traffic analysis software. Considering these results
and the research conducted by sources cited in this report, the Sacramento County
Department of Transportation should proceed with the implementation of PPLT.
50
APPENDIX A
Crash Data from Washington County, Oregon
(Source: Steyn, Quayle, and Boudart, 2013)
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APPENDIX B
Cupertino Protected-Permitted Signal Phasing Removal
(Source: Erwin Ching of the City of Cupertino Department of Public Works,
personal communication, September 25, 2013)
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APPENDIX C
Introducing the New Flashing Yellow Arrow Left-Turn Signal
(Source: Michigan Department of Transportation, 2013)
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APPENDIX D
Intersection Analysis Reports
(Data provided by Doug Maas, Senior Transportation Engineer
at the Sacramento County Department of Transportation)
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