Update on Yellow Light Timing Study

This report provides an update on the Street Transportation Department's progress on
a field study to investigate and evaluate the "before" and "after" impacts of a focused
implementation of the new Institute of Transportation Engineers' guidelines on yellow
change and red clearance intervals in comparison with Phoenix's current practices of
setting yellow change and red clearance intervals at signalized intersections.

THIS ITEM IS FOR DISCUSSION AND POSSIBLE ACTION.

Summary
The Street Transportation Department (Streets) is conducting a field study in
partnership with the University of Arizona to examine whether implementing Institute of
Transportation Engineers' (ITE) 2020 guidelines on yellow change and red clearance
intervals can enhance safety at signalized intersections. This report addresses
relevant background information, including why it is important to study road users’
compliance with traffic signal change intervals, the procedure for study site selection,
data collection and analysis, baseline condition, and how the study design is being
implemented.

Background
Red-light running (RLR) is one of the riskiest behaviors at signalized intersections.
According to a report published by the American Automobile Association (AAA)
Foundation for Traffic Safety, more than two people were killed every day due to
noncompliance with red signal indications (AAA Foundation for Traffic Safety, 2020).
According to a report published by the Insurance Institute for Highway Safety, RLR
violations caused 928 fatalities in 2020 in the United States. In addition, an estimated
116,000 people suffered injuries in RLR collisions (IIHS, 2022). Similar to cities across
the nation, RLR-related violations in the Phoenix metropolitan area have become one
of the most severe causes of fatal crashes, with 113 fatalities and 9,320 injuries
reported from 2014 to 2020.

The basic purpose for the yellow change interval is to inform the driver that the green
phase has ended. The yellow change interval provides time for the driver to either stop
before entering the intersection or to proceed and clear the intersection.


Page 5


The basic purpose of the red clearance interval is to provide the driver who decides to
enter the intersection during the yellow change interval adequate time to clear the
intersection prior to a conflicting (or opposing) green phase beginning at the
intersection. The red clearance interval is typically called the “all-red” time.

Streets' Current Signal Timing Practices
Streets has a standard operating procedure (SOP) for determining the duration of
traffic signal intervals. The SOP utilizes the pre-2020 ITE kinematic equation and also
states that any deviations from the SOP must be approved by the Streets’ Deputy
Director over the Traffic Services Division. The current City of Phoenix equations for
calculating the yellow change and red clearance intervals is included in Attachment A.

ITE 2020 Guidelines
The updated ITE guidelines for calculating traffic signal timing were released in March
2020. Since 1965, ITE has developed a variety of methods for calculating yellow
change and red clearance intervals, all of which are based on the kinematic equation
method.

According to ITE, the kinematic equation method is the most popular and widely
accepted technique for determining yellow change intervals (Noble, 2020). In
comparing the new ITE 2020 guidelines to the prior ones, there are three key
modifications.

· Change in the speed at which a reasonable driver approaches an intersection. If a
speed study is not completed and the 85th percentile speed is not available, the
85th percentile approach speed for through movements may be estimated and
substituted by the value of “Posted Speed Limit +7.” For the left-turn movements,
the “Posted Speed Limit” can be used as the 85th percentile approach speed.
· Change in the method for yellow change interval calculations for left-turn
movements.
· Change in maximum yellow change interval for left-turn movements. ITE advises
use of 7.0 seconds as the maximum yellow change interval for left-turn movements.

The current 2020 ITE-recommended equations for calculating the yellow change and
red clearance intervals are included in Attachment A.

The challenges and difficulties associated with determining traffic signal timings have
been extensively discussed among scholars and professionals in the field of traffic
engineering. It has long been an area of study to determine the proper traffic signal



Page 6

timings to ensure intersection safety while maintaining an acceptable level of travel
efficiency. However, there is still no broad consensus on the most appropriate method
for calculating yellow change and red clearance intervals.

ITE indicates they believe there has been sufficient theoretical work, research, and
practice information to reach a consensus recommendation. However, ITE
acknowledges that this is not true for all potential elements or aspects of the process
and so they recommend areas for further research. It is important to note that many
practitioners do not accept the ITE 2020 guidelines due to lack of specific field
research. There are a number of studies underway such as the Federal Highway
Administration’s Pooled Fund Study: Traffic Signal Change and Clearance Interval to
address this concern. As Streets has the same concerns, the City of Phoenix, with
Council approval, is participating in this study, with the hopes it will provide relevant
industry and peer guidance to the City of Phoenix.

In order to effectively reduce the total number of RLR-related crashes and maintain a
safe journey for all road users, it is crucial to explore RLR behavior at local
intersections, understand the impact of different signal timing parameters (e.g., yellow
change and red clearance intervals) on RLR frequency, and ultimately develop
appropriate countermeasures.

Signal Timing Field Study
Objectives
Streets’ purpose in performing this field study is to examine the ITE 2020 guidelines for
yellow change and red clearance intervals and identify the relationship between signal
timing parameters and RLR violations. Initially, based on several criteria, including the
frequency and severity of RLR-related crashes and infrastructure feasibility, twelve
intersections were selected to be part of the study. Then, at each study intersection,
smart sensor equipment was installed. Finally, to determine the relationship between
signal timing parameters and RLR violations, an experimental design for before-and-
after analysis is being conducted and implemented.

Study Site Selection, Smart Sensor Equipment, and Field Implementation
In order to evaluate and select the intersections for the study, the City of Phoenix’s
signalized intersections were ranked using the following criteria.

· RLR-related crash frequency and severity;
· List of 100 intersections ranked by crash risk (provided by Maricopa Association of
Governments);
· Former RLR camera locations; and



Page 7

· Infrastructure feasibility.

As part of this identification process, the Arizona Crash Information System (ACIS)
was queried for five years of crash data (from 2016 to 2021) to identify prospective
intersections. From that data, only crashes at four-way intersections resulting from
disregarding traffic signals were utilized.

Intersections were then ranked based on the frequency and severity of the crashes.
The list of 100 intersections and the intersections where RLR cameras had previously
been installed were two additional datasets that were considered. The list of 100
intersections contains the top 100 intersections ranked by crash risk and intersection
safety score provided by the Maricopa Association of Governments (MAG). The
document "MAG Network Screening Methodology for Intersections" provides a
detailed description of the Intersection Safety Score (MAG, 2010).

After ranking the intersections, the feasibility of smart sensor equipment installation
was considered. Due to the infrastructure limitation, intersections that required new
signal cabling or new conduits in one or two legs of the intersection were eliminated
from further consideration. The twelve intersections selected as the final candidate
locations for the study were the ones that had the highest rank that also did not require
any upgrades for smart sensor equipment installation. Selection of the twelve
intersection locations for the study was completed in June 2022.

Concurrently, Streets staff was working to procure and install the necessary smart
sensor equipment (Miovision’s SmartView 360). The smart sensor equipment provides
traffic-related data, signal timing information, and high-resolution event-based data.
The data and performance measures provided by the equipment include RLR, simple
delay, and approach volumes. Despite some equipment delivery delays, all smart
sensor equipment was installed and activated in August 2022.

Data Collection and Analysis
Signal timing information, turning movement counts, as well as red light and yellow
light running data are being collected from the smart sensor equipment at each study
intersection. The signal timing data contains information about traffic signal cycle
lengths, yellow change intervals, and green durations. The turning movement counts
represent various approach movements (left-turn, through, and right-turn movements)
that pass through each intersection over 15-minute intervals. For red light and yellow
light running data, each vehicle that ran the red light or yellow light has its own
timestamp, vehicle classification, and approach movement recorded. The data
collected will also go through a quality control check process to identify and filter out
outliers. As part of this process, information collected by the smart sensor equipment


Page 8

is reviewed, analyzed and compared to ground-truth video recordings at the
intersections.

Baseline Condition Data - Intersection Through Movements
At each of the twelve intersection study locations, baseline data was collected on
weekdays between Sept. 1 and Nov. 21, 2022. Over this twelve-week period, a total of
144,795 RLR incidents were observed. Among the study sites, two intersections
experienced the most RLR incidents at more than 300 per day, eight of the
intersections experienced between 200 and 300 RLR incidents per day, while the
remaining two intersections had less than 200 RLR incidents per day.

It was found that the majority of RLR violations (88.5 percent or 128,164 events),
occurred during the red clearance interval, while the remaining (11.5 percent or 16,631
events) RLR violations occurred after the red clearance time elapsed when opposing
green phase movement began. The figure in Attachment B shows the percentage of
RLR incidents that passed through the intersection on a red light both during and after
the red clearance time.

As noted earlier, Streets’ current practices calculate yellow change intervals based on
posted speed limits for the streets approaching an intersection. Therefore, streets with
lower speed limits have a lower calculated yellow change interval than streets with
higher speed limits. The baseline data showed that RLR frequency per 1,000 vehicles
during the yellow change intervals at each intersection was higher for intersections
with streets with lower approach speed limits and shorter yellow change intervals. In
summary, the approaches with shorter yellow intervals have greater RLR frequencies
per 1,000 vehicles than those with longer yellow intervals. However, it is worth noting
that side streets and approaches coming out from neighborhoods are most often
associated with lower speed limits and shorter yellow intervals.

Study Design to Implement and Analyze ITE 2020 Guidelines
The “before-and-after” aspects of this study is intended to statistically determine
whether the ITE 2020 guidelines for yellow change and red clearance intervals can
improve intersection safety by reducing the number of RLR violations. Moreover, the
potential relationship between different signal timing parameters and the potential
impact of signal timing parameters on RLR frequency will be identified. Finally, a
statistical method will be developed to identify the appropriate amount of yellow
change and red clearance intervals that will result in reduction in RLR frequency.

With respect to the twelve study intersections, for the through and left-turn movements,
some intersections will be used as the control sites, and the yellow change and red
clearance intervals for these control intersections will not be changed from their current


Page 9

timing. The control sites are used to eliminate the effects of traffic volume and pattern
variation during the holiday seasons in our statistical analysis.

The remaining intersections are then considered as treatment sites, and the yellow
change intervals will be changed for all the treatment sites. The new ITE 2020
guidelines were used to calculate the yellow change intervals for through and left-turn
movements for the experimental design at the treatment sites.

To make the study conclusions statistically sound, the study timeline includes twelve
weeks to collect sufficient data, which will occur over six, two-week time periods as
outlined below.

· Period 1: Nov. 21, 2022 - Dec. 5, 2022;
· Period 2: Dec. 5, 2022 - Dec. 19, 2022;
· Period 3: Dec. 19, 2022 - Jan. 2, 2023;
· Period 4: Jan. 2, 2023 - Jan. 16, 2023;
· Period 5: Jan. 16, 2023 - Jan. 30, 2023; and
· Period 6: Jan. 20, 2023 - Feb. 13, 2023.

Through Movements - Treatment Site Changes
In accordance with the ITE 2020 guidelines, due to the absence of approach speed at
the intersections, for this study, the “speed limit +7” guidance was used in lieu of the
through movement's 85th percentile of approach speed. Using the “speed limit +7”
guidance results in an increase between 0.4 seconds and 0.6 seconds for the yellow
change interval in comparison to the current yellow change interval for through
movements.

For evaluation of through movements at the twelve study intersections, three of the
intersections will be used as the control sites, and the yellow change and red
clearance intervals for these three control intersections will not be changed from their
current timing. The remaining nine intersections are considered treatment sites. The
yellow change intervals for the through movements will be changed for all the
treatment sites. To better understand driver behaviors and the influence of the yellow
change interval over the short-term and long-term, the nine treatment intersections are
divided into three equal groups for implementation methodology as outlined below.

· Incremental Intersections Group: For this group of three intersections, the increase
in the yellow change interval is implemented at the selected treatment sites over
five two-week time periods. Therefore, at each of the three incremental sites, the
yellow change interval is increased by 0.1 - 0.2 seconds at the start of each two-


Page 10

week time period, depending on how much the total calculated yellow change
interval increased.
· Periodically Intersections Group: For this group of three intersections, the yellow
change interval is adjusted at the beginning of each two-week time period. That is,
the yellow change interval will alternate between the new calculated yellow change
interval and the baseline (current) yellow change interval every two weeks to study
the drivers’ compliance behavior over a short period. During the first, third, and fifth
two-week time periods, the new yellow change intervals will be implemented at sites
for the entire two-week time period. During the second, fourth, and sixth two-week
time periods, the yellow change interval is returned to the baseline (current) yellow
change interval.
· Long-Term Intersections Group: For this group of three intersections, the focus is on
studying the impact of increasing the yellow change interval on drivers' compliance
behavior in the long-term. The new yellow change intervals are implemented at the
beginning of the first two-week time period and will not be modified for the duration
of the data collection timeframe. These sites are selected to study the long-term
impact of ITE 2020 guidelines on driver behavior.

Left-Turn Movements - Treatment Site Changes
In accordance with the ITE 2020 guidelines, due to the absence of approach speed for
left turns at the study intersection, the “speed limit +7” guidance was used in lieu of the
left-turn movement's 85th percentile of approach speed. The estimated intersection
entry speed for left-turn movements was 20 miles per hour, following the ITE 2020
guidelines, which results in an increase between 0.3 seconds and 3.2 seconds for the
yellow change interval in comparison to the current yellow change interval for left turn
movements. Since two of the selected intersections for study do not have protected left
-turn phase movements, only ten intersections are being used for studying left-turn
movements.

Of the ten study intersections for left-turn movements, two of the intersections will be
used as the control sites, and the remaining eight intersections will be used as
treatment sites. The significant increases in left-turn yellow change intervals calculated
under ITE 2020 guidelines for some intersections represent a change in left-turn yellow
change intervals that could raise safety concerns. Therefore, the eight treatment
intersections are being implemented as either the Incremental Intersections Group or
the Long-Term Intersections Group as outlined below; there are no intersections
designated for a Periodically Intersections Group.

· Incremental Intersections Group: For this group of four intersections, the increase in
the yellow change interval is implemented at the selected treatment sites over five



Page 11

two-week time periods. Therefore, the yellow change interval is increased by 0.3 -
0.7 seconds at the start of each two-week time period, depending on how much the
total calculated yellow change interval is increased.
· Long-Term Intersections Group: For this group of four intersections, the focus is on
studying the impact of increasing the yellow change interval on drivers’ compliance
behavior in the long-term. The new yellow change intervals are implemented at the
beginning of the first two-week time period and will not be modified for the duration
of the data collection timeframe. These sites are selected to study the long-term
impact ITE 2020 guidelines on driver behavior.

Red Clearance Intervals - Treatment Site Changes
The red clearance interval is intended to allow a reasonable driver who approaches
the intersection before the yellow interval ends enough time to clear the intersection
before conflicting traffic enters the intersection.

For evaluation of the ITE 2020 guidelines related to red clearance intervals, a study
has been designed similar to the yellow change interval study. However, to avoid
impacting the results of the yellow change interval study currently underway, the new
red clearance intervals will be implemented near the end of the data collection effort
for the yellow change interval study.

Next Steps
Following the initial baseline data collection efforts, the study’s twelve-week phased
implementation of the new yellow change intervals calculated using the ITE 2020
guidelines began on Nov. 21, 2022. Preliminary evaluation and results of
implementation data collected in the first few two-week time periods were presented to
the Transportation, Infrastructure and Planning Subcommittee at its Jan. 18, 2023
meeting.

All data collection will be completed in February 2023, and the final study findings will
be presented to City Council later this Spring. Additionally, Phoenix will participate in a
national pooled study.

Attachment C provides a list of references for this report.

Concurrence/Previous Council Action
This report was provided to the Transportation, Infrastructure and Planning
Subcommittee on Jan. 18, 2023.




Page 12

Responsible Department
This item is submitted by Deputy City Manager Mario Paniagua and the Street
Transportation Department.




Page 13
Attachment A


Current City of Phoenix equations for calculating the yellow change and red clearance
intervals:

Yellow Change Calculation

1.47𝑉𝑉
𝑌𝑌 ≥ 𝑡𝑡 +
2𝑎𝑎 + 64.4𝑔𝑔

Where:
𝑌𝑌 = minimum yellow change interval (in seconds) with a maximum of 5 seconds (if the
calculation exceeds 5 seconds, any excess time from the calculation is added to the red
clearance interval);
𝑡𝑡 = perception-reaction time (in seconds); the time needed for an approaching driver to
“perceive” the yellow indication and to “react” by braking to a stop or deciding to pass
through the intersection. Default value of 1.0 second.
V = intersection entry speed (mph); the approach speed limit is assumed.
𝑎𝑎 = deceleration (ft/second2); the rate at which it is assumed a driver will slow down
upon seeing the yellow signal. Default value of 10 ft/second2.
𝑔𝑔 = grade of approach (downhill is negative grade)

Red Clearance Calculation

𝑊𝑊
𝑅𝑅 = � �
1.47𝑉𝑉

Where:
𝑅𝑅 = red clearance interval (seconds);
V= intersection entry speed (mph); the approach speed limit is assumed.
𝑊𝑊 = distance to traverse the intersection (width), stop line to far side no-conflict point
along the vehicle path (ft.);





Page 14
Current 2020 ITE-recommended equations for calculating the yellow change and red
clearance intervals:

Yellow Change Calculation

1.47(𝑉𝑉85 − 𝑉𝑉𝐸𝐸 ) 1.47𝑉𝑉𝐸𝐸
𝑌𝑌 ≥ 𝑡𝑡 + +
𝑎𝑎 + 32.2𝑔𝑔 2𝑎𝑎 + 64.4𝑔𝑔

Where:
𝑌𝑌 = minimum yellow change interval (in seconds);
𝑡𝑡 = perception-reaction time (in seconds); the time needed for an approaching driver to
“perceive” the yellow indication and to “react” by braking to a stop or deciding to
pass through the intersection. Default value of 1.0 second.
𝑉𝑉85 = 85th percentile approach speed (mph); the speed at which a “reasonable” driver is
assumed to approach the intersection.
𝑉𝑉𝐸𝐸 = intersection entry speed (mph); the speed at which a “reasonable” driver is
assumed to cross the stop line of the intersection when they have been slowing
down in preparation for making a left turn.
𝑎𝑎 = deceleration (ft/second2); the rate at which it is assumed a driver will slow down
upon seeing the yellow signal. Default value of 10 ft/second2.
𝑔𝑔 = grade of approach (downhill is negative grade)

Red Clearance Calculation

𝑊𝑊 + 𝐿𝐿
𝑅𝑅 = � � − 𝑡𝑡𝑠𝑠
1.47𝑉𝑉𝐸𝐸
Where:
𝑅𝑅 = red clearance interval (seconds);
𝑊𝑊 = distance to traverse the intersection (width), stop line to far side no-conflict point
along the vehicle path (ft.);
𝐿𝐿 = length of vehicle (ft.); 20 ft is often used as the representative length for vehicles
entering the intersection.
𝑡𝑡𝑠𝑠 = conflicting vehicular movement start up delay (seconds); an optional parameter with
an initial value set at 0.0 seconds, values may be used based on engineering
judgment or as supported by an engineering study.





Page 15
Attachment B



Percentage of red-light running incidents that passed through the
intersection on a red light both during and after the red clearance time




Page 16
Attachment C


References

Bonneson, J. A., & Zimmerman, K. H. (2004). Effect of Yellow-Interval Timing on the
Frequency of Red-Light Violations at Urban Intersections. Transportation research
record, 1865(1), 20-27. https://doi.org/10.3141/1865-04

Chiou, Y.-C., & Chang, C.-H. (2010). Driver responses to green and red vehicular signal
countdown displays: Safety and efficiency aspects. Accident Analysis &
Prevention, 42(4), 1057-1065.

Freas, R. (2022). 2022-12 Traffic Signal Timing Report.

IIHS. (2022). Red light running. https://www.iihs.org/topics/red-light-running

Karimpour, A., Anderson, J. C., Kothuri, S., & Wu, Y.-J. (2021). Estimating pedestrian
delay at signalized intersections using high-resolution event-based data: a finite
mixture modeling method. Journal of Intelligent Transportation Systems, 1-18.

Lee, J., Abdel-Aty, A., & Park, J. (2018). Investigation of associations between marijuana
law changes and marijuana-involved fatal traffic crashes: A state-level analysis.
Journal of Transport & Health, 10, 194-202.

Lum, K., & Halim, H. (2006). A before-and-after study on green signal countdown device
installation. Transportation Research Part F: Traffic Psychology and Behaviour,
9(1), 29-41.

MAG. (2010). MAG Network Screening Methodology for Intersections.
http://azmag.gov/LinkClick.aspx?fileticket=L0lMsuxQNYU%3d&tabid=527&portali
d=0&mid=3809

Noble, D. E. (2020). Guidelines for determining traffic signal change and clearance
intervals. ITE journal, 90(3), 28-31.

Retting, R. A., Ferguson, S. A., & Farmer, C. M. (2008). Reducing red light running
through longer yellow signal timing and red light camera enforcement: Results of
a field investigation. Accident analysis and prevention, 40(1), 327-333.
https://doi.org/10.1016/j.aap.2007.06.011

Retting, R. A., & Greene, M. A. (1997). Influence of Traffic Signal Timing on Red-Light
Running and Potential Vehicle Conflicts at Urban Intersections. Transportation
research record, 1595(1), 1-7. https://doi.org/10.3141/1595-01




Page 17