Title
Results on Yellow Light Timing Field Study
Description
This report provides the field study results on the Street Transportation Department's efforts to investigate and evaluate the "before" and "after" impacts of a focused implementation of the new Institute of Transportation Engineers' guidelines on yellow change intervals at signalized intersections in comparison with Phoenix's current practices. The report further requests Council approval of staff's recommendation on the implementation of new yellow change interval timing based on the results of the field study.
THIS ITEM IS FOR DISCUSSION AND POSSIBLE ACTION.
Report
Summary
The Street Transportation Department (Streets) conducted a field study in partnership with the University of Arizona to examine whether implementing Institute of Transportation Engineers’ (ITE) 2020 guidelines on yellow change intervals can enhance safety at signalized intersections. This report addresses relevant background information, how the field study was designed and implemented, the field study results and recommendations.
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 across the U.S. 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 (IIHS), 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 144 fatalities and 10,685 injuries reported from 2014 to 2021.
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. 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 calculates yellow change intervals based on posted speed limits. The current City of Phoenix equations for calculating the yellow change and red clearance intervals are 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 miles per hour. 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.
Traffic signal timings have long been an area of study to determine the proper traffic signal 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.
In order to effectively reduce the total number of RLR-related crashes and maintain a safe journey for all road users, it was crucial for Streets to perform a field study 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 was to examine the ITE 2020 guidelines for yellow change 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 as well as infrastructure feasibility, 12 intersections were selected to be part of the field 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 was conducted and implemented.
Baseline Condition Data - Intersection Through Movements
At each of the 12 intersection study locations, baseline data was collected on weekdays between Sept. 1 and Nov. 21, 2022. Over this 12-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, and 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.
Study Design to Implement and Analyze ITE 2020 Guidelines
The “before-and-after” aspects of this study were 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 was 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 12 study intersections, for the through and left-turn movements, some intersections were used as the control sites, and the yellow change and red clearance intervals for these control intersections was not changed from their current timing. The control sites were used to eliminate the effects of traffic volume and pattern variation during the holiday seasons in our statistical analysis.
The remaining intersections were then considered as treatment sites, and the yellow change intervals were 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 included twelve weeks to collect sufficient data, which occurred 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 Posted Speed Limit + 7 miles per hour guidance was used in lieu of the through movement's 85th percentile of approach speed. Using the Posted Speed Limit + 7 miles per hour guidance resulted 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 were used as the control sites, and the yellow change and red clearance intervals for these three control intersections were not be changed from their current timing. The remaining nine intersections were considered treatment sites. The yellow change intervals for the through movements were 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 were divided into three equal groups for implementation using the methodology outlined below.
- Incremental Intersections Group: For this group of three intersections, the increase in the yellow change interval was implemented at the selected treatment sites over five two-week periods. Therefore, at each of the three incremental sites, the yellow change interval was increased by 0.1 to 0.2 seconds at the start of each two-week 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 was adjusted at the beginning of each two-week period. That is, the yellow change interval alternated 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 periods, the new yellow change intervals were implemented at sites for the entire two-week period. During the second, fourth, and sixth two-week periods, the yellow change interval was returned to the baseline (current) yellow change interval.
- Long-Term Intersections Group: For this group of three intersections, the focus was on studying the impact of increasing the yellow change interval on drivers' compliance behavior in the long term. The new yellow change intervals were implemented at the beginning of the first two-week period and were not modified for the duration of the data collection time frame. These sites were 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 Posted Speed Limit + 7 miles per hour 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 resulted 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 the study did not have protected left -turn phase movements, only ten intersections were used for studying left-turn movements.
Of the ten study intersections for left-turn movements, two of the intersections were used as the control sites, and the remaining eight intersections were used as treatment sites. The significant increases in left-turn yellow change intervals calculated under ITE 2020 guidelines for some intersections represented a change in left-turn yellow change intervals that raised safety concerns. Therefore, four of the eight treatment intersections were implemented as Incremental Intersections Group and four as Long-Term Intersections Group as previously outlined; there were no intersections designated for a Periodically Intersections Group. For the incremental Intersections Group, the yellow change interval was increased by 0.3 to 0.7 seconds at the start of each two-week period, depending on how much the total calculated yellow change interval was increased.
Field Study Results
In order to evaluate the comparative differences between the before and after study groups, a statistical T-test was employed. The T-test is a common method used to assess the significance of differences observed between two sets of data. The statistical T-test has been widely used in previous before-and-after studies. In this study, a 95 percent confidence level is employed to conclude the statistical significance of the difference in RLR rates between the before and after groups. To avoid bias in the analysis and ensure a fair comparison regardless of traffic volumes, the RLR frequency was normalized by dividing the number of RLR violations by the number of vehicles. This calculated the RLR rate, which was measured as the number of RLR violations per 1,000 vehicles per day.
Through Movements - Treatment Site Changes
- Incremental Intersections Group: Table 1 in Attachment C shows the statistical T-test results for the RLR rate at the Incremental Intersections Group sites. When comparing Periods 3, 4, 5, and 6 with the Baseline Condition Data, it is evident that the average RLR rate decreased significantly at all treatment sites. However, the results during the initial implementation (Periods 1 and 2) were inconclusive.
- Periodically Intersections Group: Table 2 in Attachment C shows the statistical T-test results for the RLR rate at the Periodically Intersections Group sites. These results indicate a significant decrease in the average RLR rate during periods where the new yellow change intervals were implemented (Periods 1, 3, and 5), compared to the periods with the current yellow change intervals (Periods 2, 4, and 6).
- Long-Term Intersections Group: Table 3 in Attachment C shows the statistical T-test results for the RLR rate at the Long-Term Intersections Group sites. The results indicate a significant decrease in the average RLR rate during all six periods, particularly in Period 1 when the new yellow change interval was first implemented.
Left-Turn Movements - Treatment Site Changes
- Incremental Intersections Group: Table 4 in Attachment C shows the statistical T-test results for the RLR rate at the Incremental Intersections Group sites. It is evident from the comparison of Periods 3, 4, 5, and 6 with the Baseline Condition Data that the average RLR rate decreased at all treatment sites. However, the reduction was not statistically significant for some periods and study sites.
- Long-Term Intersections Group: Table 5 in Attachment C shows the statistical T-test results for the RLR rate at the Long-Term Intersections Group sites. The results indicate a significant decrease in the average RLR rate during all six periods, particularly in Period 1 when the new yellow change interval was first implemented.
It should be noted that due to a malfunction of the smart sensors at some of the treatment intersection sites during several of the field study periods, data was not captured for those field study locations or periods.
According to the statistical analysis of the data, the results for both through and left-turn movements are summarized below.
- For the control sites, which were kept at the current yellow change intervals, there was no significant reduction in the average rate of RLR for the through movements and left-turn movements when comparing with the Baseline Condition Data.
- For all the treatment sites for the through and left-turn movements, the average rate of RLR significantly decreased during the periods that used the ITE 2020 yellow change intervals compared to periods with the current yellow change intervals.
- The yellow change intervals for the left-turn movements implemented at the incremental sites did not show a significantly increased benefit beyond Period 3 when additional yellow time was added. Period 3 closely matched the recommended intervals from the National Cooperative Highway Research Program (NCHRP) Report 731 Guidelines for Timing Yellow and All-Red Intervals at Signalized Intersections. The NCHRP Report 731 recommends using the Posted Speed Limit - 5 miles per hour as the 85th percentile approach speed for left-turn movements. This results in a shorter yellow change interval for the left-turn movement as compared to the ITE 2020 guidelines. However, the NCHRP yellow change interval is longer than the City's current yellow change intervals for the left-turn movement. The comparison of all three methods is shown in Attachment D. The results suggest that using the NCHRP yellow change intervals can be effective, without the need to implement the ITE 2020 guidelines for calculating yellow change intervals for the left-turn movements.
Based on the results of the field study, several recommendations were proposed to improve safety at signalized intersections by better understanding road users' compliance behavior with the yellow change interval. These recommendations include:
- Implementation of the ITE 2020 Guidelines for the yellow change interval for the through movements. For calculating through movement yellow change intervals, the Posted Speed Limit + 7 miles per hour as the 85th percentile approach speed is recommended when field-measured speed data is unavailable.
- Implementation of the NCHRP Report 731 Guidelines for the yellow change interval for the left-turn movements. For left-turn movements, it is recommended to use Posted Speed Limit - 5 miles per hour as an estimation for the 85th percentile left-turn approach speed.
- As the field study recommendations were based on only 12 weeks of data, further research is necessary to understand the long-term effects of yellow change intervals on driver behavior. Further research is currently being conducted to investigate the impact of red clearance intervals, both individually and in conjunction with yellow change intervals, to gain a more comprehensive understanding of their combined effects on intersection safety.
Attachment E provides a list of references for this report.
Implementation Timeline
In order to implement the field study recommendations with current staff resources in the Traffic Management Center (TMC), it is estimated that it will take approximately six years to implement the new yellow change intervals for the 1,200 traffic signals in Phoenix. Based on the results of this study and the improved safety benefits related to RLR, Streets proposes implementing the field study recommendations on an expedited timeline with a goal to complete the retiming all 1,200 traffic signals within three years.
However, additional staff resources will be needed. Streets is recommending adding five positions to the TMC, which would include one Traffic Engineer I, one Traffic Engineer II, two Signal Systems Specialist I's, and one Signal Systems Specialist II. The new positions would be funded by existing Streets' revenue from the Arizona Highway User Revenue Fund (HURF), at an estimated cost of $445,502 per year.
Update on the MAG Top 100 Locations
At the Jan. 24, 2023, Policy meeting, Council directed staff to install smart sensor equipment at the most dangerous intersections in Phoenix taken from the Maricopa Association of Governments (MAG) Top 100 Intersections by Crash Risk. Staff has evaluated the list and identified 64 Phoenix intersections from that list. Streets is currently working on a procurement to enable the purchase of the necessary smart sensor equipment and anticipates having a contract in place by Fall 2023. Concurrently, staff is evaluating the infrastructure feasibility of all 64 locations in order to develop a prioritized list for implementation.
Recommendation
The Street Transportation Department requests City Council approval of the following actions based on the results of the Yellow Light Timing Field Study:
1. Adoption and implementation of the ITE 2020 Guidelines for calculating the yellow change interval for the through movements for traffic signal timing;
2. Adoption and implementation of the NCHRP Report 731 Guidelines for calculating the yellow change interval for the left-turn movements for traffic signal timing; and
3. Approval of five full-time City staff positions to expedite implementation using existing Department HURF revenues.
Concurrence/Previous Council Action
- An update on the Yellow Light Timing Study was provided to the Transportation, Infrastructure and Planning Subcommittee on Jan. 18, 2023.
- An update on the Yellow Light Timing Study was provided at the Policy meeting on Jan. 24, 2023.
Department
Responsible Department
This item is submitted by Deputy City Manager Alan Stephenson and the Street Transportation Department.