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The National Academies

NCHRP 03-125 [Active]

Evaluation of Change and Clearance Intervals Prior to the Flashing Yellow Arrow Permissive Left-Turn Indication

  Project Data
Funds: $300,000
Staff Responsibility: David Jared
Research Agency: University of Wisconsin-Madison
Principal Investigator: David Noyce
Effective Date: 9/21/2016
Completion Date: 9/20/2019

OBJECTIVE
 
The objective of this research is to develop recommendations for the use of solid yellow arrow change and solid red arrow clearance intervals after a leading solid green arrow transitioning to a permissive FYA or FRA based upon their human factors and efficiency implications.

STATUS

The contract is just underway.
 
TASKS
 
Task 1. Analyze, describe, and critique pertinent research on driver behavior during change and clearance intervals, particularly those following protected left-turn signal phases. Include completed research and research currently underway. Extract the pertinent information on timing change and clearance intervals from the ITE Proposed Recommended Practice: Guidelines for Determining Traffic Signal Change and Clearance Intervals. For the purposes of this research, it is assumed that the durations for the change and clearance intervals from the ITE Proposed Recommended Practice are appropriate.
 
Task 2. Survey state and local transportation agencies that have implemented the FYA or FRA for left turns to determine their policies on change and clearance intervals following the protected phase, their rationale for these policies, and their general experiences with the operational and safety implications associated with these policies.
 
Task 3. Identify critical human factor variables (e.g., distracted driving, driver age, driver aggressiveness (and changes due to cycle failures), familiarity with FYA/FRA) and varying intersection characteristics (e.g., number of opposing lanes, conflicting pedestrian and vehicular volumes, supplemental signs, approach speeds, environmental conditions) that may influence driver behavior during the solid yellow arrow change and solid red arrow clearance intervals. Define and justify behaviors (e.g., failure to clear the intersection before the onset of opposing traffic through green, unnecessary stops that may increase the potential for rear-end crashes) that can be used as metrics to evaluate the effect of the alternative sequences during the human factors study in Task 6 and the appropriate parameters that should be used in Task 7.
 
Task 4. Develop detailed work plans for the human factors and operational studies in Tasks 6 and 7, including the scenarios that will be used. Since use of the FYA is much more prevalent than the FRA, FYA should be emphasized in the study designs. As the budget permits, field data should be collected to calibrate and validate any simulation-based analysis.
 
Task 5. Prepare an interim report on the information developed in Tasks 1 through 4.

Task 6. Carry out the human factors study as presented in the interim report and modified based on the discussion at the interim meeting.

Task 7. Carry out the operational study as presented in the interim report and modified based on the discussion at the interim meeting. It is expected that the study will use microscopic simulation, field data, or another appropriate method to quantify the impacts of including a solid red arrow clearance interval after a solid yellow arrow on delay, capacity, and other operational performance measures.
 
Task 8. Submit a final report documenting the entire research effort and, if appropriate, providing practitioners with a process for determining whether a solid red arrow should be included as a clearance interval at a traffic signal. Any recommendations for modifications to standard references (e.g., MUTCD, ITE Proposed Recommended Practice: Guidelines for Determining Traffic Signal Change and Clearance Intervals, Traffic Engineering Handbook, NCHRP Report 812: Traffic Signal Timing Manual) should be written in an appropriate form for consideration in their updates and included as an appendix. In addition, prepare a stand-alone technical memorandum titled “Implementation of Research Findings and Products."

BACKGROUND
 
The flashing yellow arrow (FYA) has been the focus of many research efforts over the past 20 years. Those efforts culminated in the inclusion of the FYA as a permissive left-turn and right-turn indication in the 2009 Manual on Uniform Traffic Control Devices (MUTCD). Implementation of FYA across the country has rapidly increased with hundreds of installations by cities and states. An alternative to the FYA is the flashing red arrow (FRA) that is available when stopping before turning is desired. Some agencies have adopted the FRA as their standard permissive left-turn indication.
 
One of the issues with the implementation of the FYA/FRA indications that has not been effectively addressed is the change and clearance interval sequence prior to the permissive interval that most effectively communicates the desired action to road users. For example, many jurisdictions use both the yellow arrow change indication and the red arrow clearance indication when transitioning from the protected movement to the FYA. Although not currently required by the MUTCD, some traffic engineers believe that the solid red arrow is needed to effectively communicate to drivers that the left-turn movement has switched from protected to permissive. Other traffic engineers allow an immediate transition from the solid yellow arrow change interval to the permissive indication. The human factors and operational impacts of these alternative sequences have not been explored and traffic engineers need guidance.
 
Tradeoffs between safety and operational efficiency are common in all aspects of transportation engineering. Determining an appropriate balance is often difficult due to the number of confounding factors. In this case, a direct assessment of the safety performance of the alternative indication sequences is cost prohibitive due to the difficulty in determining whether the change and clearance intervals contributed to a crash and the likely small number of these types of crashes. A human factors approach is therefore being taken. This research is intended to provide greater clarity on the performance of these alternative sequences so that, to the extent feasible, a tradeoff assessment can be made using data rather than perceptions.
 
Some agencies are using a solid red arrow between the protected and permitted intervals for purposes other than clearance. Examples include providing time for an opposing queue to process or as a leading pedestrian interval. These types of applications are not being addressed in this research.
 

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