The Flashing Yellow Arrow (FYA) has made a big splash in the traffic engineering industry. They allow flexibility in left turn phasing operation, are better understood than standard protected/permissive signals, and eliminate the dreaded “yellow trap”. This makes FYAs a very powerful and efficient tool to use in signal timing that usually decreases overall delay while increasing driver safety. Due to these benefits, many agencies across the Country are retrofitting existing signals with FYA operation.
Recently, the City of Saint Louis Park asked us to complete a comprehensive FYA review of their 15 existing signal systems. The purpose of the study was to determine which signals could be retrofitted and which ones should be (i.e. what locations would get the most benefit from using FYAs).
Many guides are available discussing the what, why, and how to use FYAs. However, the ‘when’ is a piece that is lacking detail. We started with the Minnesota Department of Transportation’s Traffic Signal Timing and Coordination Course Manual, adding national guidance where we could find it to develop the following seven questions for FYA analysis:
- Is there a shared left turn lane?
- Do the opposing left turn paths overlap?
- Is sight distance deficient?
- Is the cross-product of left turns and opposing thru traffic greater than 80,000 vehicles per hour (100,000 vehicles per hour if two or more opposing lanes)?
- Does the left turn face three or more opposing lanes?
- Does the left turn have 2 or more lanes?
- Is the speed limit greater than 45 MPH?
Questions 1 through 3 are for FYA compatibility, determining if a signal is eligible for FYA operation at any point in a day. If the answer to any of the questions is “yes”, it is not safe to run FYA operation. Although not a question above, we also checked the crash history to determine if protected only operation was implemented due to safety concerns.
Questions 4 through 7 check FYA operation benefits and feasibility, determining when protected only operation should be considered throughout the day. Using the hourly average of the 48-hour turning movement counts we collected, each hour of an average day was checked against the volume threshold. Then the characteristics of the left turn movement/intersection were examined. As before, an answer of “yes” to one or more of these questions raised concerns about the use of FYA for that particular hour or approach. Since the “yes” answer does not necessarily rule out FYA (just raises potential concern), we also made first-hand observations of existing operations in a number of instances. Thus, ‘engineering judgment’ was a supplement to the simple analysis. The outcome of the analysis and judgment was how many hours of the day a left turn approach would be recommended to run protected only operation compared to protected/permissive using the FYA.
Finally, the current left turn phasing and volume of each approach was checked to determine what benefit would occur from FYA installation. By determining the number of hours the signal was within the suggested protected operation period, and comparing this with the current operation and volume of the left turn movement, three tiers of benefit were determined:
- High Benefit. Signals that were currently running protected only and could operate as permissive of protected/permissive, based on volume, during certain periods of the day. These are signals where an immediate benefit would occur.
- Medium Benefit. Signals that were running protected/permissive and could operate permissive only during certain time periods of the day. These are signals that also may need to run protected only in the future, as traffic volume increases with time. These signals would have some immediate benefit and the potential for more future benefit when protected operation becomes necessary due to traffic volumes.
- Low Benefit. Signals that are running permissive only left turns with low volumes or signals running with protected only volume for most of the day. These signals would see little immediate benefit as the traffic volume either does not warrant left turn phasing or exceeds protected left turn phasing operation volumes for a significant amount of the day.
The result was a simple table that identified recommended FYA installation and which locations have characteristics that suggest high, or low, benefits from that installation. By using these seven questions, in conjunction with some engineering judgment, we were able to quickly identify and analysis the potential for FYA at 15 intersections.
Want to see our final report? – Download it here. FYA Submittal_4.22.16
