Flashing Yellow Arrow: Tool for Time of Day Use

Guest Post by Max Moreland, PE, Director of Operations at Traffic Data Inc.

flashing-yellow-arrow1-300x200EDM (electronic dance music), man buns and Flashing Yellow Arrows.  Undeniably three big trends.  Due to an unfortunate word count restriction on the blog, only Flashing Yellow Arrows will be discussed in this post.

Flashing Yellow Arrows (FYAs) are becoming more prevalent, and have become the standard for Minnesota at signalized intersections.  FYAs allow the option to switch between protected and permitted left turn movements throughout the course of the day.  If traffic is heavy, the FYA can operate in protected mode giving left turning vehicles their own phase to get through the intersection.  If traffic is light, the exclusive left turn phase can be skipped and the FYA can operate in flashing mode to allow for left turning vehicles to turn when there is an appropriate gap in the opposing traffic.

Once an FYA is installed, the first question that comes up is when should it be in protected mode and when should it be in permitted mode?

I recently went to a presentation about a tool that helps determine time of day usage for FYAs.  This presentation was put on by the Minnesota Local Road Research Board and presented by Vahid Moshtagh of SRF Consulting Group with some help from Professor Gary Davis of the University of Minnesota.

The decision to operate an FYA in protected or permitted mode has both operational and safety impacts such as signal optimization and left turn crash risk.  The developers of this tool determined what to include through a matched case-control design, which describes how the risk of operating an FYA in permitted mode changes as dominating conditions change.

Using a sample of 328 intersections with permissive left turn movements from the seven county Twin Cities area, the development of this tool factored in different geometric characteristics including sight distances, crash data and turning movement counts to develop crash prediction models.  To use the tool, a user just needs to select a base condition, enter turning movement volumes, and fill out some geometric characteristics of the intersection.  A relative risk diagram is then computed which plots the relative risk of a left turn crash at different times throughout the day.  This plot will give an agency a quick idea of what time of day pattern they can use for an FYA.

It appears that work is still going to be done on improving this tool from where it is now.  Based on the available crash data, only some intersection types had enough data to fit reliable statistical models meaning that this tool cannot currently be applied to every intersection.  Also, other factors such as conflicting pedestrians have not been incorporated into the tool.

Though there are still improvements to be made, this spreadsheet tool can be useful for giving signal operators a good idea of what time of day operations could look like at an FYA.  As of now I wouldn’t use this as the final decision for time of day operations, but the simplicity of the tool allows for a quick and easy check to get operators in the right ballpark.

A lot more information on this project as well as a download of the spreadsheet tool can be found here:

Development of Guidelines for Permitted Left-Turn Phasing Using Flashing Yellow Arrows

 

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