Sunday, February 23, 2014

Troubleshooting Traffic Signals with Arrows

A colleague sent me this Boston Globe article on a traffic signal display.

Portland has 15 or so traffic signal locations with Flashing Yellow Arrows in order to display protected permissive left turns. The definitive research on the topic was completed by my colleagues at Kittelson & Associates and is available online here. I had a small part in the project and it was a great learning experience. The experience with implementation was very good initially. Portland's first location was NE 82nd & Tillamook. This location isn't a very busy pedestrian location. 

As their implementation widened, (our neighbors in Washington County - suburbs to the west of Portland) had trouble with left turning movements yielding to the pedestrian. Research that was done on NCHRP  did not consider pedestrians explicitly. The most basic crash that was assumed was the pedestrian approaching from "behind" the left turning motorist in the crosswalk. Our local transportation reporter touched on this issue of problems with the Flashing Yellow Arrow

There was some follow up research work, so the response based on science was underway.

This work by Oregon State University in collaboration with Portland State is a nice study on the topic: and there is also a video interview of the topic

There has been some follow up work that our regional traffic engineering group has done to protect pedestrians at these locations. Depending on the equipment at the intersection, their may be the ability to delay the flashing yellow until after the WALK is complete (like a leading pedestrian interval), or more conservatively, start the Flashing Yellow Arrow only after the Flashing Don't Walk has completed to make sure you can get a pedestrian all the way across the intersection before asking someone in their car to choose a gap. 

The one remaining problem with the specific example at this intersection is the two arrows in the same signal head or face. I would recommend that the left turn is separated from the right turn arrows so the display may be a little more clear to the user. 

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