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The Monday Morning Commute: Ask the traffic engineer; left-turn arrows before or after?

Posted by   /  February 18, 2020  /  No Comments

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By Paul Basha, traffic engineer, Summit Land Management

How is it that some Phoenix area cities have green left-turn arrows before the straight traffic green, and others have the green left-turn arrows after the green circle? Paul Basha, with Summit Land Management, was the Scottsdale traffic engineer from 1984 to 2000. In the mid-1980’s, Paul was told by several Scottsdale residents that they much preferred the lagging arrows that the City of Tucson had implemented in 1984. The Tucson collision and delay data were convincing that lagging arrows were superior to leading arrows. After a six-month trial period and City Council authorization, lagging left-turn arrows were implemented citywide in Scottsdale in 1989.

As a consultant in the early 2000’s, Paul investigated eight years of collision data in both Scottsdale and another Valley of the Sun city. The other city had leading left-turn arrows. Paul’s extensive analysis was documented in an August 2007 article in the Institute of Transportation Engineers Journal, a professional peer-reviewed monthly trade publication. The extensive data analysis discovered that left-turn collisions in Scottsdale had a statistically significant 40% lower collision rate than left-turn collisions in the city with leading arrows. Furthermore, there was no statistically significant difference in non-left-turn collision rates between Scottsdale and the city with leading left-turn arrows. Sometimes traffic flow situations suggest that leading left-turn arrows are more appropriate than lagging left-turn arrows. However, at most intersections, lagging left-turn arrows have much lower left-turn collision rates than leading left-turn arrows.  

Curious? Call or e-mail Paul at (480) 505-3931 and pbasha@summitlandmgmt.com.

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