Graham: Lagging-left makes Scottsdale transit go right

In Scottsdale, we turn left after a green light. Drivers in other cities turn left before a green light. It’s one of the ways we distinguish ourselves on this side of Scottsdale Road.

It also happens to be more efficient and safer.

Barry Graham

You may have noticed recent improvements to the signalization along Scottsdale Road and Hayden Road, as well as Indian School Road and Shea Boulevard. After a year-long collaborative effort by the city and consultant Kimley-Horn, the city has synchronized lights to reduce travel times.

Concurrently, you may have noticed the city using more “leading” left-arrows. Perhaps you wondered if “leading” or “lagging” left-arrows are better.

Scottsdale has a long history using lagging left-turn arrows. They were implemented in 1989 as part of the city’s wider and successful efforts to improve signal timing during the 1990s.

In the 2000s, after some personnel changes, overall signal timing deteriorated while the city continued to use lagging left-arrows. The completion of the 101 freeway in 2001 compounded the signal problems, and lagging left-arrows were largely blamed.

In 2009, the city attempted to fix traffic signal timing by transferring it from the Transportation Department to the Public Works Department.

It was thought that Public Works could signalize traffic lights because they install and maintain them.

Since then, the city has hired consultants to assist with signal timing that have resulted in improvements across the city. Simultaneously, many lagging left-arrows have been replaced with leading left-arrows, giving the impression that leading left-arrows are better.

The data, however, shows that lagging left-arrows are more efficient and safer.

Consider this scenario regarding efficiency: 15 southbound cars on Hayden Road are waiting at the red light to cross McDonald Drive. Three northbound cars at the same intersection are attempting to turn left (west) onto McDonald.

With a lagging left-arrow, the 15 cars proceed before the three northbound cars make a left-turn. If there were additional southbound cars behind the 15, the three cars wait to receive a lagging arrow. If not, they cross the intersection and a left-turn arrow is no longer necessary.

With a leading left-arrow, the 15 southbound cars wait while the three northbound cars receive a left-arrow that may not have been necessary.

Regarding safety, add to the above example 10 eastbound cars on McDonald Drive waiting to cross Hayden Road. With a lagging left-arrow, even if one of the three northbound cars turning left (west) on McDonald enters the intersection on a yellow left-turn arrow, there is ample space between them and the 10 eastbound cars when the light changes.

With a leading left-arrow, the three northbound cars turning left (west) are very close to the 15 southbound cars when they receive a green light, increasing the chance of collision.

The difference between the two methods may seem trivial, but these scenarios occur tens-of-thousands of time each day in Scottsdale.

Although each intersection is unique, “lagging” left-arrows have repeatedly been shown to be more efficient and safer.

Scottsdale’s reliance on lagging left-arrows has made our intersections more efficient and safer for years. I applaud the city’s continued improvements to traffic signalization, and I remain hopeful that lagging left-arrows continue to be used in Scottsdale.

Barry Graham is a Scottsdale resident and has served on the Transportation Commission since 2014.

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