What’s with Scottsdale’s left turn signals?
Question: Why do the left turn arrows on traffic lights in Scottsdale appear at the end of a green light, rather than at the beginning?
Answer: Let’s start by explaining two types of left-turn arrow traffic signals. Traditionally, the left-turn arrow appears before the green light for through traffic. This is called a leading left-turn arrow. The reader refers to a lagging left-turn arrow, which comes after the green light.
The lagging left is used not only in Scottsdale, but in cities such as Goodyear and Gilbert. Tempe uses a mix of both depending on the time of day.
“Other cities in metropolitan Phoenix use primarily leading left-turn arrows. However, most freeway interchanges throughout metropolitan Phoenix use lagging left-turn arrows,” said Scottsdale Transportation Director Paul Basha.
All signals at intersections with light rail in Phoenix, Mesa and Tempe use both varieties, depending on the presence of light rail vehicles, he added.
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Scottsdale lagging since 1989
Basha was Scottsdale’s traffic engineer in 1989, when the city implemented the lagging left turn arrows.
The reason, according to Basha, is “there are fewer collisions than with leading left-turn arrows.”
So how did Scottsdale arrive at this conclusion?
Tucson had implemented the lagging left-turn arrow five years earlier, said Basha, who is now Scottsdale’s transportation director.
“Scottsdale studied the idea in 1987 and 1988, and it appeared to result in less delay and fewer collisions,” he said.
By late 1988, Scottsdale conducted its own six-month experiment at five intersections on Thomas Road. The study confirmed Tucson’s results.
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The lagging left is about safety
Basha prepared a professional journal article in 2007 based upon a collision-comparison analysis of Scottsdale’s left-turn arrows and the leading left-turn arrows in a neighboring city.
The analysis examined eight years of collision data at 13 intersections and nine in the neighboring city, he said.
The findings?
- Intersections with lagging left-turn arrows have 23 percent fewer total collisions than intersections with leading left-turn arrows.
- Intersections with lagging left-turn arrows have 40 percent fewer left-turn collisions than intersections with leading left-turn arrows.
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