Early data shows longer yellow lights reduce red light running in Phoenix
Preliminary data from a Phoenix traffic study shows a reduction in red light runners at certain intersections where yellow lights were extended by roughly half.
Using crash data and working with University of Arizona researchers, Phoenix added sensors to 12 intersections to monitor when cars enter on yellow and red lights.
The baseline data, covering a 12-week period, showed 128,164 cars entered the intersections during a period known as “all red” and 16,631 cars entered after “all red.” The term “all red” is used for the interval meant to provide adequate time for a driver who enters the intersection during a yellow light to clear the intersection before the green light begins in the opposing direction.
Early results for three intersections over a four-week period show red light running decreased when yellow lights were extended.
“We do show that it is promising,” said Briiana Velez, assistant director for Phoenix’s street transportation department. “Obviously, we want to make sure we look at — because what tends to happen is drivers’ behaviors — they notice differences, right, and then it changes their behavior again.”
"We do show that it is promising."
— Briiana Velez, assistant director street transportation
She told a council subcommittee it is critical to evaluate data from all 12 intersections for the entire 12-week period, which runs through early February.
Beyond safety, Kini Knudson, street transportation director, said longer yellow lights have other consequences, “Either you’re going to have to increase the signal length of the overall signal at that intersection or you’re going to have to reduce green light and both those have impacts on our traveling public. An emphasis on safety is important, but we also, you know, obviously got to understand the impact of what this may do across 1,200 signals across our intersections.”
Final results, analysis and recommendations will be presented to the full City Council.
Nationwide, the topic of extending yellow lights has been controversial among engineers since 2020 when the Institute of Transportation Engineers released updated guidelines on yellow changes and red clearance intervals.
“I’m agnostic at best when it comes to what the results of the studies are,” Knudson said, “If it shows that we think that this has a safety impact to make our streets safer, it makes sense for us to consider and to do this. But we want to be able to be judicious about that and to use that engineering judgment to see whether this is going to have that impact we expect.”
Although other cities have followed the 2020 ITE guidelines, Knudson said he doesn’t think any city conducted a study beforehand as Phoenix is doing.
According to a council report, the Insurance Institute for Highway Safety found red light running violations led to 928 deaths and 116,000 injuries across the United States in 2020. Between 2014 and 2020, red light running violations in metro Phoenix injured 9,320 people and killed 113.
The preliminary results below show the daily change in red light running incidents between the baseline (before yellow lights were extended) and during the first four-week period when yellow lights were extended: