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Report Documents Benefit Of Longer Yellow in California
A compilation of data from six California cities shows red light violations go down when the yellow signal is increased.

Longer yellow in Redlands, CA
Longer yellows reduce red light running, according to a newly released analysis. Safer Streets LA compiled citation data in six California cities to evaluate the effect of extending the duration of the yellow interval at intersections with red light cameras. Jay Beeber, the group's executive director, says when drivers have more advance warning that the signal is about to change to red, they are more likely to stop.

Yellow times increases increased between 0.3 and 1.0 seconds in Fremont, Loma Linda, Oakland, Redlands, Santa Clarita and West Hollywood. At the low end, violations dropped in half with the smallest adjustment. With a full second added, citations plunged up to 93 percent.

"Transportation officials and engineers know that the proper timing of the signal change interval is essential to intersection safety," Beeber explained. "The results show that where there is a high number of red light running events, the yellow interval is likely too short for the needs of approaching traffic. Regardless of the yellow interval time that may have previously been calculated, red light running can be reduced simply by increasing the yellow interval."

In Fremont, the California Department of Transportation (CalTrans) ordered city engineers to increase the yellow time at the intersection of Mission Boulevard and Mohave Drive from 4.3 to 5.0 seconds. The number of monthly tickets issued immediately plunged and stayed down by an average of 77 percent in the two years of data available. At the rest of the city's intersections, where the signal timing remained the same, the average number of tickets issued did not change.

The story was the same in Loma Linda where Mayor Rhodes L. Rigsby, a professor of medicine, personally championed longer yellows over the course of five years. An initial change of 0.3 seconds proved so successful that Rigsby asked for more.

"On this arbitrary basis, we added another 1.0 second to each yellow light at each camera-controlled intersection," Rigsby wrote in an email to the state Senate Transportation Committee. "We made no other engineering changes and made no change in our enforcement. As the chart shows, the violations decreased by another 90 percent overnight."

After Santa Clarita increased yellow times in the left turn lanes at three intersections, the 0.5 second increases saw 58 percent reductions and 1.0 second increases saw 70 percent improvement. West Hollywood increased yellow at five intersections by 0.3 seconds and saw an overall reduction of 61 percent. Beeber showed in his analysis of Oakland intersections that one could predict the effect of longer yellows by looking at how many violations are issued to drivers who miss the red light by less than a second. That is the number of violations that disappear when the yellow time is extended by a second.

"Note that the high numbers of violations did not simply shift 1.0 second later, they were eliminated," Beeber wrote. "Overall, there was an 87 percent reduction in the number of violations within the first second of the red interval."

After seeing a loss of revenue, Oakland shortened the yellow time and the violations went right back up. The city has since dropped red light cameras.

"If the yellow signal time was increased at red light camera locations, violations would be greatly reduced resulting in a significant increase in safety as well as eliminating the needless ticketing of tens of thousands of otherwise law-abiding motorists every year," Beeber concluded.

Source: PDF File Compendium of Yellow Interval Increase Studies (Safer Streets LA, 6/3/2015)

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