In a previous post, we provided undeniable proof that longer yellow signal times reduce red light running violations and that drivers do not adjust to the increased signal times. We highlighted an intersection in Fremont, California where CalTrans officials increased the yellow signal time for the straight through movement by 0.7 seconds above the minimum time required by state law to a full 5.0 seconds. The intersection experienced a significant reduction in violations and the lower citation rate did not return to earlier levels.
As can be seen in the updated chart below, the positive safety results achieved by lengthening the yellow signal time have now remained in place for more than 22 months. Overall, there has been an average 71% decrease in citations during the study period and we have yet to observe any rebound to previous violation levels.
In addition, we charted the violation rates at all other red light camera locations in the city and found no increase in violations at these locations either.
The absence of an increase in red light running violations both at intersections with longer yellow times and at intersections without increases, clearly indicates that motorists do not adjust their driving behavior to account for longer yellow times as critics of this safety countermeasure have often claimed. Furthermore, the immediate and lasting reduction in violations which occurred upon lengthening of the yellow signal time strongly suggests that a large majority of the red light running incidents that had been occurring previous to the timing adjustment were inadvertent, not willful.
We note again that the data includes both straight through and left turn violations but only the yellow time for the straight through movement was increased. Yellow time for the left turn movement remains at an insufficient state minimum of 3.0 seconds. It is almost a certainty that if the left turn yellow time was also increased, the violation rate at this intersection would be further reduced.
More importantly, if the yellow signal time was increased at other red light camera locations in Fremont (and everywhere else), violations would likewise be reduced at those intersections, 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.
Special thanks to Roger Jones for obtaining the data used in the above charts.