I have been studying this issue for what seems like half my career, both as the Principal Investigator for the Federal Highway Administration's Signal Timing Manual and as a practitioner who has worked all across the country from Seattle, WA to Lee County, FL and now is responsible for a City traffic signal system. Safety is the first goal with signal timing. As a consultant, I didn't always appreciate that but now as someone responsibly in charge of traffic signal timing, that is clear to me.
The operative question is this: Do higher speeds and longer clearance intervals improve safety of the transportation system?
In a complex urban environment with multimodal travel, the answer appears to be No. Safety is drastically influenced by driver behavior and the built environment and controlling speeds is difficult. This most recent article that came to my attention highlights some of the challenges that I experienced in Florida working in the many high speed streets of southwest Florida and while this article focuses on the safety of bicycling, it is a reminder of the challenges given an 85th percentile speed limit setting and what that can do when serving multimodal transportation.On the flip side, downtown Portland (I am sure many of you have similar experiences) is an example where all modes converge and generally speaking on average the safety record is better than average. Our statutory speed limit is 25 MPH, but the built environment manages speeds even lower than that for the most part. In fact, most dense urban areas are safer because of this fact.
But back to the topic at hand (clearance intervals). The research I point to most about this topic (and cited in the Signal Timing Manual) was completed for the Minnesota DOT. The document entitled: "Effectiveness of All-Red Clearance Interval on Intersection Crashes" focuses on All Red Intervals and the long term effects of changing them. I encourage folks to read it as they consider the NCHRP 731 research. The research exposes the harmful effect of increasing all red times over the long term. Much of the research focuses on short term effects (increasing the yellow and studying it a day or two later shows positive safety benefits), which doesn't account for changing driver behavior. I believe Florida DOT experienced similar changes in driver behavior given the long clearance intervals being used on their high speed, wide arterials cited in the aforementioned News Press article about safety in Florida communities.
We should also consider what the changing technology landscape presents in terms of opportunities for increasing safety. A Connected Vehicle/Traffic Signal world presents some amazing opportunities where an increased clearance interval will unnecessarily reduce the efficiency of an intersection. These are exciting times.