“At Yellow Lights, Timing is Everything,” by Jo Craven McGinty. The Wall Street Journal, 10/31/2015.
Busy Concord Pike is only a block away from my home, so I spend a lot of time both on it and waiting at a traffic light to get on it. When I am waiting, I can tell when the light is about to change by the cars on the Pike abruptly speeding up to get through the yellow before it changes. When on it, I sometimes cringe as I continue on through a yellow light, but see in my rear view mirror five more cars well behind me have also gone through what must be an obvious red light. (Experienced drivers know they still have a few seconds of grace before the cross traffic light turns green. Even the best-intentioned plans fail as drivers learn what they are. A green light to many drivers has become green plus yellow plus a second or two of red.)
A recent Federal study shows the importance of the length of the yellow light. If it is too short, drivers are likely to run the red light by mistake. Too long and they will intentionally risk running it. (Many commuting drivers pass the same lights twice a day and soon learn better than the engineers just what they can risk.)
The current Federal guideline simply says the yellow light must be between 3 and 6 seconds long. But now with increasing penalties from automatic red-light cameras, more precise standards are needed. A new study shows drivers need 1 second to react plus enough time to decelerate at a comfortable rate of 10 feet/second/second (6.8 mph less each second). As we can all appreciate, the speed used in the calculations has to be the actual speed of the approaching vehicles, not the posted speed. Since localities often do not have this data, they can approximate by assuming an actual average speed of 7 mph over the posted limit (a scary concept in itself). The road grade also has to be considered since it obviously affects the rate of deceleration. All of this has been quantified in a formula. “If the formula suggests you need 4.5 seconds, and if you are providing drivers with 3 seconds, you’re putting drivers into a case where they either have to stop abruptly or proceed through and run the risk of running the red light,” observed one official.
This is one government study that actually makes sense. How quickly we have to stop at a red light should be uniform everywhere. The old comic-book image of a fat cop on a motorcycle hiding behind a billboard has been replaced by a brilliant flash of light and a camera, and we all need to know what is expected. The rule of our high school driver’s ed. teacher, Mr. Brown, that a yellow light means to clear the intersection is too general today. (I often see older drivers making a left turn on a highway stop in the middle of the intersection waiting for the yellow signal to clear it. Younger drivers do not do this.)