Years ago, a friend of mine in Newburyport, Massachusetts, got a ticket for running a red light. But the yellow light at that intersection seemed unreasonably short. So he went back and shot video at the intersection, demonstrating that the brief duration of the yellow light was a safety hazard, causing many drivers either to mash the accelerator or go into a panic stop to avoid running a red. The judge dismissed the ticket.
For my friend, that was the end of it. But for Mats Järlström, an Oregon electrical engineer, a red-light ticket issued to his wife set off a multiyear quest to reform the whole methodology behind traffic-signal timing. It wasn't easy. Along the way, he fought a court battle against the Oregon Board of Engineers---they'd fined him $500 for practicing engineering without a license---and prompted a huge study by the Institute of Traffic Engineers. Last week, the ITE issued the results of that study. And the short version is: Järlström was right.
So what was the study about? Take it away, ITE: "The report describes the sources of methods and values presented in the recommended practice to address the goal of the engineering profession to determine the appropriate duration of the yellow change and red clearance intervals that provide for intersection safety while retaining a high level of operational efficiency." Yes, they're stingy with their periods and commas over there at the ITE.
Järlström's updated equation, dubbed the extended kinematic equation, accommodates the fact that turning traffic slows down---to an average of 20 mph for left-hand turns across an intersection and 12 mph for right-hand turns.
The formula to calculate the duration of yellow lights was created in the 1960s and is known as the kinematic equation. And the kinematic equation works fine to eliminate what's known as the dilemma zone (the area where a vehicle can neither stop safely nor make it through the intersection before the red light), as long as the vehicle speed stays constant. As in, if you're doing 45 mph when the light turns yellow, and you keep doing 45 mph, everything's fine. But what if you slow down, as if to turn? Then the equation falls apart and you might be hanging out in the middle of the intersection when the light turns red. Järlström's updated equation, dubbed the extended kinematic equation, accommodates the fact that turning traffic slows down---to an average of 20 mph for left-hand turns across an intersection and 12 mph for right-hand turns.
Applying Järlström's math generates conclusions that are heartening for anyone who has ever encountered a yellow light that felt like a no-win situation. For instance, if traffic engineers want to set a yellow-light interval that accounts for drivers slowing down rather than hitting the NOS bottle, that means extending yellow-light durations for up to seven seconds. Not all yellows would last seven seconds. But even the ones that did shouldn't result in a "stale yellow," which is when a driver is stopped and staring up at a light that hasn't yet changed.
Ultimately, the report acknowledges that there are so many variables at play---intersection width, driver age, vehicle length and condition, weather, reaction times, grade---that there's no perfect solution. And therefore, the ultimate judge of whether you ran a red might just be . . . a judge. According to the ITE, "the range of variables . . . makes zero-tolerance enforcement inappropriate." So, good news and bad news there, we suppose, 108 years out from the invention of the yellow traffic signal. Now when can we get those yellow lights that come on ahead of the green?
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