Seeing Yellow before Red—Importance of Yellow Light Timings & the 85th Percentile

By Gary Biller, NMA President

Editor’s Note: This post originally appeared as Newsletter #248 from October 2013. The information is still pertinent for those who continue to advocate against red-light camera programs in their communities. For additional information on Yellow Light Timings, check out the NMA’s ShortYellowLights.com website.

At the heart of the red-light camera debate is the signal timing for the yellow light. Do the cameras improve intersection safety by modifying driver behavior through punishment, or are they a financial windfall for local and state governments and the for-profit camera companies? The answer depends largely on whether the yellow light is timed properly.

Few dispute that a tiny tweaking — even by fractions of a single second — of the yellow light duration can make a significant difference in the number of photo tickets issued and revenue collected. Some consider the NMA position to increase the yellow to be almost arbitrary, as evidenced by the cover article of the October/November 2013 issue of Traffic Technology International. The co-authors of that article, “Blinded by the Truth,” noted:

“Anti-camera organizations such as the National Motorists Association lobby government to add one second to the yellow. Many organizations, such as the FHWA and NTSB, want engineers to use the 85th percentile velocity rather than the posted speed limit in the calculation. Attempts such as these add at most one second to the yellow. But this just skims the surface.”

This provides us with an excellent opportunity to explain the scientific basis of the NMA position regarding the proper setting of the yellow light interval. The TTI article covers the derivation of the yellow light timing formula, commonly referred to as the kinematic formula in the traffic engineering community, in significant detail. The NMA has focused on two key parameters in the formula that, when misapplied, are the primary cause of short yellow lights: the approach speed of vehicles to the intersection and the perception/reaction time of drivers to traffic signal changes.

The federal Manual on Uniform Traffic Control Devices (MUTCD) provides a bailout for states concerning the yellows, requiring only that the yellow duration be set in the range of 3.0 to 6.0 seconds. That level of imprecision on such a critical safety matter is astonishing, kind of like having authorities tell the public that the appropriate police response time to an emergency call falls somewhere between one and 45 minutes. Many cities, such as Chicago and New York, set most of their yellows at the bare minimum of 3.0 seconds as a matter of policy and are rewarded with tens of millions of dollars in annual traffic citation revenue from drivers who wouldn’t be ticketed if the yellows were set per the kinematic formula.

That brings us back to the key parameters that form the basis of the NMA position on yellow lights. In 1985, the Institute of Traffic Engineers (ITE) proposed that the vehicle speed variable in the kinematic formula be the 85th percentile speed of free-flowing traffic approaching the signalized intersection. Many states, such as California, currently allow the posted speed limit to be used instead. A significant disparity exists between the ITE recommendation and the use of a posted speed limit. Traffic studies consistently have shown that posted speed limits are typically 5 to 15 mph below 85th percentile speeds. A 2011 report based on extensive field data showed that posted limits on the approaches to signalized intersections are, on average, almost 7.5 mph below actual vehicle speeds.

The NMA position on the approach speed to be used in the kinematic formula is straight-forward. Use the 85th percentile speed based on traffic surveys or, because cities may balk at the cost of conducting extensive testing, plug in the posted speed limit plus 10 mph. The kinematic formula is used to calculate a minimum yellow light interval; we recommend “plus 10” because there are intersections with approach speeds that are much higher than the posted limits.

The NMA has participated in discussions with traffic safety experts at the Federal Highway Administration and the most recent National Committee on Uniform Traffic Control Devices conferences. There is an acknowledgment that reform in this area is necessary. The technical community is leaning toward requiring 85th percentile speeds or alternatively the posted limit plus 7 mph rather than plus 10. Even this would be a significant step forward for safety.

The ITE has long recommended the use of 1.0 second for the driver perception/reaction time variable in the kinematic formula. A study within the past two years has verified 1.0 second is the mean value of P/R time among drivers. But again, because the kinematic formula determines a minimum yellow light interval, the NMA argues that data from the same recent study should be used to justify 1.4 second as the P/R time in the formula, essentially one standard deviation above the mean value. As I testified earlier this year before the California Traffic Control Devices Committee:

“Because the yellow light change interval is such a critical parameter for intersection safety – studies have shown that an increase in the change interval by as little as 0.5 second will reduce red-light run­ning violations by 50 percent or more – and because the driving population dominated by the baby boomer generation is rapidly aging – estimates have 40 million boomers as senior drivers on the roads by 2030, with 5.8 million of them being over the age of 70 – state regulations must err on the high side of key variables like driver perception/reaction time. 

“The NMA urges the CTCDC to require jurisdictions to use a driver perception/reaction time of 1.40 second (the mean of 1.00 second plus one standard deviation rounded to the nearest tenth) just as the Florida Department of Transportation recently announced it would be doing later this year.”

The TTI “Blinded by the Truth” article states that the Florida DOT transferred the extra 0.4 second from the all-red clearance interval to the yellow light duration at selected red-light camera locations. That is not entirely correct. FDOT mandated adding 0.4 second to the yellow at all camera-equipped intersections within a prescribed period of time and generally explains it as increased perception/reaction time that will accommodate a growing proportion of older drivers.

All of this rewinds us to the statement in the third paragraph that the NMA simply recommends adding one second to the yellow light change interval.

Let’s take a look at an intersection with a posted speed limit of 30 mph. Assuming a flat grade, using a driver perception/reaction time of 1.0 second, and using the 30 mph posting in the kinematic formula, the minimum yellow light change interval is 3.2 seconds. Now add 0.4 second to the perception/reaction time and 7 mph to the posted limit. The yellow light change interval is increased to 4.1 seconds, resulting in a differential that rounds to one second.

When the NMA is quoted as wanting an additional second added to existing yellow lights, you can be confident that the rationale behind that recommendation is rooted in science.

Most important, that fractional increase in the yellow light change interval will make all the difference in the world. Intersections will be safer, and drivers can be assured that subsequent violations are serious matters, not just opportunities for local and state government to fill their budget gaps with ticket revenue.

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