Sunday, May 26, 2013

The Yellow Light Dilemma

At a state level, most Departments of Transportation in the United States have very similar mission statements that list goals such as safety, efficiency, and economically viable as their priorities for their state transportation agency.  However, different states often choose to prioritize certain goals over others. Recently, the state of Florida has been getting much grief for its decision to slightly shorten the yellow light time of traffic signals as a revenue generating mechanism. The response has been quite negative from the transportation planning community, stating that such a move prioritizes revenue over safety since the increase in revenue would come from more people running red lights. Florida Today reports that red light cameras generated more than $100 million in revenue in 70 Florida counties in 2012, and estimates over $120 million in revenue in 2013 with the yellow light changes. Studies have shown that even an adjustment of a yellow light by portions of a second can cause a difference in the number of red lights ran at an intersection and therefore the number of crashes (Retting, 1999). This leads us to believe that U.S. drivers have become accustomed to a certain length of yellow light as it relates to the speed limit they are driving at.

Through this situation that has left many planners appalled, I see a great opportunity. Florida’s Department of Transportation may be on to something, but the problem is they are only halfway there. Although the shortening of the yellow light has been traditionally viewed as reducing safety, I believe there is an opportunity here to do just the opposite while still raising revenue. I would argue that by shortening the yellow light signal in a state and publicizing the shorter lighting scheme, you can create a more cautious driving environment and create a custom for slightly lower speeds at intersections by having drivers be more prepared to stop as they anticipate the slightly shorter yellow lights. By adjusting ITE’s recommended yellow light times by speed to be slightly less than what they currently are, and having appropriate signage telling people to slow down as they approach an intersection, many unexpected interactions at intersections may be avoided with the more cautious drivers. However, this cannot occur without taking care of the major issue with Florida’s current red light scheme: the lack of grace period after the light turns from yellow to red. Most intersections have a very little delay if any between a red light and a green light for a different direction. By taking the time reduced from the yellow light in one direction, and adding it to a grace period after the light goes red but before the next green, there is no added danger of intersection crashes due to the same delay time between yellow lights and green lights as was present before.

The largest safety concern this approach may raise is a potential increase of rear-ending crashes by more sudden breaking caused by shorter yellow lights. However, if traffic calming mechanisms and signage are properly added to encourage drivers to slow at signals knowing they have slightly slower yellow light times, this can be avoided. One signage mechanism that could be used here includes the indication of a “dilemma zone”. In a 1960 study by Gazis et al, the authors introduce a concept of the “dilemma zone”, which is, the area on the road during which a driver going at the speed limit is not sure if they have enough time to break or to make it through the light. By adding an indicator line at intersections (much like the line present at where cars should stop at an intersection) that signals to drivers that they should just go through the light if they pass that line, or stop if they haven’t passed it yet when they see the yellow light, some rear end crashes may be avoided.

Using some of these techniques would allow for those who do not follow the signs and run a red light to continue to raise revenue for the state, encourage drivers to drive slower at the signal next time, all without causing greater safety concerns given the post-red light grace period before the next green. Although these suggestions may take great education efforts for people to understand the changes, they may provide safer intersections in the long term with better guidance for judgment calls by drivers on what to do at a yellow light.

Academic Sources:

Gazis, D., R. Herman, and A. Maradudin. "The Problem of the Amber Signal Light in Traffic Flow." Operations Research 8.1 (1960): 112-32.

Richard A. Retting, Robert G. Ulmer, Allan F. Williams, Prevalence and characteristics of red light running crashes in the United States, Accident Analysis & Prevention, Volume 31, Issue 6, November 1999: 687-694.

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