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The Yellow Light Dilemma

Drivers face split-second decisions at intersections - an accident might not be as simple as it first appears.


August 1, 2015   by Joseph Jakym, Partner, Roar Engineering


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As children, most of us have played the game “red light, green light”. The rules were obvious, when red light was called you stopped, and when green light was called you proceeded forward. However, sometimes during this game a child would announce “yellow light”, and the actions that would follow would often be split amongst the participants, with them either stopping, slowing, or proceeding forward.

Even as adults, while driving our vehicles, it can be unclear whether or not we are supposed to stop or proceed forward when we are unexpectedly faced with a yellow light. When the choice is not clear, this concept is known as being in the “dilemma zone.”

To better explain what the dilemma zone is, it is easiest to start with what it isn’t. If you are far away from an intersection (i.e. 10 seconds away) when its traffic signals change from green to yellow, the decision to stop your vehicle is obvious and there is no dilemma. Further, if your vehicle is positioned nearly within the intersection when the light changes from green to yellow, the decision to proceed into the intersection is also obvious since it is physically impossible to stop your vehicle before encroaching into the intersection.

However, there is a region between these two areas in which the decision to stop or go is less clear; this region is the dilemma zone. For instance, if you find yourself in the dilemma zone, you may find the vehicle next to you proceed into the intersection, while you chose to stop your vehicle in response to the amber traffic signal.

The dilemma zone is a concept that has been mathematically modeled in research. This allows accident reconstruction experts to determine the probability of whether drivers in a particular situation would be expected to stop or proceed through an intersection. For example, an accident reconstruction expert can use this research to come to the opinion that based upon the circumstances that were facing a particular driver, we would expect a certain percentage of drivers in their position to continue through the intersection.

Some of the factors that might be taken into consideration when determining the probability of entering the intersection include the following:

• The distance from the intersection when the light changes from green to yellow

• The speed of the vehicle

• The type of vehicle

Drivers of light passenger cars are more likely to stop compared to drivers of heavy vehicles (i.e. transport trucks)

• Whether there are adjacent vehicles that are also travelling through the intersection

If the vehicle beside you enters the intersection on a late amber, you are more likely to go through than if your vehicle was alone.

The concept of the dilemma zone can be very useful in circumstances where the colour/state of a traffic signal is a matter of importance. Imagine the scenario where your client/insured drives a heavy vehicle, such as a tractor-trailer. The tractor-trailer enters an intersection, while a passenger car, initially travelling the opposite direction as the tractor-trailer, turns left into the tractor-trailer’s path. The two vehicles collide in the intersection. Several witnesses to the accident indicated that the tractor-trailer entered the intersection right as the traffic signal turned red. The initial outlook regarding liability on this matter may be bleak. However, the concept of the dilemma zone can be used to determine how reasonable it is that your driver entered on a fresh red light.

It might be determined in this scenario from the mathematical models that 60% of drivers in the tractor-trailer’s position would have elected to continue through the intersection, based on the tractor-trailer’s likely position when the light changed from green to amber. If that was the case, most drivers in his/her position would have entered the intersection against a red traffic signal. This type of information can be useful when determining the percentage of liability in a civil case.

This approach can also be useful if your client/insured is driving the left-turning vehicle. For example, imagine the scenario where your client/insured is waiting within an intersection to turn left. The traffic signal changes to red, and then proceeds to make their left turn. Video surveillance showed that about 3-seconds into their left turn, they are struck by a van which entered against a red traffic light.

Based on a reconstruction of the speeds of the vehicles, the approximate position of the van when the traffic signal changed from green to yellow can be determined. Based on the van’s initial positioning, it might be determined through the dilemma zone models that 100 % of drivers in the van driver’s position would be expected to stop in response to the yellow signal before entering the intersection. This conclusion may initially seem to be obvious; however, these mathematical models provide a tool which can be used in litigation to support these common sense answers.

In the end, there may be more to the story when a vehicle enters an intersection on a fresh red light or a stale amber. Accident reconstruction experts are often asked to determine whether a driver’s actions or inaction was reasonable. This is determined by comparing how well a particular driver performed against research of drivers facing similar situations. In some cases, it might be reasonable to expect a driver to have entered an intersection on a stale amber or a fresh red light, whereas in other cases it may not be reasonable.

Therefore, the next time you are reviewing a case where there may be an issue of a vehicle entering an intersection against a red traffic signal, there may be more to the story than you had originally thought. Consequently, the liability of a particular case may not be as black and white as it first appears… or as green and red.

Joseph Jakym, P.Eng., is partner at Roar Engineering, responsible for managing the accident reconstruction department. He has over 8 years of experience in the forensic engineering/accident reconstruction field.


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