By Michael Peck, P.Eng

A very common request as part of an investigation into an intersection collision is to determine who went through a red light. Unless there is video footage of the incident from a nearby security camera or dash camera showing the actual status of the traffic lights, there often is no remaining physical evidence to answer this request. There may be eye-witnesses who can provide some good information about what they observed, but what if these statements are not available or contain inconsistent, unreliable or conflicting information?

A report on how the traffic lights operate at an intersection can often be requested from the local city transportation department or municipality. These reports can come in a variety of forms, but generally describe how long the light phases are for the various traffic lanes at the intersection. How the lights at an intersection operate and the duration of many of their light phases can be very complicated. For example, the signal program often changes throughout the day to accommodate rush-hour traffic volumes, or the intersection may be equipped with traffic sensors either built into the roadway or mounted in the form of detection cameras above the intersection. These and other factors can all influence how the light phases behave and should be considered, depending on the particular nature of the collision being investigated.

One of the more common and useful pieces of traffic signal information that is used in a reconstruction of a collision is the clearance interval, which is mostly made up of an amber phase and sometimes an all-red phase. A driver who is waiting in an intersection to turn left will need this clearance interval to ensure opposing traffic is stopping and that it is safe to complete their turn. What happens when this left-turning vehicle collides with an oncoming vehicle? The answer can be revealed when the timing of events based on a reconstruction of the vehicle movements is paired with the timing of the traffic lights. Nowadays, many vehicles are equipped with event data recorders (EDRs) that can record vehicle parameters such as speed, accelerator and brake application by the driver in the seconds before a collision. This information can paint a relatively accurate picture of how those vehicles moved and what those drivers were doing just prior to the collision.

As a simple example, let’s assume that a left-turning driver stated they observed their light change from green to amber and they checked for oncoming traffic before proceeding from a stop. Data from the left-turning vehicle’s EDR showed how long the vehicle accelerated before impact, meaning that the time from the start of the amber phase to impact could be reasonably estimated based on the driver’s reaction to the changing light and the data from the vehicle’s EDR. A reconstruction of the event could also determine how long it would take the oncoming vehicle to reach the impact area from where it entered the intersection (stop line). The differences between these time estimates can then be compared to the amber phase of the traffic signal timing report to tell if the oncoming vehicle entered the intersection while the light was still amber or if the light was red. 

Ultimately, the example above relies on witness evidence to be able to know where the traffic signal timing information fits in with the sequence of events based on the reconstruction; however, if the accuracy or credibility of the witness information is not in question, the traffic signal information is an important piece of the puzzle in determining if one or more vehicles at an intersection collision violated a red traffic light.  

If you have any questions or would like to learn more about this topic, please contact our Collision Reconstruction team at 877 686-0240 or

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