An attorney once asked me if the process of reconstructing an accident at an intersection with a traffic signal was any different than reconstructing one at an intersection controlled by a stop sign. My answer was immediate — yes! While it is true that many of the typical accident reconstruction concepts and terms are the same for both signalized intersections and unsignalized intersections, there are just as many that are unique to signalized intersections. Traffic signals are complex electrical devices that provide visual indications to drivers, thereby indicating who has to yield the right-of-way at an intersection. The decision-making processes that are needed at stop sign controlled intersections in order to proceed — such as paying attention to who arrived first, vehicles approaching on the other streets, or who is turning in which direction — are removed from the driver’s list of duties at a signalized intersection. It would seem that a traffic signal simplifies the decision-making task. Yet while the list of decisions may seem to be shorter on the surface, I would argue that the list actually becomes longer. Some of the decisions have even become more complex. For example, at stop signs the driver must stop. Period. At traffic signals, a driver can receive a yellow signal, forcing the driver to make a judgment call whether to stop or continue through the intersection. It is these judgment calls that often lead to collisions. And it is these collisions that need to be reconstructed to understand the actions at the scene. This text is not intended to be an exhaustive book on the precise details that go into traffic engineering. It is intended to serve as a manual for reconstructing traffic accidents that occur at intersections controlled by traffic signals. Understanding how signals operate and the framework that likely was considered in their design is crucial for reconstructing collisions that occur at signalized intersections.
