Traffic Signal FAQs

The police officer said there is no way to tell who ran the red light. Can you help? Most often — yes! It is often possible to, based on driver and witness statements and a working knowledge of traffic signals and specific knowledge about the particular signals involved in the crash, to reconstruct the sequence of events leading up to the crash — including who ran a red light. The majority of police officers and other crash investigators do not have training in how traffic signals work. That is why Daren teaches classes to officers and investigators across the U.S.

My client had a green light, and several independent witnesses support her claim of having a green light. The defendant driver has admitted to running a red light, but says it changed right before the intersection, and he could not stop in time. Is this true?In the vast majority of cases where one driver did in fact have a green light, the opposing driver received a yellow and red light at a significant distance before they got to the intersection.

It is extremely rare that a driver received a red light right before entering the intersection and is then involved in a crash with a green light driver. Preparing exhibits for testimony to illustrate this is a powerful way to prove the defendant driver’s negligence.

Aren’t all traffic signals the same? No. Each traffic signal is typically unique with respect to lanes controlled, types of left turn signals, and phasing.

Are the traffic signals in Florida different from the ones in Wyoming? No, not really. Granted there may be some minor jurisdictional differences between different agencies, but traffic signals in the U.S. are designed, constructed, operated, and maintained in accordance with the Manual on Uniform Traffic Control Devices. The MUTCD is a federal publication, thus there is uniformity to traffic signal phasing and timing regardless of where the intersection may be.

What is a preemption? When the normal operation of a traffic signal is interrupted and special phasing operates, for example when a train crosses one or more legs of the intersection, we call this a preemption.

Can two opposing lanes get a green light at the same time? Sure! And the traffic signal will jump into flashing mode very, very fast. So fast that drivers will not likely see or perceive the conflicting green indications.

The green time varies depending on the time of day. Does the red time vary throughout the day? No. Red and yellow clearance times are usually based on intersection dimensions, speed, and grade, and do not change. Slowing to a stop from 45 mph takes the same time regardless of how long the green time was.

Is it a requirement that licensed professional engineers design traffic signals? In most jurisdictions it is. There are typically design plans sealed by the engineer of record.

How does the traffic signal know that a car is waiting? Most traffic signals respond to traffic when vehicles pass over wires buried in the pavement. Sometimes microwave or video detection may be used in place of wires.

What is a signal system? When two or more traffic signals are within a half mile or so of each other it is sometimes advantageous to coordinate the flow of traffic through the two signals. Drivers do not like to get a green light at the first signal, and then have to stop on a red light at the adjacent signal. If the two (or more) traffic signals are interconnected with wire, fiber optic cable, or radios, they can communicate with each other, and can remain in coordination with each other to provide green signals that complement each other.

Contact dmarceau@trafficsignalexpert.com for more information.