Yes, police can control traffic lights, and the law gives them several ways to do it. Officers can override a signal by standing in the intersection and directing traffic by hand, by triggering an emergency vehicle preemption system from their patrol car, by switching the signal’s control cabinet into manual mode, or by coordinating with a city traffic management center to retime signals along a corridor. Whichever method they use, an officer’s direction legally outranks whatever the light is showing.
Where the Authority Comes From
Every state’s vehicle code is built on the Uniform Vehicle Code, which requires drivers to obey traffic control devices “unless otherwise directed by a police officer.” That single clause is what makes everything else possible. Once an officer is actively directing traffic, the signal on the pole stops governing the intersection and the officer does.
The authority isn’t limited to hand signals. It also covers activating preemption equipment, opening the signal cabinet to change phases manually, and asking a traffic management center to adjust timing remotely. The principle behind all of it is the same: officers can step in whenever normal signal operations aren’t keeping people safe.
Emergency Vehicle Preemption
The most common technology-based method is emergency vehicle preemption, or EVP. The Federal Highway Administration defines preemption as “the transfer of normal operation of a traffic control signal to a special control mode of operation.”1Federal Highway Administration. Traffic Signal Timing Manual: Chapter 9 An emitter on the police car, fire engine, or ambulance talks to a receiver at the intersection and asks the signal to clear a path.
Older systems, including Opticom and Strobecom, use coded pulses of infrared or visible light sent from the vehicle to a detector mounted near the signal head. They work well but need line of sight and can be affected by bright sunlight or sharp curves. Newer systems use GPS and cellular data instead. The vehicle broadcasts its location, speed, and heading, and the signal’s controller calculates when to begin preemption. GPS systems work around corners and in bad weather, and traffic engineers can define specific approach corridors so a cruiser on a parallel street doesn’t accidentally trigger the wrong intersection.
Preemption does not snap the light to green. Federal guidelines require that yellow and all-red clearance intervals cannot be shortened or skipped during the transition, which protects drivers already moving through the intersection.1Federal Highway Administration. Traffic Signal Timing Manual: Chapter 9 Pedestrian walk signals can be cut short, though, so a walk indication is not a guarantee of a safe crossing when emergency vehicles are approaching. The green for the emergency vehicle can also lag the request, since the signal has to finish its safety sequence first.
Manual Control at the Cabinet and From a Traffic Center
Not every situation calls for vehicle-mounted equipment. Most signalized intersections have a controller cabinet, usually a metal box on a nearby pole or pedestal. Officers with the right access can switch the signal into manual mode, cycle phases on demand, or put the intersection into flashing red. This is common at crash scenes, where the normal timing would feed traffic straight into the hazard.
Many cities also run traffic management centers where staff monitor intersections through cameras and networked controllers. During major incidents, officers can coordinate with these centers to retime an entire corridor. If a freeway shutdown pushes thousands of cars onto surface streets, the center can extend green times along the diversion route while officers handle the worst intersections in person.
When an Officer Stands in the Intersection
The most visible form of police traffic control is an officer physically in the intersection using hand signals. It tends to happen in a few recurring situations:
- Crash scenes, where officers route traffic around wreckage, emergency vehicles, and the investigation area to prevent secondary collisions.
- Signal failures, where a dark or malfunctioning light legally becomes an all-way stop and can gridlock a busy intersection within minutes.
- Special events such as parades, marathons, concerts, and sporting events, which create traffic patterns no signal was programmed for.
- Emergency vehicle assistance, where officers post up at key intersections along a known route to hold cross traffic manually.
- Severe congestion during rush hour or construction, where an officer can give extra green time to the direction carrying heavier demand.
What You Have to Do as a Driver
When an officer is directing traffic, you follow the officer. If the light is red and the officer waves you through, you go. If the light is green and the officer holds up a hand, you stop. Officer direction overrides signals, signs, and pavement markings, and this rule appears in every state’s vehicle code, modeled on the Uniform Vehicle Code’s requirement that drivers not refuse to comply with any lawful order of an officer authorized to direct traffic.
Disobeying an officer directing traffic is a misdemeanor in most states. Fines typically run from around $100 to $250 depending on the jurisdiction, and a conviction often adds points to your driving record. The larger risk is the safety one. An officer is out there because the signal is not adequate for what’s actually happening in the intersection, and following the light instead can put you into the path of emergency vehicles, oncoming cars, or pedestrians the officer is trying to protect.
Civilians Cannot Legally Use Preemption Devices
Because a preemption transmitter can override a live traffic signal, federal law tightly limits who can have one. Under 18 U.S.C. ยง 39, selling a traffic signal preemption transmitter to an unauthorized person is a federal crime punishable by up to one year in prison, and using one without authorization carries up to six months.2Office of the Law Revision Counsel. 18 U.S. Code 39 – Traffic Signal Preemption Transmitters The statute defines the device broadly as anything that can change or alter a traffic signal’s phase time or sequence.
Authorized users are people acting on behalf of agencies or contractors that provide fire protection, law enforcement, emergency medical services, transit, or government maintenance. Everyone else is a “nonqualifying user,” and buying a transmitter online to mount in a personal vehicle is exactly the conduct the statute targets. Many states add their own penalties on top of the federal charge.