Each municipality has different rules, but vehicles that can commonly change traffic lights include:
- Fire Trucks
- Police Vehicles
- City Buses
The reality is that a request for traffic signal preemption can only be triggered with a special coded message, which is delivered to the traffic signal’s controller via radio or infrared light; or in some cases, over a cellular connection to the cloud and then back to the intersection. Only specific vehicles – such as emergency and public transit vehicles – can make requests for green lights.
First responders aren’t just given green lights at will. They can only request that the traffic controller gives them a green light; but those requests aren’t always granted, depending on conditions at that point in time. For example, if a pedestrian is in the crosswalk, the green light won’t be granted until the crosswalk is cleared. Preemption is generally triggered when the lights and sirens of police cars are activated, but there are many ways in which the Opticom system can be configured. Some towns are moving toward incident-based preemption, where the preemption system may or may not be automatically activated, depending on the time-sensitivity of the situation.
Like the rest of the Opticom priority control system, there are numerous configuration options. But generally, buses either operate according to a schedule or based on the ideal time between buses at the stops on a route. If the agency is trying to get its buses through its routes as quickly as possible, it may opt to have the transit signal priority (TSP) system activated throughout the route. If the agency is trying to adhere to a predefined bus schedule, it may opt to have the Opticom system determine whether it is on time or ahead of schedule, and then turn the system on and off to maintain schedule adherence. Regardless, when TSP is activated, it is either requesting an extension of the green light (by up to six seconds) or requesting a truncation of the red light in the opposing direction. Like emergency vehicle preemption (EVP), TSP isn’t always granted. Cities can put rules in place, for instance, where and when TSP can be used.
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