A Mobile Infrared Transmitter (MIRT) is an electronic traffic preemption device that city buses and emergency vehicles use to control the traffic control equipment for intersections they are approaching, in order to pass through the intersection as efficiently or safely as possible.
A MIRT device consists of a timer circuit connected to an infrared LED array. The timer causes the infrared LEDs to strobe at specific frequencies, such as 10Hz for low priority (buses) or 14 Hz for high priority (emergency vehicles)[1]. Low Priority transmitters will control the intersection to perform a normal light change, while High Priority tramsitters will change an entire intersection immediately [2].
Certain cities use specially encoded infrared pulses to prevent the use of home made transmitters.
It is a federal offense in the United States for an unauthorized person to use any traffic preemption device. This is due to the Safe Intersection Act of 2005.