The devices used to change traffic signals are called Mobile Infrared Transmitters (MIRT), they are used by city buses and emergency vehicles to control traffic signals at intersections to allow them to pass quickly and safely. A MIRT consists of a infrared LED light array connected to a timer, the infrared LED's are mounted on the emergency light bar. When an emergency vehicle activates its overhead emergency lights the MIRT is also activated, the timer causes the LED's to strobe at a specific frequency, typically 10Hz for a low priority change like a city bus or an emergency vehicle responding at a code 2 (lights but no siren) and 14Hz for a high priority change, which would be an emergency vehicle responding at a code 3 (lights and siren). A low priority frequency will cause a normal light change, the light will go from green to yellow then red in the opposing lane before changing to green for the transmitting vehicle; a high priority frequency will immediately change the lights in both directions, the light in the opposing direction from the emergency vehicle will go straight from green to red. Some cities use specially encoded transmitters on their own unique frequency to prevent the use of homemade transmitters. In 2005 the Safe, Accountable, Flexible, Efficient Transportation Equity Act included a provision that established a minimum sentence of six months in jail for anyone illegally using a MIRT, and a minimum sentence of one year in prison for anyone selling a MIRT illegally.
