What is the difference between “Low Priority” and “High Priority” preemption? “Low Priority” is generally used by mass transit authorities to increase the time duration of green lights or and/or decrease the time duration of cross-traffic red lights. “High Priority” takes precedence over “Low Priority”, and the purpose is to secure control of the intersection by maintaining a single direction of traffic flow by holding the traffic signal green until the emergency vehicle has passed through that intersection. “Low Priority” is generally considered to be 10 hertz (10 flashes-per-second) while “High Priority” is 14 hertz. The MIRT is only available in “HIGH PRIORITY” 14 hertz.