What is the difference between priority control and preemption control, such as for emergency vehicles?
Section 4D.27 provides standards, guidance and options for the preemption and priority control of traffic control signals. Priority control can be given to certain non-emergency vehicles such as buses and light-rail vehicles. It is typically less disruptive to normal signal operations, and most normal rules for signal sequences and timing apply. Preemption control can disrupt normal operations and institute special signal phases, and shortening or omission of certain normally-required intervals is permitted. When preemption occurs, it may take several signal cycles to resume normal operation in a coordinated signal system. Therefore, preemption control typically is given only to emergency vehicles and to vehicles that cannot be easily stopped, such as boats (at drawbridge signals) and trains (at signals at or near railroad crossings).
