This field-to-vehicle application area covers the interface between a traffic signal controller and an emergency vehicle desiring signal preemption or a transit vehicle desiring signal priority. The granting of preemption or priority to the vehicle may be based upon passive detection of the vehicle type, or upon a request from an active device on the vehicle. Signal preemption typically implies that the controller switches the light to green in the direction of the preemption request, overriding the current timing. (However, there are other possible scenarios, such as setting all directions to flashing red.) Signal priority typically implies that the controller delays the phase transition so that a transit vehicle can get through the intersection. With signal priority the request for priority may or may not be granted, based on the overall traffic situation at the intersection.
National ITS Architecture Interface
This application area includes the interfaces between the Transit Vehicle Subsystem (TRVS) and the Emergency Vehicle Subsystem (EVS) and the traffic signal control equipment in the Roadway Subsystem (RS). The traffic signal controller in the RS receives a preemption request from the EVS or a priority request from the TRVS. The signal preemption interface for emergency vehicles is similar to the signal priority interface for transit vehicles; the difference lies in the level of priority given each type of vehicle. The vehicle requests signal preemption or prioritization using dedicated short-range communications to communicate with a controller for a particular intersection.
Applicable Standards
In general, the following standards are applicable to Signal Priority deployments.
To determine which specific standards are applicable for a deployment you will
need to determine which architecture flows will be needed for the Signal Priority piece of your deployment.
Contact your local FHWA
ITS division specialist or an
ITS Standards Program Field Support Team contact for additional help.
Standards development statuses as of January 23, 2009
