SPaT

Signal Phase and Timing (SPaT) is a standardized data message used in intelligent transportation systems to communicate the current state of traffic signals at an intersection and the timing of upcoming changes. SPaT messages are part of the SAE J2735 vehicle-to-infrastructure (V2I) message set and are commonly transmitted from roadside equipment to vehicles using communications technologies such as DSRC or C-V2X.

A SPaT message describes one or more signal faces or approaches, indicating the current signal phase (for example, green, yellow, red) and the time remaining before the phase changes or the cycle restarts. In a typical multi-approach intersection, SPaT provides synchronized information for all approaches to support coordinated signaling. SPaT is typically used together with a MAP message, which conveys the geometric layout and lane configurations of the intersection.

SPaT data enables a range of applications, including speed advisory and green wave optimization, red light violation warnings, and other safety and efficiency tools. It supports both driver assistance systems and automated driving functions, as well as advisory signaling in smart city deployments. SPaT information is designed to respect privacy, since it conveys signal timing rather than individual vehicle data.

Deployment and standards for SPaT vary by region. The specification is part of ongoing standards development for V2I communications, and practical implementations depend on compatible traffic controllers, roadside units, and user devices. Limitations include the need for robust communications coverage and alignment with local signal timing plans, which may change due to traffic conditions or construction.