ISO 22951:2009 relates to systems that use priority signal control functions to help emergency vehicles operate. This type of system is composed of a traffic management centre, in‑vehicle units, roadside communication units, and roadside units. Public transport vehicles such as buses are also targeted to receive priority signal control service.
The scope of standardization includes message sets and data dictionary related to the communications as follows:
- between a roadside communication unit and each in-vehicle unit,
- between a roadside communication unit and other roadside units,
- between in-vehicle units and roadside units.
ISO 22951:2009 concerns only information related to priority signal control and does not deal with information provision such as that of the situations at scenes. Since it is necessary to handle public transport vehicles in accordance with the conditions of individual cities and regions, the section in the messages and the data dictionary that are concerned with priority signal control for the vehicles are treated as an option. Furthermore, the standardization does not depend on the type of communication medium used.