ITS stands for "Intelligent Transportation Systems". Another common acronym for this marketplace is IVHS - Intelligent Vehicular Highway Systems. This application considers adding "intelligence" to a vehicle to improve safety or efficiency of operation.
Radio applications that form a part of ITS include:
Remote toll paying through the use of RF/ID tags.
Navigation using Position Location (including GPS) and/or dead reckoning.
Radar to avoid collisions (including backwards looking radar for trucks and busses as well as forwards looking mmwave radars for passenger vehicles)