Car-to-car communications protocols have passed field testing trials The frequency range 5.875 to 5.925 GHz is being used for Cooperative Transport Systems using modified WiFi technology.
The evaluation of a large Car2x communications field test proves that the technology is now ready for mass deployment, say its promoters. The simTD (Safe Intelligent Mobility - Test Field Deutschland) test involved 123 vehicles - including cars and motorbikes - in real-world traffic. Results indicate that the number of accidents could be significantly reduced if the technology were widely deployed.
Siemens - Cooperativity in Motion -http://www.mobility.siemens.com/mobility/global/SiteCollectionDocuments/en/road-solutions/urban/cooperativity-in-motion.pdf
The Vehicular Radio Channel in the 5 GHz Band -http://publik.tuwien.ac.at/files/PubDat_188736.pdf