It is very easy to eavesdrop the communication medium, and to send false messages. There are potentially very large number of nodes. Disclosure of secret data. Jamming the radio channel. The convoy driving service, for example, relies on continuous correct communication between the cars in the convoy.
Malicious information can result in a disaster.
The third threat, disclosure of secret data, is not very important for most services mentioned above. At least, the threat doesn't affect traffic safety. Protection against masquerading. The identity of a vehicle is in many instances not relevant, but instead the relative position of a vehicle. Protection against unauthorized access.