I had this idea a couple years ago…
There’s a couple layers to this idea.
First, get all the vehicles to have an automatic WiFi type connection with all the other vehicles. Any signal sent into one (or more) receiver would automatically be sent out to all the other connections nearby in effect the same way the internet connects to other servers. This network could be used to boost cell phone connections, and if each server/automobile appends it’s ID, locating stolen/missing vehicles could be found almost instantly by the authorities simply by examining the connections listed near it (or maybe also have GPS location appended).
Now make the ID based on (or connectable to) the license plate number (& state). Now add a simple communications gizmo to the vehicle’s mini-server to allow drivers to send messages to other drivers within sight, but still be relatively anonymous. Give this device some pre-programmable messages (for ease of use while driving), and you can alert other drivers of things like a broken tail light, or "thanks for cutting me off!" messages. The police could notify you of an impending speeding ticket, or an ambulance could tell you to move over to the right, or a message could be broadcast to you of construction ahead and which lane(s).
The idea brings quite a few other ideas to mind.
Here’s something similar to this idea: