[eximius]

Spoofing, Jamming, and Sybil attacks are what concern me, more than failing parts. Really, you'd need some sort of special hardware that makes the protocol hard to spoof with off the shelf parts. Maybe a very high frequency band (since you're only communicating in LOS at extremely short distances) so that you can't just use any old SDR to hop on. Then use some kind of gossip protocol to achieve consensus and identify bad peers.

[AWildC182]

Just because there aren't $200 SDR transceivers available that can handle 20+GHz right now doesn't mean there won't be in the next few years. These protocols have to be designed with more foresight than simple availability barriers because the second you mass produce something, it and it's constituent parts are suddenly available.

That said, if you want to lose sleep at night, I suggest you look into the strict authentication and jamming resistance of ADS-B (effectively the aircraft equivalent of such a network).

[toss1]

I would not specify that the communication can be considered fully Line Of Sight.

There are many intersections where the car would need to communicate with the crossing car and the intersecting road is behind a hill, trees, or building.

Yes, attacks are an enormous concern, but comms and hardware failure are also very significant & non-trivial to solve.

[eximius]

LOS was a poor choice of words. But on roads or highways within 500m is even smaller than LOS