Cars should not depend on information from other cars, they should treat them just as suggestions, or intents, not actual events. In your example, the reported speed of the car will depend of many factors like tire size, quality of the sensor or even humidity or temperature, so it can't be trusted by other cars.

> Why do the cars not communicate their speed, so your car can adjust speed to maintain a safe distance?

How do you deal with spoofing info for/from another vehicle?

What happens when someone decides to see what happens when they `inter_car_radio.broadcast(myspeed=0)` or `inter_car_radio.broadcast(myspeed=120)` when the speed limit is 80?

The naive solution would be to make it illegal to modify the communication system of your car, but wouldn't just making it illegal to broadcast false data be enough? And then if an accident is caused because of that, you can tack on some other charge like an autonomous version of reckless driving.

A better solution would be to provision each car with its own key pair and the car maker's root CA just before it rolls off the factory floor, that way it can constantly beam 2 signals:

- a signed (regulator-mandated) signal for cars from other manufacturers;

- an encrypted signal intended for cars from the same manufacturer.

The first signal would be signed by the root CA of the car maker to indicate that its communications can be trusted.

The encrypted signal would of course carry an information-rich payload for coordinating a convoy to travel at the same speed, brake, signal a turn, etc simultaneously, for instance.

Signed messages only ensure that they weren't modified by a MITM. If the attack controls the sending vehicle, the signature doesn't help.

Tamper proof modules that store the key? Like Secure Enclave?

Surely the (already) observable relative difference is more important than what the car thinks it's speed is? Both cars might read 60mph and go at different speeds. Therefore I don't see much need for that to change.

I think more interesting would be smarter traffic control systems which communicate with cars in cities to advise traffic flows - a simple example being a (wirelessly) counting down light rather than red/amber/green.

Cars already communicate their speed through the EM spectrum, and it's very hard to spoof this. LIDAR can easily determine if it is approaching an object at even 1 cm/s.