Let's say you've got 50 miles of freeway where it is completely full of cars driving 80mph two feet apart. A dog runs into the road. Locally, the traffic jam is

Let's say you've got 50 miles of freeway where it is completely full of cars driving 80mph two feet apart. A dog runs into the road. Locally, the traffic jam is inevitable and bad. But they transmit this condition via radio such that cars downstream can start braking early. What happens when one car doesn't get the message on time? The car in front of it brakes, and this car brakes much harder in reaction to visual cues in lieu of the radio cues, forcing the car behind it to react to visual cues and so on, causing another shock wave. Don't even get me started on the idea that we can rely on wireless communication in order to make reaction times fast enough that we can drive closer than we can adequately stop. Even locally that level of reliability is decades beyond current capabilities...non visible distances are a non-starter.