Slow roads are easier because you can rely on a simple emergency breaking system for safety. You have a radar that looks directly in front of the car and slams on the breaks if you’re about to crash. This prevents almost all accidents below 35mph.

The emergency breaking system gives you a lot of room for error in the rest of the system.

Once you’re going faster than 35mph this approach no longer works. You have lots of objects on the pavement that are false positives for the emergency breaking system so you have to turn it off.