From a strictly mechanical engineering perspective....
Cars travelling at 100 kph are travelling about 28 mps (meters per second.) If the average reaction time for a human driver is 150 mS then he needs to be at least maintain at least 5 meters of space just too account for how long it will take before he can actuate his brakes. And you get a chain reaction as the cars behind him need the same amount of time to react again and again. The chain of human driven cars react at a rate of 1 every 150-200 mS.
By the time autonomous cars are driving in convoys the reaction time should be well under (for example) say 1 mS. Which is about 28 cm. You still need to also allow for distance for actually braking. Cars in that chain react at the rate of 1 every 1-3 mS. So in the time that just the first human driver could react. Something on the order of 50 autonomous vehicles could react and be applying their brakes.
Making (the modest) assumption that autonomous cars can signal other information (e.g. I see a possible obstruction, I may need to brake) allows following cars to increase their follow distance to increase their follow distance which increases their safety margin. All within a few mS of the possible obstruction being seen.
The current roads are built to be safe for cars driven up to about 120kmh by drivers with about 150-250 mS reaction times by drivers that focus their attention ahead and only periodically (once per many seconds) check conditions to the side and rear.
Drivers (i.e. autonomous vehicle drivers) that can react in under 5 mS to changing conditions in any direction will be far far safer.
Just put lots of sensors on the vehicle. And lots of computers to analyze the data. Moores Law will make them cheap over time.