The average speed in a city is probably around 20km/h, at least in the city I am in (Berlin, Germany). With perfect coördination of driverless cars, you could possibly raise that to 60km/h – which is the speed cars drive when they're not in traffic or stopped at traffic lights. That leaves a lot of road usage to disappear even if car usage actually increases.
That would increase the average kinetic energy on the roads 9 times. 9 times more severity and frequency of accidents with an unclear responsibility. 9 times more noise and energy consumption. 9 times less pedestrian and bicycle and humans friendly cities. Driverlessness does not change the physics of transportation.
