Faster is cheaper, in terms of how much road we need.

Road-hours are absolutely fixed, for a given set of roads - 24 hours in a day times physical road miles divided by car spacing is a hard limit.

Slow cars are on the road longer.

Ideally all the cars would go the speed of light and use the roads for milliseconds per trip.

Surely we're not optimal now - speed is set by arbitrary limit signs placed largely politically. Then people go whatever speed they feel is right anyway.

Just saying, the measure of autodriving is a larger space than accident-rate.