When I was a young kid, the city had a night crew working which would go to the scene of an accident after the bulk of the wreckage was towed away and clean things up. They would pick up and remove the bits of debris scattered about and then sluice off the road way of any blood or fluids. By morning, you might be lucky to find a few small bit of glass or reflector lens on the road.
I assume they still do it because after accidents here, the spots are usually identified by the plowed up grass and bent guard rails along the roadside with a little bit of wreckage left. If the fire department showed up, they high pressure blast the road off with their hoses, and they show up at most automobile accidents these days.