If you are going to come off your bike, then downhill is the best palce. You will slide down the hill, decelerating slowly, with a few cuts and bruises, and road rash to bare skin.
It is sudden stops which do the real damage.

Last year in the TdF, one guy came off his bike during a fast descent, and flew right off the side of the mountain. He bounced down the hill, on scrub and bushes, and walked back up to the road.