Took a slide off of a road bike at 30 mph. It was no joke, and ground most of the skin off of my right side and replaced it with asphalt. 20+ mph off of a mountain bike took a 3/8 of an inch of skin and meat out of a silver dollar sized patch of my left forearm. Additionally, the average bicyclist isn't going 30 mph. I'd estimate more like 10-15 mph sustained. 20+ mph only if they're in shape and pushing.
No real point to this other than to point out that asphalt hurts, no matter the speed, but ultimately, wear what you want.