I've gone down maybe 5 or 6 times. About half of these, I went down to the side - tires going out from under me on a curve or something else at low speed, and I kept my arms on the bars, landing on my shoulder and/or hip, with very little injury - some scrapes.

on three occasions, I've gone over the bars, and you better believe I used my arms to break my fall. The fastest of these was at about 19 MPH. The other two were around 15 and 18 MPH respectively (Yeah, I've got the garmin data, I'm not just making up the numbers). The slower two, I got a bunch of road rash, and the faster one, I broke my hand. No head/pavement contact for any of them.

If am going over the bars, you better believe my arms are going out in front if I have the time.

BTW, I've had several minor concussions, though not from bike riding. I'll bet that anybody who's had any post-concussive symptoms, temporary or permanent, would tell you that if you gave them the choice, they'd have preferred a broken bone to a head injury.