[ahaferburg]

You don't hit your head on the bike, you hit it on the concrete or a curb. Roads can be slippery, material failure is possible. The speed of the bike isn't that much of a factor, it's the object under you preventing you from effectively mitigating the fall, or making it worse.

[machinawhite]

I fell twice recently because of slippery roads just as you're saying. Both times my head was far far away fron the floor, I only hurt my palms and knees both times.

I still don't know how you're supposed to fall head first onto a curb unless you tie your arms behind your back.