wearing a crash hat on bikes

I'd endorse the careful approach. Any experienced biker looking at someone wearing next to nothing and zipping around will simply think such behaviour is very childish - no sense, no appreciation of the issue, potentially no skin left. To anyone who knows, it really dooesn't look cool - just stupid. Remember when your sliding along the road, the tarmac simply becomes a cheesegrater. And guess what the cheese is made of.

If you must ride without full protective gear, think pushbike - ie you should only be pottering at the sort of average speeds you might do on a cycle - say 15/20 mph (not including those Tour de France style folk, who seem to survive some horrible falls at pretty quick paces, but then at least they're training for that and are aware beforehand of the risks). And always wear the helmet - you might only injure your leg (which has nothing to do with wearing a helmet) but the failure to wear this item gives your insurer grounds to viod your policy.