I've never managed to get my helmet to interact with a road surface or car, and sometimes I confess I lose track of why I wear it, other than for decorative effect.
Yesterday evening, I found out exactly what it is for. As I was cycling down the innocent path through niddrie looking forward to a lovely twilight ride, I came upon a couple of local youths who managed to launch a golf ball sized rock at my head which struck me with a massive thwack right in the front centre area of my helmet. I was impressed that they scored a direct hit and even more impressed that my helmet successfully prevented any serious damage to my skull.
It could have been very serious if it was even a bit lower, at least leading to some bloodshed and potentially to a scar or broken tooth.
Needless to say I took the road on the way back.