Same difference when it comes to the items listed by skye and digger as well as the incidence of crashes. Crashes are, for the most part, random events that you can't anticipate no matter where or how you ride. You can plan for them by having good equipment and good skills. Only my latest crash was on dirt. The two crashes before that were on ice while commuting. And both involved head strikes. Having a helmet, for me and a lot of people, is just part of having good equipment and good skills.