Taking examples from other sports, in boxing the fighters don't use helmets. When demanded (i.e. olympic games), they use a protection, but still they have to protect themselves with their hands (broken noses are still common, for example). In rugby, the players don't use helmets either and they want to have limited protections, so that everybody has full awareness of the extent they can hurt themselves or others (special exemption for the red lions
).
The problem with these protections is that they may cause a false sense of safety and distract the drivers in some ways (oil is the first one that I thought of), causing other types of freak accidents.