If someone is in a traffic accident and ends up with severe physical or mental disabilities because they didn't take basic safety precautions (wearing a helmet or seatbelt), costs are incurred by the rest of society.
Then create a cost for not wearing a helmet. Raise the insurance rates to account for the extra costs. Or give appropriate insurance discounts for wearing a helmet. If you are caught without a helmet and don't have the "helmet-free" insurance, then you get fined, your insurer can drop your policy, and/or your insurance skyrockets.