If you ride a motorcycle even following the speed limit and you fall, even if you don't hit any vehicle you are risking: - hitting the ground (front or back) so having a concussion or smashing all your teeth - hitting sidewalks or guard rails Moreover the helmet lowers the noise given by air flowing, helps you see better at sunrise/sunset, protects you from insects. I think there's no good reason to avoid wearing an helmet while riding a motorcycle.