It's my observation that blows to the head are more likely on a bike than in either of those situations for two reasons - the speed and the position. When you're biking, you're going much faster than you would walk (I commute every day and often hit 40 mph). Second, in an accident, you often to go straight over the handlebars - the bike stops but you don't. That means head impacts are common. Cars have the same issues, but they have seatbelts that keep you from flying through the windshield or slamming into the steering column. As for whether it's dangerous - in general I agree with you. I've been commuting by bike 2000 miles a year for 10 years, and have had only a couple of incidents, but when things happen, I'm glad I have a helmet. Similarly, I wear a seatbelt when driving, even though I've never had an accident that necessitated one.