If you could fall off your bike and smash your head on the ground, you should be wearing a helmet. Doesn't matter what kind of bike, or how slowly you're moving, or how short a distance it is. You could pull out of your driveway, and the guy next door could be mad at his wife and accelerate out of his driveway -- BOOM -- major head trauma. No bike ride (in which the bike is moving forward) is safe without a helmet. I heard an impressive number--that helmets prevent 80% of major head trauma.

My brain. It's not the best, but it's the only one I have!