I'd guess there are a disproportional number of flats on the front simply because the front tire encounters road debris first but the rear tire has more weight. I'm not holding my breath for a study on the subject to cut through the anecdotal evidence. I do know front flats are a greater concern. My wife had a blow out flat while coming down a 9% grade. She's so good. She simply took solid control of the steering and slowed down slowly. The tire stayed in place. I was proud of her bike handling skills.