I think a lot of the folks on bikes are pretty uncoordinated and out-of-shape, such that turning their head to look over their shoulder is a big issue for them. Too many people here are out of shape, across every spectrum of the community. I've seen so many people riding bikes that look like retards without ever looking up or looking around them.

This from someone who rides to work a few times a week and has been an active volunteer for biking advocacy groups.