I commuted 10 mi. total daily for several years till Dec. 1997. (Now about to take up cycling again.) I found pedestrians to be the biggest danger, and the type of incompetent cyclist you describe, a close second. I puzzled during my commuting years the same as you, and I'm still at a loss to know how this problem can be solved, particularly since, as you point out, the police don't seem interested even in ILLEGAL cycling! (I might add that, in Kansas City, we have bicycle cops, and they violate the vehicular laws regularly, too, particularly by riding on sidewalks.)