I always thought it was the sheer number of people that wanted to participate - 10,000 runners and at least twice that number of spectators (assuming that every

I always thought it was the sheer number of people that wanted to participate - 10,000 runners and at least twice that number of spectators (assuming that every runner has at least one person interested in watching, some bring a whole family). I don't know what marathon traffic is like in most cities, but I've driven across San Francisco many times during the marathon and it's never been as bad as during normal rush-hour traffic. (edit: I guess I understated the number of runners/spectators, the SF marathon claims 27,000 runners and 80,000 spectators)