As the title of the below video suggests professional bike messengers are a bit crazy.

I have a fixed gear myself and never had the courage, let alone the skill to ride through the city so carelessly and two doorings over 10 years (one not my fault and one regrettably so) have done wonders to affect any potentially detrimental courage I may have been cultivating. Minus the ability to stop quickly (as is the case on a fixed gear) these riders are relying solely on the ability to maneuver at high speeds and in tight confines to avoid accidents.

Just yesterday I was riding up Walnut Street on the right side of a moving car in the center lane as a messenger was on the left side of the same car. That car, without using any turn single, abruptly pulled left into a parking garage entrance and ran the messenger off the road. Fortunately he didn't crash and no one was hurt. But as you can see below, accidents still occur. In this case, it appears to be the fault of a reckless cyclist who attempts to cut across the front right bumper of a moving van. Though it is admittedly hard to tell.