Usually, the bike then skids up to 20 feet before halting completely. As a last resort, the rider can grab the front wheel with his hand.

When traffic makes all that impossible, the other option is to crash into something.

Messengers with track bikes said experienced riders had enough control to stop when they needed to.

''After a while, you get used to it,'' said Mozart Augustin, a messenger for Rapid Couriers, who said he grew accustomed to his bicycle after several mishaps, such as hitting a taxicab. ''Most of the guys who have them have been using them for a long time and are really good.''

Owners also said that track bikes were cheaper to maintain because they had fewer parts to break and that their light frames allowed riders to go faster and make more deliveries. Leather and Spandex

Although many courier services said they required their messengers to ride bicycles with hand brakes, messengers said that the rules were only loosely enforced and that the number of track bikes on the streets was increasing rapidly.