Having a fixed is effective enough. The robber will either fall on his first turn, when he tries to coast.

Enough bikes have been posted as stolen on this board to show that this line of thought is beyond flawed. Lock it up.

I've toyed with the idea of trying to rig up a system for a fixed gear anti-theft system. Mount a rear brake (or if you ride brakeless, front brake) lever on your bars, and have it activate a canister of pepper spray contained inside the bars. Use brake cable to direct the spray at where the thief's face will be when he's riding.

Of course, I haven't worked out the mechanical details of the system since ya know, I'm never actually going to do it.

that's awesome. you might want to add a safety/off switch you can activate while riding. then take it off safety when you lock it up.

oh yeah. never thought of that but of course it makes sense