There are no street lights in Black Rock City, so traveling around after dark is dangerous. If you cannot be seen, you run a high risk of colliding with a pedestrian or another bike, or worse, a huge mutant vehicle.
You are responsible to make yourself visible whether you are walking to the PortaPotties or riding your bike on the streets or the open Playa.
It's best to have plenty of LEDs, glow sticks, or EL wire on your bike and your person, but something made of reflective fabric (such as on this fun vest) will give you an extra safety edge.
A nice feature of this vest is that you can put it on over almost anything else without getting bulky or restrictive. It weighs hardly anything. Take it off and stuff it in your bag when you don't need it.
This tutorial shows you how to make the vest pictured here. You can improvise on the idea for your own creation.