Originally Posted by Verbose

After watching that video of the people on bikes dumping paint onto the intersection (to make cars "paint"), I've thought it would be very interesting to rig up a canister of colored chalk near the rear wheel. Brake lever could open the canister releasing a bit of chalk and painting a path behind you.

Another option would be a rear brakelight. Ideally something with a push/pull style activation so you don't have to really wire anything.