This idea was by Pepin Gelardi, a Finalist in pedal power design competition.
THE ROAD: A bicyclist on the road often feels that they are under assault by passing vehicles. Particularly in cities where bicyclists seem rare, motor vehicles aggressively dominate the road, discouraging potential bicyclists from joining in.
THE CONTRAIL: Contrail is a small bright bike accessory that allows bicyclists to color in their own space on the road. It’s like playing with sidewalk chalk, but faster.
THE WORKS: Contrail holds 200 grams of chalk, enough for about 20 miles (32km) of riding. It functions much like a carpenter’s chalk line: A small amount of powdered chalk is filtered through a brush inside the device and picked up on a felt wheel. The felt wheel transfers this chalk onto the rear tire leaving a fine layer. As chalk builds up, the tire leaves a faint line of color on the roadbed. This is you contrail. It is a colorful and ephemeral representation of your path.
JOIN IN: Making your mark is easy. (1) The device snaps onto the rear seat tube just below the seatstays with the felt wheel resting lightly on the rear tire. A custom molded rubber band holds Contrail in place. Rubber shims may be used to insure a tight fit. (2) Once you begin to ride, you’ve already started to contribute to the strength of the community and the safety of your fellow riders. Your path becomes evident and lets others know where you’ve been. (3) The more riders present, the brighter the community’s paths become. Motor vehicles become more aware of our presence and potential riders find themselves encouraged to join in.
I think you could have a lot of fun adding colour to roads and cycle paths, it would certainly brighten things up!
Although you would need to be able to turn it off or people could follow you home...