Contrail

From Doobybrain:

Contrail is a concept bicycle device that attaches above the wheel of a bike and covers the bicycle wheel with a layer of chalk. The chalk then creates a trail or mark on the surface of the road, turning the bike into a sort of large drawing utensil. 

The concept, developed by Pepin Gelardi of Studio Gelardi focuses around the idea of safety in numbers. By using this device, bicyclists will have a clearer path on which to ride safely and out of the way of vehicular traffic. At the same time, as more bicyclists using the Contrail go over a line created by a cyclist before them, the line gets brighter allowing drivers to clearly see a marked bike path where there might be none. It’s sort of similar to what happens when a dirt path appears in a grassy field after lots of people have taken the same shortcut over a period of time.