(via nickmcglynn)

Contrail - Biking Community Tool

The concept was created by Studio Gelardi, and it consists of a small apparatus that applies a thin layer of colored chalk to a bike’s tire. As you ride, the chalk is transferred to the road, leaving a colorful trail behind you. Over time these trails on the road get more pronounced, which not only encourages other bikers to take the same route, but it also lets drivers know what roads are frequented with bike traffic, hopefully making them a bit more cautious.