Contrail is a tool for developing bicycle communities. As you ride,
contrail leaves a faint chalk line behind your bike. The goal is to
encourage a new cycle of biking participation by allowing the biking
community to leave a unique mark on the road and to reclaim this crucial
shared space.
https://www.treehugger.com/files/2009/03/bike-contrails-diy-bicycle-path-chalk-design-concept.phpoldId.20090307193749487
Biking in Baltimore