Found this over on Bicycle Design. The Contrail - Biking Community Tool encourages bicycling by making a chalk line on the road where a Contrail equipped bike has passed. As more Contrail bikes pass over the area, the chalk lines becomes thicker and brighter. The lines create interest and shows where to ride bikes safely.
Its an interesting idea but it'd be a long stretch for this to work outside of areas where there are lots of bicyclists to create and maintain these chalk pathways.
But it sure is an interesting idea. Until it rains.