Bike accessory leaves a trail of chalk behind you

Contrail is a device that applies chalk to the rear tire of your bike, leaving a trail behind you. It leaves trails for motorists and other cyclists to see, enabling bikers to "reclaim this crucial shared space." I'm not sure where you can get one or if it's even been physically realized, but I like the idea. It's like a cross between the prototype for the projected bike lane symbol and Bikes Against Bush, which sprayed chalk according to text messages. Via Cool Hunting.