Very cool article on Wired.com about the rear facing camera/radar on the Pratt & Miller C6.R. I tried to embed the video here, but not sure if it will show up or not. At any rate, they use a radar and camera to track cars to the rear and indicate which side the car is coming up on and it can differentiate between different car classes and let the driver know if it is a P1 car coming up or not.