If they succeed, DARPA will be able to follow your car everywhere you go - even if you get into downtown Manhattan's traffic - making movies with car chases absolutely pointless. If Steve McQueen were alive, he would be very sad today.
The new radar is called Multipath Exploration Radar, and combines three-dimensional urban maps with a Ku-band radar running at frequencies high enough to resolve vehicle details. This makes the MER capable of fixing a target on anything that moves on the ground. The system will depend on unmanned air vehicles, bouncing their radars on ground and buildings and comparing the patterns with 3D maps. The result would be a complete picture of the movement in a city. Once they nail the chasing of one vehicle, DARPA wants to enable multiple target tracking.
Enjoy your freedom while it lasts, citizen. [New Scientist]