There are three cameras fitted to a police vehicle -- marked or unmarked. In the video after the jump, the vehicle is unmarked, and by just driving down the road, every plate from oncoming traffic, and cars on the side of the road (or the next lane), is logged and analyzed. There's also a side-facing camera which comes into play when cruising parking lots. The system can handle up to 3,000 plates a day, and by making it a passive activity, it turns nabbing bad guys into something akin to playing the lottery.
The system can be anywhere and everywhere, from stationary systems to car-mounted mobile units, there is no hiding from the long arm of the law. All an officer needs to do is drive around. With the system keeping an ever-vigilant watch on all the plates and comparing them with a database of suspect plate numbers, police time is freed up, so they can go about protecting and serving. Heck, while on their way to another call, they may just get lucky and pass a vehicle that "hits", turning it into a bonus score for the day.
Thanks for the tip, Dave!
Video after the jump
Automatic Licence Plate Recognition System - video powered by Metacafe