Barcodes? We don't need no stinkin' barcodes.
Link The agency where I work has a couple of these systems now. Wish I could find a pic of a cruiser equipped with this thing. It has cameras all over it. It scans plates forward, sideways (you can drive along a row of parked cars for example) and even traveling the opposite direction. It runs every plate and if it's expired or the vehicle (or tag for that matter) is reported stolen, the officer gets an alert.
Cue up the red light camera whiners...they'll have a field day with this too I'm sure.