Houston is way ahead of you. I don't live there anymore, but circa 06-08 they introduced dodge charger cop cars with 'ghost' paint, which is basically only visible if you're parallel with the car. they're primarily used for traffic enforcement, always assumed that it was just to increase revenue at the cost of reducing speeding deterrence, but from your comment it looks like they're threading that loophole nicely.

Pic: http://www.stcofhouston.org/PoliceCarPhotos/Houston_Police_G...

Article: http://m.policemag.com/blogpost/1474/houston-pd-s-traffic-en...

Unrelated: I had no idea police had a magazine.