If you live in one of the more than 20 states where Opus Inspection, through its subsidiary Envirotest, operates emissions testing programs, you may have seen its “RapidScreen” trucks and, more lately, unmanned stations set up beside the road, scanning the tailpipe emissions of passing vehicles. You may even live in a state where receiving a clean bill of health from one of these trucks a couple of times a year can exempt you from having to present yourself and your car for an emissions test at registration time (though you still have to pay the inspection fee, naturally). The...