This is why we should move from mandatory emissions checks on all vehicles, to spot checks of the worst vehicles (but we'd need to get the police excited to run a test.)
You'd have to get everyone excited to allow those tests to be run. If you're spot checking the worst vehicles that will more than likely lead to a large proportion of checks occurring in low-income areas.
I don't see why you couldn't do a red-light camera sort-of thing. Spot emissions testers at ground level along with a photo of the offending license plate.
This is my biggest concern with emissions tests- it disproportionately affects lower income people. Not sure how to get around that, other than offering subsidies for them to upgrade/repair to a more efficient vehicle.
But as others have said, there is better low hanging fruit to go after rather than older cars.
