My intuition tells me that there are quite a lot of dangerous cars on the roads in states where it's a free-for-all, so I'd be interested in the research you're referring to.
If it's just an annual emissions test (as I assume many are, but I'm not well informed), then there's unlikely to be any significant prevention of mechanical failure on the road.
If it's just an annual emissions test (as I assume many are, but I'm not well informed), then there's unlikely to be any significant prevention of mechanical failure on the road.