I think that's the big thing that is preventing a national adoption. And I'm actually surprised that there isn't an accurate test out there, besides the sniff test. Once you have portable weed testers, along with very accurate station testers. I think the movement to nationally legalize pot will gain traction.
More than likely, the higher range will be used first. But if you find a high number of accidents, that number will be decreased. Of course, if you get into a serious accident, like alcohol, they will test you and see if you have a trace amount in your system. And like alcohol, they can use below the legal limit.