All that would prove is that someone smoked within the last month. They don't have any evidence of driving under the influence. If this is so cut and dried, why aren't they doing this now? There's already baked drivers everywhere (just like there's drunks). The real risk are the people who take regular prescription drugs - just about all say don't drive if you're taking these things but everyone does anyway. That's a bad thing.
Just because they are not caught driving while impaired does not mean they are not driving while impaired.