Actually, if they’re testing for actual THC, rather than metabolites, that should correspond to actual current THC levels. It’s (from my understanding) the metabolites that can show up long after (regular) use.
I’m not convinced that the .5 ng/ml figure represents actual impairment, either, especially for regular users.
I share your doubts about the relevance of this figure. And we know the police will distort statistics to support a viewpoint. Like including passengers or drivers who were 100% not at fault in drinking driving statistics.
I studied a lawsuit in Seattle which tests the same and the motorist tested toxic even two weeks after his last smoke