Uh, this already exists dude, maybe do a little googling next time. You get pulled over, officer may claim they detect marijuana, you get tested or refuse and get detained. And since there's no way to differentiate recent use from currently being high, you're pretty much at their mercy even if you're 100% sober at the time of the incident. I might have been high last night, but the next afternoon a test will just show the presence of THC, regardless of my current state.