It's been done for years as even though it has yet (Cannabis) to be completely legalized, and if you think it's the only thing other than alcohol shown in direct correlation to cognitive impairment, add in all the others, such as hallucinogenics, analgesics, anesthetics, etc. Operating a motor vehicle and exhibiting signs similar to those of alcohol impairment. Of course, most officers have some funny experiences with stops like this. You get stopped, I walk on up. I tell you to roll down your window and it's like Cheech and Chong times 20 have been inside the vehicle along with you....
Reasonable suspicion begins at first observation, such as swerving, sitting there at an intersection and the light cycles more than once and you haven't moved, failure to maintain your lane integrity and others, then continues during personal contact once the fumigation cloud clears up and beside the driver. Then you get the usual SFST's. HGN, VGN, pupil dilation, Divided Attention, and observations pertaining to interaction, slurred speech, blood shot eyes, and performance results based on the tests administered. Granted, nystagmus isn't always present in THC use alone, however, from my standpoint, you may be over lapping (Impairment from more than one substance)
Implied consent for either an alco sensor and or intoxilyzer, and or by warrant (If not by consent) for blood and oral samples for concentration levels. THC concentrations usually between 5 and 10 ng/ml can be shown to establish a pretty good case regarding THC limit for showing threshold impairment combined with all the other results and the officer's testimony.
Many variables exist which make it more difficult to detect, such as body mass, diet, frequency of use, tobacco users etc. But most officers, based on their training and experience have been down this road....a lot.
If you think this is something new, think again.
