If they pull you over for erratic driving, maybe. States vary from one to the next. Every law-abiding citizen would benefit from a quick, non-intrusive (no blood or urine) assay for drugs. There are too many impaired drug users on the roads now, and as MJ is legalized and marketed to everyone the roads are even more dangerous. The technology is improving, and that’s good for road safety.
So the people who used to not use MJ because it was illegal, are now using and driving under the influence although that remains illegal? Why should we believe this dubious claim?