Just 'cuz it's legal to partake ... doesn't mean you get a free pass for driving while high.
As Canada preps for recreational legalization (in just 3 days!!) police in the city of Sault St. Marie are readying to use a new device that can check for THC in saliva. Of course no one is encouraging cannabis-related DWIs -- but there are some concerns about how accurate the device actually is.
We can probably expect similar efforts here in the states as recreational use continues to gain traction ...