People can only be as good a driver as they try to be. In my experience, people high on cannabis are cautious to the fault of perhaps driving a tad slower than the speed limit. Other than 400 series highways there is no minimum speed. Where there is, cruise control solved this many years ago. 
By the way, the saliva test is just like the more obtrusive blood test in that both can determine the presence of THC in the system but neither can indicate how stoned or impaired the driver may or may not be. THC may reside in the body for many weeks: FAIL