The Supreme Court of Canada has ruled that although arbitrary stops are a violation, the police have the authority to stop a vehicle to check vehicle fitness and driver fitness at any time. That really falls under the scenario you're giving, that we've stopped a person because of a licence plate you can't read, or it's just a simple road check where we're running vehicles through, but by the time the person gets to you, they may have been twelfth to thirteenth in line, and we'd have no driving evidence.
If we have some kind of suspicion that the person may have consumed alcohol—and that actually goes back to another Supreme Court ruling. During the road check the common stuff we do is pull somebody over, and when they come up to you, ask them, “Have you been consuming alcohol tonight?” If they admit to consuming alcohol, we ask them a few more questions, and if during that conversation we feel that a person should be checked to see if they're fit to drive, we have that authority to do so.
Again, it's on a voluntary basis currently.