If someone is a danger, they get pulled over, and if the officer sees that the driver is impaired, they take care of it the usual way. I don't really see how that's any different than now. There's two groups of people that would smoke when legal... those who already smoke, so they are already a possible 'danger' for impaired driving... and those who might give it a try since it's legal, but before hand were worried since it wasn't.... these people probably aren't the type to drive while impaired.
Not really convinced that's a good argument for keeping marijuana illegal, if anything that's an argument for banning all substances that can impair driving (which also includes various legal medications, not just stuff like alcohol).