Just to add, as many have already said, a driver who has smoked can be very different from another driver who has smoked. Whereas, someone who has drank even just a few, almost always behaves the same as someone else who has drank just a few. In almost all cases, anyone who has been drinking will do everything to appear as if they are sober while driving. Depending on how much they have consumed, they might even be more aggressive, or they might just try and drive the exact posted speed limit.
I feel much much safer in a car with someone who has smoked, given the idea they I know that person well enough and know how they drive. Most people I have driven with after they have smoked are actually better drivers. They are more alert and pay more attention to their surroundings, usually drive the posted speed limit or a little below, and are very at ease with their driving, in that they just seem to be natural, kind of "one with the car" type thing. Honestly. It always seems like they know where every other car is, they maneuver the car very smoothly and is a comfortable ride (not that that really matters), and I have been in a car twice with someone who has had to navigate the car out of potential harm, i.e. someone pulling into the roadway in an unsafe manner, or someone just driving like an idiot on the roadway (crossing a few lanes at the last second realizing they need to turn the other direction at an upcoming intersection then they were prepared to do). In all honesty, based on the people I have been around, I'd argue that the roads would be safer if everyone had smoked *some* prior to driving.

Now, I do have to argue against that very same notion though, because some people, when extremely stoned, almost lose themselves in the surroundings. Sometimes they 'space out', miss a turn, or if in an unfamiliar area, easily get lost because they don't know where to navigate and lose their sense of direction. So the amount smoked, and other variables, can make the driver more hazardous.
And since every person is different, 1) with their driving skills while sober, and 2) with their coordination and motor-control after smoking, I'd have to say it's definitely better for everyone to be sober and that smoking can become a hazard. Yes, it's different for everyone, and unlike drinking, it's far more easy to get away with smoking and driving then drinking and driving. And for those drivers, I don't care, because if they can get away with it, even when driving near an officer, then most likely they are good drivers. But as a rule of thumb, being under the influence is more hazardous then not - regardless of what has been consumed. Everyone is different and people will always be able to get away with it, right or wrong, and good driver or not.