I think it does impair you but not in the same way as alcohol. Pot smokers seem to get confused about whose turn it is at a 4 way stop, they drive slower, they might stop suddenly if they see a pedestrian a half a block away, they wait longer to pull out into traffic and then may pull out after its too late, they leave too much space between them and the car in front of them at a red light so the car behind them can't get into the left turn pocket. In other words they're spaced out and they think they're being more safe. That's how people drive in Bellingham and Seattle Wa. where I'm from. More so in Bellingham. In Seattle you have to kind of know what you're doing or get out of the way. Drivers in both places are a little overly polite though. Especially to pedestrians crossing against the light. I've noticed that in Boise Idaho if you're a pedestrian, you better get out of the street before the light changes. Maybe not as many pot smokers there. Pot smoking pedestrians are pretty bad too. they'll walk right out in front of you without even looking. Then when you stop short to avoid hitting them they look at you like you did something wrong and when the car behind slams into you because you stopped short, they just continue on their merry way in their socks and sandals and those knit caps that look like they have braids.
It will impair you and make you more likely to crash, but not in the same way as alcohol. Alcohol makes you more aggressive and reckless, whereas marijuana makes you overly cautious and driving-defensive, almost like a disoriented, elderly person. Both impairments are bad news, though a drunk driver is even more dangerous than a high one.
Depending on the person, one could become paranoid and over think their driving... They could become confused and overwhelmed in a high traffic situation that requires sharp thinking. If they stick to the back country roads we'll be ok :-)