I think driving skills are impaired when people are stoned, but not to anywhere near the same extent as they are when they are drunk. I hate being in a car with a driver who has been drinking, and avoid it as much as possible. On the other hand, I don't feel very much at risk if I am in a car with a driver who is stoned. Personally I never drive drunk, but driving stoned doesn't feel as dangerous to me, especially if I am not baked out of my mind.