Most studies find that marijuana use doesn't actually significantly impair driving ability.

I've conducted such studies, albeit in less than scientific environments. Truth be told, even without the bias of wanting to see the day where the vox populi regarding recreational use has swung the other way, driving after a few puffs of marijuana has always made me a more considerate driver.

This isn't the same as "I drive better when drunk" but instead "I tend to care less about some guy going before me at a four way stop when it's not his turn" et al

There isn't a scientific explanation, again but it's purely anecdotal; I become more relaxed and forgiving behind the wheel and honestly? Less angry drivers the better.