There's been several studies done saying that people who drive while extremely overtired have the same reaction time as someone who is inebriated.
And driving while stoned/drunk is just not cool, period. 
How do you get your car back from the party??
Responsible people leave their cars locked and call a cab. She lives in Canada, you know.
It depends on how many drinks you've had. I know from experience when I can't safely drive.
It's been studied again and again that people are not nearly as capable behind the wheel as they think they are if you've been drinking/smoking up.
If I know I'm going to drink, I cab it, have someone pick me up, or I stay the night. If I'm at a friend's house for dinner or something, or I drive to a party, I have one glass of wine and that's it. I absolutely cannot risk my life or someone elses just because I was having a good time at a party. For me, it's not worth the risk. I could never live with myself if I was the source of an accident that caused harm to someone else.
My lines are firmly drawn. That's just me.
