Just curious. Any numbers on this?
That would be hard to say since with any herbal drug or suppliment, the dosage (i.e. THC for pot) varies due to plant age, local growing conditions, genetics, etc. If you were to express it in terms of, say, mg of THC you could find the correlation. But working only from plants, you couldn’t find a good correlation.
Vlad/Igor
The best response to this question is: Don’t drive after smoking pot.
SGT Schwartz
I would think this nearly impossible to answer because one beer for a 300 lb. guy is different than one beer for a 100 lb. guy. But one toke (or whatever?) would affect a person the same whether they weighed 100 or 300 lbs. I think. Also, beer intoxication depends on how much you’ve had to eat and marijuana intoxication doesn’t change based on what you’ve eaten.
Thirdly, how do you even measure “how much marijuana” - it depends on the type of marijuana and the way it is ingested doesn’t it?
Sorry, I’m asking more questions than I’m answering. Is that illegal in “General Questions”? 
Pfffttt… That’s your response for everything.
I don’t see how you can really compare the effects anyway because they’re so completely different. Marijuana doesn’t really cause the same kind of “impairment” that alcohol does, no matter how much is smoked.
Exactly. Alcohol affects fine motor skills and judgment much more than marijuana, which is precisely what makes drunk driving so dangerous.
Aren’t there studies which show that people who drive under the influence of marijuana actually tend to drive more safely because they consciously overcomensate for any perecived impairment? I think I saw something once that showed that people under a low to moderate influence of alcohol will show significantly impaired ability but will believe that their abilities haven’t changed. People under a low to moderate influence of marijuana show just the opposite- a negligable to non-existent impairment of abilities but they think they’re more impaired than they actally are and compensate for it by driving more slowly and cautiously (and they don’t have the same impairments of either motor skills or judgement that are caused by alcohol).
My own extensive anecdotal experience has led me to conclude that while pot won’t impair your actual driving ability very much, if at all (especially if you’re a habitual stoner) but it does greatly increase your odds of forgetting where the hell you were going once you get out on the freeway.
My thoughts exactly: it may take you 15 tries to find the Kroger (dude, chips are so good), but you’re hardly a hazard to anyone while you’re looking. The effects of marijuana are cognitive and sensory.
Now, driving on mushrooms or LSD would be hazardous. :eek:
It’s kind of like asking “how many beers does it take to give you the same buzz as two cups of espresso?”
No, a toke can affect people of the exact same weight quite differently, there is no way to tell how it will affect a person. Even the same person toking before or after a meal can have different effects.
With the recent heavy crackdown on drunk driving I’ve always liked Adam Carolla’s take on the whole issue. Which states that he believes you should register as a level I, II, or III on impairments. He claims that he has more tolerance than most people so he should be allowed to drink more. Fill him with 6 beers and he’ll clear a whole obstacle course of pylons, while his mom would drive straight into an orphanage after a sip of champagne.
We don’t need a driver, man, we’re going miniature golfing!
There is a factual answer, assuming that marijuana does increase ones odds at ending up in an accident. True body size and whatnot might vary those odds, but you can still look at the ratio of drinks to accidents over the whole population and the ratio of marijuana joints to accidents and you’ll know what on average is going to be true.
If it takes 3 beers to increase the odds of an accident by 10% and 10 joints then so be it, but that is a calculable relation.
Here’s a study that seems to have sufficient numbers to determine back to an answer for the OP. That is, it tends to use blood-alcohol level and THC level, so one would need to do some math to figure out what that means in terms of glasses and joints.
This also seem to vary by person, though also presumably from trip to trip.
Insofar as any randomly chosen experience on psychedelics could generate markedly different emotional and cognitive states, I’m probably confessing here to having been a public menace at the time I chose to drive, but in retrospect, in all the times I’ve driven under the effectss of LSD, I had very ordinary trips (in the LSD) sense resulting in very safe trips (in the automotive sense), both for me and for anyone on the same roads at the same time. Very relaxed, unfrustrated, experiencing vigilance as pleasant and entertaining, enjoying the feel of the road and the responsiveness of the controls in the car, visualizing the drivetrain components and how they were humming along, attributing personality traits to the drivers of different cars based on their driving and how they interact with each other on the road, etc etc.
This is not to be construed as endorsing the practice of driving on LSD, and I do not currently drive at all (citydweller, no car, good public trans), offer not valid where prohibited.
I’ve always thought it funny that I know I could easily pass the current driving test here in Florida completely smashed.
I have nothing to add to this post except to say that reading it gave me a total flashback. 
I’ve never driven drunk or stoned. But I have been a passenger in the front while stoned. (the driver was totally sober) I found myself getting distracted easily by road signs and traffic lights. Staring at them for far too long and that kind of thing. I could see that this would be a danger if I was driving.