NORML’s Principles of Responsible Use has stated since 1996:
The responsible cannabis consumer does not operate a motor vehicle or other dangerous machinery while impaired by cannabis, nor (like other responsible citizens) while impaired by any other substance or condition, including some medicines and fatigue.
Although cannabis is said by most experts to be safer than alcohol and many prescription drugs with motorists, responsible cannabis consumers never operate motor vehicles in an impaired condition. Public safety demands not only that impaired drivers be taken off the road, but that objective measures of impairment be developed and used, rather than chemical testing.
The question, then, is what defines “impaired”? Is the consumption of marijuana equal to impairment? What about an hour after consumption? What if you’re a frequent heavy consumer and have developed a tolerance to impairing effects of marijuana?
These are the questions that are difficult to answer. Our Paul Armentano does a remarkable job of compiling and updating all the latest studies in Cannabis and Driving: A Scientific and Rational Review. However, never does Paul or NORML make the assertion that smoking marijuana and getting behind the wheel (or vice versa) is going to make you a better driver.
This online auto insurance website, however, claims “marijuana users are safe drivers”. Included in their claims is a “Top 10 Reasons Marijuana Users Are Safer Drivers”. They cite many of the studies we’ve covered at NORML, but make some exaggerated claims about the study conclusions. For instance, the drop in driving fatalities in medical marijuana states, while laudable, is likely attributable to the drop in beer sales. Sure, there may be less drunk drivers because some have substituted marijuana for beer, and yes, marijuana-using drivers are safer than beer-using drivers, but it doesn’t follow that marijuana users are safe drivers. It only shows less drinking equals less fatalities and marijuana equals less drinking. If marijuana were as ubiquitous and legal as beer, we don’t know what the fatalities would be like.
The glaring problem with any sort of studies regarding marijuana and driving is that marijuana is illegal. Do pot smoking drivers drive slower, stay in their lane better, give more distance between cars, have fewer fatalities, etc., because they are safer drivers, or because the threat of any interaction with police will result in arrest and a drug record rather than just a ticket or a warning?
Reformers arguing for marijuana legalization need to be very careful around the driving issue. For example, here is one of the worst arguments you can make, from that insurance website’s Top Ten:
7. Most marijuana smokers have fewer crashes because they don’t even drive in the first place and just stay home thus concluded more than one of these tests on pot smoking and driving.
And why are the marijuana smokers staying at home? Because smoking marijuana is illegal! If you pin your argument on this point, all a prohibitionist needs to respond with is, “So, if you do succeed in legalizing drugs, aren’t you then going to have addicts driving to the cocaine nightclub and the marijuana coffeehouse and the heroin injection clinic?”
By focusing on any safety statistic we can find on marijuana using drivers, we run the risk of prohibitionists pivoting that point to prove how keeping marijuana illegal keeps potheads from wreaking havoc on the roads. Let these studies help you show that marijuana doesn’t make you a poorer driver, but don’t try to say it makes you a better driver. With the exception of some patients whose spasticity, seizures, or pain would make driving impossible without using cannabis, using marijuana will impair your reaction times and coordination as compared to not using, that’s science. Also, arguing for the idea that smoking pot makes you a good driver is a vote loser, that’s politics.
But back to the question – what defines “impaired”? If I went to dinner and had a beer with dinner, then had dessert, and chatted with friends, and an hour after that beer I got into the car in the parking lot of the restaurant and drove home, would anyone think I’m impaired? A six-foot tall, 250 pound man and one 16-ounce American beer? Very few would consider my drive home a crime.
What if the beer is instead a joint shared with friends at dinner, and I’m leaving my friend’s home an hour after smoking it? I’m certainly not impaired, even less so than if I had drank a 16 ounce beer (I’ve experienced both, you’ll have to trust me.) How does one establish a guideline for responsible marijuana use that depends on the driver’s evaluation of self-impairment? My belief I’m unimpaired an hour after a joint turns into a half hour for someone else and soon I’ve got tokers telling me they’re just fine to drive while smoking a joint.
And the problem for me is that I believe them. I’ve known marijuana smokers who think nothing of sparking up a joint while driving on a long road trip. I grew up with road musicians who traveled all across this country smoking doobies and never had so much as a speeding ticket or fender bender. Cell phones in cars scare me more on the highway than a spliff. My reason for imploring consumers not to toke and drive is because that’s the number one way tokers get busted, not because I’m terrified they’re going to cause mayhem on the freeways.
I’ve often lamented the “relative impairment” standard we have in America when it comes to drugs and alcohol and driving. Simply put, it means that we punish drivers who aren’t as fit to drive as they can be, not whether they’re fit to drive. For instance, Danny might be an “A+” driver and Granny might be a “C-”. Danny is a motor sports athlete with 20/15 vision. Granny’s so old her reflexes are shot and her senses are poor. But if Danny has a couple of beers, he becomes a “C” driver. If Granny gets into a collision, she’s just an unfortunate old lady, but if Danny gets into a collision, he’s a “drunk driver”. We don’t care whether Danny drives better than Granny or whether either of them can drive at a “C” level. We care that Danny knowingly reduced himself from an “A+” to a “C” and then drove.
In that analogy, the difference between Danny the drinker and Tammy the toker is this: If both Danny and Tammy are “A+” drivers, when Danny drinks, he becomes a “C”, then a “D”, then a “F”, but always thinks he’s a straight “A” student. Tammy goes from an “A+” to an “A”, then a “B+”, and then she stops smoking and starts studying, because she can see her grades are slipping. Part of the impairment of drinking alcohol is judgment of how impaired you are and even delusions of how unimpaired you are. In contrast, marijuana users are very aware of their impairment and generally have good judgment.

Contact your elected representatives and urge them to 'Stop Arresting Marijuana Smokers'. 
I would like to uslysht a bit additional on this subject
It’s not a big deal driving stoned, it enables one to drive longer with more comfort. I’ve driven well over ten thousand miles back and forth accross the U.S. while stoned, with no problems.
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I’m optimistic that we’ll all be able to ride around in self-driving cars before too long. Google’s current car is already safer than an average driver (probably safer than good drivers, too) so with any luck it should be a non-issue in a few years time.