It’s recommended that you should let at least four hours pass between the time you smoke marijuana and the time you drive a car. The window for taking marijuana orally is six hours. However, these guidelines come with a few caveats.

The Trouble With Guidelines
However, it’s difficult to set hard parameters for safe time passed between smoking marijuana and getting behind the wheel because the effects of marijuana vary widely according to potency and tolerance. They are, in short, dose-dependent. So while it is helpful to suggest a window of hours between smoking and driving, it’s even more useful to get into the details.

Don’t Be High
No one should drive while high. If you’re “a little high,” as measured by how strong your buzz is, you shouldn’t get behind the wheel. Many of us think we can drive with a slight high. The first parameter for safe time passed between smoking marijuana and driving is that you shouldn’t be high at all.

Don’t Be Overconfident
Smokers who drive can underestimate the time they need by forgetting these factors:

There is a period during which you might not be feeling the “high” — you’re no longer recreating with marijuana — but your senses and reflexes aren’t yet what you’d like them to be for getting behind the wheel.

There are levels of being “straight” after smoking that are appropriate to normal driving conditions but not for emergencies. Skidding on black ice or having to swerve away from a pedestrian stepping into the street from between two parked cars, for example. It’s usually recommended that eight hours should pass between the feeling of euphoria has passed and the moment you drive.

Notice that there’s a certain inconsistency between that recommendation and the one listed further above (that you shouldn’t drive for four hours after smoking).

When recommendations clash, it’s better to go with the more conservative option and decide that you shouldn’t drive for eight hours after smoking marijuana.
A smoker might handle his or her vehicle faultlessly but still be in legal trouble if something goes wrong completely beyond your control.

Marijuana, Driving, and the Law
Don’t make the mistake of thinking that because marijuana might now be legal in your state it’s legal to drive while high. Even if there hasn’t been an accident, if you’re pulled over while driving high you’ll be cited for Driving Under the Influence, just as though you had had a martini.

Laws vary from state to state. Some have a zero-tolerance policy, together with broad interpretive powers granted to the police officer who has pulled you over. Don’t be surprised if the mere presence of weed paraphernalia in your vehicle is enough for a judge to decide in favor of the officer even though you hadn’t smoked in several days.

Conclusion
Marijuana is criticized for leading to a sedentary lifestyle. When it comes to driving, let this criticism become sound advice. The safest place to be high is on your couch with your One Hitter