A DrinkingAndDriving.Org Article

The Times They Are a Changin’

Marijuana users are coming out of the shadows as laws in various states begin to allow usage medically and even recreationally. One of the questions this raises is how do drivers who use marijuana reconcile their lifestyle with safety behind the wheel? What do you do if you are a pot smoker who wants to drive safely and avoid getting arrested for driving under the influence of drugs?

Rules for pot and driving

There needs to be a basic rule of thumb that can be applied to smoking weed and driving. When it comes to alcohol, we have a rule of thumb. We emphatically recommend waiting a calculated amount of time after drinking before driving. The reasoning behind this is fairly simple …

For most people, when we drink our impairment rises and ebbs predictably along with our blood alcohol concentration (BAC). As we drink more, our BAC raises along with our impairment and it takes longer for the effects to wear off. The more one drinks, the longer one must wait before they can safely drive.

The second reason for waiting is both chemical and legal. BAC is a primary yardstick used by police to determine if you are under the influence. They measure it with a breathalyzer and if you refuse the breathalyzer, you will likely lose your license anyway! So by waiting an appropriate time for impairment to pass, we can drive without fearing the breathalyzer.

The desired outcome from waiting after drinking before driving is that symptoms of impairment will have passed and we can drive without endangering others or worrying about being arrested for DUI/DWI.

It turns out a variation of this approach may be the best road to take with marijuana, though it’s a bit of a minefield out there right now. As states are changing their stance toward pot, they are tasked with refining laws on what constitutes a pot impaired driver and what the consequences are.


Let’s be clear. If you are in a state where smoking pot is still illegal, a joint smoked a couple of weeks ago can cause you to lose your license. You are basically expected to choose between smoking pot and being able to drive. It is our first Rule of Thumb. If arrested for impaired driving in these states, you could be tested at the police station by a blood draw or urine sample. Testing on blood or urine can reliably show the presence of THC in the body weeks after smoking. Be sure you understand what the state and local laws are wherever you drive if you were to test positive for cannabis.

The issue this Zero Tolerance approach raises is that impairment from smoking weed tends to last only a couple of hours despite how long it stays in your body. To understand this disparity, let’s take a simplified look at what happens to THC when you smoke a blunt.



NIH - Four different THC moleculesSmoking anything introduces a large number of chemicals to your system. The National Institutes of Health reports that smoking cannabis releases over 2,000 chemical compounds. The usual goal of a pot smoker is to interact with the psychoactive chemical (the one that impairs you) - THC. But THC itself is complex with several molecular configurations, each with different properties.

Out of all this, there are two compounds of importance to this discussion of pot and driving, Delta-9 THC (THC) and Delta-9 Carboxy THC (THC-COOH).

THC exits your body primarily through feces, urine, and saliva. But Delta-9 and Carboxy THC leave the body at extremely different rates of speed.

Delta 9 THC is the compound associated with the (impairment) high. After smoking, Delta 9 levels rise and ebb along with impairment within a few hours. The peak is actually short-lived and much of the high tends to come and go within the first hour.

NIH - Delta 9 THC concentrations rise and fall quickly

Delta 9 Carboxy THC-COOH rises after smoking, however it takes many days for the levels to drop back down. Levels of THC-COOH bear no relation to current impairment. A urine test used by law enforcement will detect THC-COOH and therefore imply marijuana usage sometime in the past week or more.

In recent years, saliva tests which detect Delta 9 THC have been developed for use in the field by law enforcement. (example 1 Oral Cube, example 2 DDT5000 and DrugWipe) Testing positive for THC with these saliva tests indicate smoking within the last 2-3 hours.

States where marijuana has been decriminalized for medicinal or recreational use are looking at this matter and these new testing devices more closely. What many legislators would like to see is an acceptable per se limit like every state uses with alcohol. Alcohol impairment for most people rises predictably along with BAC with expected symptoms being experienced at expected BAC levels. The corresponding relationship between symptoms of impairment and levels of THC has not been mapped out so clearly yet. Some opposition to laws creating a per se standard for pot are based on the lack of current science to trace this relationship as precisely as with alcohol.

In other words, there are not (yet) established links between a specific symptom of impairment and an XX nanogram THC level.

Still, setting a limit is the comfortable thing to do and we’ve seen that happen in Washington, Montana, and Colorado. Between the accuracy of the current testing methods and the laws being written, we can see that waiting for Delta 9 levels to drop before driving will greatly decrease the possibility of getting arrested for driving under the influence. DrinkingAndDriving.Org recommends waiting 3 hours after smoking marijuana before driving.

Waiting three hours after smoking before driving is also the best approach to take in terms of safety. It is our 2nd Rule of Thumb. Now, let’s look at what marijuana impairment is and how it affects the driver.


First, understand that while the symptoms of alcohol impairment may be more extreme, smoking marijuana does cause impairment and its impact on driving should never be dismissed.

Here are the impairment symptoms associated with smoking pot from the NHTSA’s page on cannabis. We’ve bolded the ones which can affect driving …

Symptoms of Cannabis Intoxication


relaxation, euphoria, relaxed inhibitions, sense of well-being, disorientation, altered time and space perception, lack of concentration, impaired learning and memory, alterations in thought formation and expression, drowsiness, sedation, mood changes such as panic reactions and paranoia, and a more vivid sense of taste, sight, smell, and hearing. Stronger doses intensify reactions and may cause fluctuating emotions, flights of fragmentary thoughts with disturbed associations, a dulling of attention despite an illusion of heightened insight, image distortion, and psychosis.


The most frequent effects include increased heart rate, reddening of the eyes, dry mouth and throat, increased appetite, and vasodilatation.

{DrinkingAndDriving.Org Note - Vasodilatation causes lower blood pressure which can lead to dizziness, blurred vision and confusion.}

The effects these symptoms can have on your driving is nothing to scoff at. According to NORML’s whitepaper on the subject, the pot-impaired driver may appear more cautious driving slower and keeping more distance between their car and the one ahead of them. But it also states they could have difficulty staying in their lane and be slow to hit the brakes. Both are dangerous behaviors. They may even experience eye movement control issues. This could cause one to visually misperceive the trajectories of other vehicles in a situation requiring instant decision making and reaction.

Then again, both NORML and the NHTSA point to studies that show the impaired pot smoker is more likely to know they are impaired than the impaired drinker. A drunk person is more likely to want to drive than the stoned person.

For the most part, no matter who you listen to, the NHTSA, NORML, or the NIH, the driver who is high on pot tends to try to drive more carefully. Surprisingly, even this can be a problem. Current autonomous vehicles being tested on our roads are routinely rear-ended by human drivers. Why? Because they come to a complete 'behind the line' stop at stop signs and traffic lights, they travel at or below the speed limit, and they very cautiously make their turns. This is probably not the way the guy in front of you drives unless they've been smoking weed. In fact, these behaviors can trigger suspicion by police that a driver is impaired by marijuana.


If you are consuming THC edibles, you will need to wait longer than smokers. It takes longer for the THC to pass through to your bloodstream and the stages of impairement become drawn out and last longer. It is literally like taking pills. We recommend waiting 6 hours after eating THC edibles before driving as our 3rd Rule of Thumb.

But there is more we need to consider. What if you are also drinking? NORML, NHTSA, and NIH all say that the effects of combining marijuana and alcohol are ADDITIVE. They compound one another. For this reason, our 3rd Rule of Thumb recommends that pot smokers who drink should treat their wait-time as ADDITIVE. Wait 3 hours for the pot PLUS the appropriate time for what you drink before getting in the car. For example, if you knock down a couple of shots while smoking, you should wait at least 4.5 hours before driving.


We have two more rules you need to adopt as driving pot smokers. Our 5th Rule of Thumb is NEVER smoke WHILE driving. If you are following our other rules, this won't happen. Yet it needs to be said. It is both dangerous and dumb. No matter how lax the laws become in your state, smoking while driving is going to give you a headache. If you are caught, at the very least it will be an infraction, but don't count on being given a ticket and sent on your way. Before they write that ticket, they will likely give you the swab test we were talking about a few paragraphs ago. As much as possible, you should keep pot and paraphenalia out of your car, period.

Our 6th and final piece of advice is not just to drivers or smokers, but to everyone. As with drunk drivers, NEVER let yourself be the passenger of a stoned driver. First, it's a real drag to be with someone who gets pulled over. But there is also a genuine risk here. Stoned drivers, like drunk drivers, can kill their passengers along with pedestrians and people in other cars.

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