Marijuana and Driving: The Science and the Controversy

When it comes to people driving while high, inconsistencies in effects and DUI testing are creating challenges.
Last Updated: March 01, 2019

One of the biggest arguments against marijuana legalization is a concern about people driving while high. With more places opting to legalize medical and even recreational marijuana, the debate over marijuana and driving has reached a fever pitch.

From commercial truck drivers to NASCAR as well as everyday people moving down the roadways, questions persist about the potential dangers of driving under the influence of marijuana. There has also been much debate about whether current forms of testing for marijuana intoxication provide fair and accurate results. Here’s what scientists and doctors know – and still don’t know – about marijuana and driving.

Effects of Marijuana on Driving

There are many different cannabinoids within marijuana, and these are the active ingredients responsible for its effects. The cannabinoid present in highest concentrations in most marijuana strains is tetrahydrocannabinol, or THC. It’s the compound responsible for causing marijuana’s signature high.

Common THC side effects include:

  • Relaxation
  • Euphoria
  • Slowed perception of time
  • Intensified visual/auditory effects
  • Dizziness

Because of the possibility of these side effects, medical experts and law enforcement have concerns about people driving while high on marijuana. The National Institute on Drug Abuse warns that “marijuana significantly impairs judgment, motor coordination, and reaction time.”  It also references statistics that show that drivers with THC in their system are twice as likely to be culpable for fatal automobile crashes.

However, some studies raise questions about the actual effect of marijuana use on driving ability as well as the validity of blood tests performed after automobile accidents. One 2009 study published in the American Journal on Addictions contains a wealth of information about the effects of marijuana on driving. The study (by R. Andrew Sewell, MD, James Poling, PhD, and Mehmet Sofuoglu, MD) explored the effects of cannabis, as well as alcohol, on the ability to drive.

It found that driving while high on marijuana causes an increase in decision time and number of incorrect responses to emergencies. This includes increased time needed to make decisions while passing another vehicle, increased time needed to brake the vehicle when a traffic light changes (or to begin moving again when the light indicates that it’s time to go), and decreased ability to monitor the speedometer. Drivers under the influence of THC are also unable to compensate for a factor known as standard deviation of lateral position (SDLP), which is how researchers measure a driver’s ability to stay in their lane.

Behavioral Compensation

Among the findings in the Sewell, Poling, and Sofuoglu study was that when people are impaired due to marijuana consumption, they’re generally well aware that they’re impaired. This often results in them utilizing strategies to attempt to compensate for their intoxication.

Researchers found that even when marijuana-impaired participants were instructed to maintain a particular speed, they frequently tended to drive much slower than that speed. Drivers under the influence of marijuana also showed a tendency to maintain an increased distance between their vehicle and the vehicle ahead of them.

Just like cannabis users are often aware that they’re intoxicated, they’re also typically aware that they’re being assessed when they participate in a study. This leads scientists to believe that results from research studies are a good indication of what marijuana-impaired drivers are capable of, but not necessarily how they may actually behave if they’re driving under the influence of marijuana while not being observed.

Variation Factors

Research studies have also showed that there is too much variation from one person to another and one method of consumption to another to determine the exact effects of marijuana on driving. Factors that create variation include:

Frequency of Consumption

Infrequent cannabis users are most likely to show signs of impairment if they smoke marijuana and drive, and mixing marijuana with alcohol greatly increases the level of impairment in infrequent users. With cannabis use alone, these drivers pay more attention to basic tasks such as steering and less attention to speed or overall driving skill.

Meanwhile, more frequent marijuana users show less impairment, both with consumption of marijuana and after combining it with alcohol. In fact, when compared with occasional cannabis users, regular cannabis users showed less of a reduction in factors such as peripheral signal detection both when combining marijuana with alcohol and when consuming alcohol by itself.

Dose and Method of Consumption

Marijuana intoxication is also highly dose-dependent. Studies show that low concentrations of THC do not increase the incidence of accidents, while higher concentrations do. Because different marijuana-based products (oils, edibles, and BHO concentrates, for example) and various marijuana strains contain different percentages of THC, intoxication will depend on what was ingested as well as how much.

Consumption methods also matter greatly. Researchers have found that smoked cannabis results in a rapid rise followed by a rapid decrease in blood THC concentration, while marijuana edibles are much less predictable. Effects from edibles set in at between 30 and 90 minutes, typically reaching their peak within two to three hours. Someone who has consumed edibles may continue to experience effects for anywhere from 4 to 12 hours, making it harder to plan ahead to avoid driving high on marijuana edibles. Studies also show that vaporizing marijuana results in a higher blood THC concentration level than smoking an equal amount of marijuana with the same THC percentage.


Young people are the group that’s most likely to use cannabis and also the group that’s most likely to be involved in traffic accidents, according to a 2018 report on cannabis and driving. Furthermore, researchers wonder if some of the factors surrounding unsafe driving under the influence of marijuana could be mostly due to the specific demographic that typically engages in frequent cannabis use – young males that have a tendency to engage in risk-taking behaviors even when they’re not intoxicated. Some studies have suggested that when participants corrected self-reported unsafe driving behaviors (speeding and following other vehicles too closely, for example), the connection between cannabis use and accidents disappeared.

Watch: Drivers stoned on marijuana test their driving skills

Driving High vs Drunk

Sewell, Poling, and Sofuoglu found that the behavioral compensation present when a driver was high on marijuana became quite the opposite for alcohol intoxication. While drivers who are high tend to drive slower, drunk drivers tend to drive faster than the speed requested by the researchers. Alcohol intoxication also impacts decision-making as it pertains to factors such as risk-taking; for example, drunk drivers show an increased tendency to try to pass vehicles ahead of them (unlike study participants who were high; they made it a point to drive well behind the vehicle in front of them and did not frequently try to overtake them).

The combination of cannabis and alcohol results in worse impairment than consuming either alone. While individual results of culpability studies were mixed (with some showing that marijuana use does not result in a higher rate of crashes and others showing that it does), they also showed that drivers who combined alcohol and marijuana suffered worse consequences than those who only used marijuana.

Controversy Over Testing

In addition to variations caused by dosage, ingestion methods, frequency of consumption, and behavioral compensation, there is also a large degree of controversy over the accuracy of testing for the active presence of THC in one’s body.

In Colorado, the DUI limit for marijuana is 5 nanogram of THC per milliliter of blood. There is no time limit for authorities to acquire the blood for a test, however (for blood alcohol levels, there is a two-hour time limit to collect a sample). Because levels of THC in the bloodstream can metabolize and drop significantly over a few hours, this means that time is of the essence to get an accurate measurement.

These types of blood tests are also problematic for frequent marijuana consumers. As detailed in an NPR story on testing for marijuana-intoxicated drivers, THC stores itself away in fat, where it is slowly released, making it present in the blood for as long as 30 days after consumption. One study found that the blood of someone who uses marijuana frequently can test well over 5 nanograms even if they haven’t consumed marijuana for several days.

Although blood tests are the legal benchmark for law enforcement, some companies are hoping to provide more accurate testing though the creation of THC breathalyzer tests. Hound Labs is one such company; they insist their breathalyzer is more accurate than blood tests because THC is only present on the breath during the peak time in which a person is impaired. The Hound Labs breathalyzer reportedly can measure whether someone has smoked pot within the last two hours and displays the number of THC breath molecules in parts per trillion. The Hound Labs device can also measure whether a person is currently under the influence of THC consumed via marijuana edibles, as THC leaves a person’s bloodstream via the lungs, which is why (like alcohol), it’s measurable via their breath.

Marijuana and Driving for a Living

What about people who drive for a living? Marijuana legalization has created a swirl of debate regarding everyday commercial drivers (such as long-haul truckers) as well as more high-profile professional drivers such as those who race for NASCAR. Should the previously-existing blanket ban on marijuana use be lifted for them, especially if they’re licensed to use medical marijuana?

Commercial Drivers

In places like Canada where no federal prohibition still exists, there is also no federal regulation for commercial drivers. Such laws are left up to each individual province. Ontario, for example, has a zero-tolerance policy for all drivers – both commercial and non-commercial – while they operate a motor vehicle. Most Canadian companies simply require that their employees arrive for work fit for duty and remain so during the hours they’re behind the wheel. In 2013, Canada’s Supreme Court deemed random drug testing an affront to an employee’s privacy and dignity, and employers must refrain from it unless given an excellent reason for doing so.

Commercial drivers in places like the U.S. must abide by a different set of laws and employer-mandated rules. Because marijuana is still considered illegal by the federal government, all drivers who are subject to federally-mandated drug testing must refrain from recreational and medical marijuana use even when they’re off-duty. Even in states like Massachusetts (which passed a law forbidding employers from firing employees for testing positive for marijuana), federal drug testing laws still reign supreme.

NASCAR Drivers

NASCAR began random drug testing of its drivers in 2009; prior to that, it only tested them if it suspected them of being under the influence of a prohibited substance. Their inclusion of marijuana on the banned substances list means that drivers could test positive and be suspended even if they never compete while under the influence. This includes medical marijuana for drivers who live in states where it’s legal.

This has kicked off calls for NASCAR to remove marijuana from its list of prohibited substances. As long as NASCAR competitors aren’t driving while high, proponents ask why they shouldn’t be able to consume it where it’s legal (in states such as California and Nevada, for example – two states that happen to be on NASCAR’s West Coast Swing series of races).

Still, the organization has maintained a blanket ban on marijuana, as well as a refusal to allow its drivers to promote any cannabis-related sponsorship. In the summer of 2018, NASCAR suspended Spencer Gallagher for failing a drug test (although the organization refused to comment on the nature of the test results). One year earlier, NASCAR forced driver Carl Long to remove a sponsorship decal from Veedverks, a marijuana vaping company based in Colorado, from the hood of his car before he was allowed to race in Kansas.

FAQ About Driving While High

Most people know the dangers of drinking alcohol while driving, but efforts to educate people about the potential risks of driving while high have only just begun. Legalization of medical and recreational marijuana in many places has led to confusion over how to make good decisions about marijuana and driving. Can you drive while high? Is driving high dangerous? The following section will address some of the most common questions about driving under the influence of marijuana.

Does marijuana impair driving?

While the effects of driving high on marijuana can vary, marijuana is generally known to impair reaction time as well as the time it takes to make decisions in emergency situations. People driving while high may also have trouble monitoring the speedometer or holding a consistent lane position.

How does Colorado test for driving while high?

Currently, Colorado trains its officers to conduct visible sobriety tests of people who are suspected of driving stoned. Drivers may also be asked to submit to a blood test; people with blood levels of over 5 nanograms per milliliter of active THC are considered legally impaired.

How long after smoking weed can I drive?

Anyone who plans to consume edibles or smoke marijuana and drive must understand that the effects of marijuana on driving can vary. Effects from smoked cannabis start sooner and fade faster, while edibles take longer to set in and take longer to wear off (often hours later).

Can you get a DUI for driving high?

Yes. Even in places where recreational marijuana is legal, driving while high is illegal. Someone may be deemed intoxicated through a visual sobriety test and circumstantial evidence (a pipe or open container of marijuana in the vehicle), a blood test, and in some places, a THC breathalyzer test.

Driving Stoned: Not a Straightforward Topic

While everyone can generally agree that driving while high is a bad idea, variations in metabolism of THC, consumption methods, and other factors can sometimes make it hard for authorities to determine if someone has an unsafe level of marijuana in their system. From rules set by NASCAR to the U.S. Department of Transportation, the matter is further muddied when it comes to employer drug testing of drivers that may show evidence of marijuana consumed during a time when the person wasn’t even behind the wheel.

As cannabis laws continue to change, rules governing marijuana and driving are likely to shift, as well. This includes use of technology like breathalyzers to determine if someone is actively driving under the influence of marijuana as well as lifting prohibition of off-duty marijuana use for commercial truck drivers, NASCAR competitors, and other people who drive for a living.

Dorothy Harris - CNBS Author: Dorothy Harris
Dorothy is far from being a pot-head. Actually, when we first met she wasn’t very familiar with cannabis - but with her excellent research skills and drive for knowledge, it didn’t take her long to master the field. Dorothy comes from a scientific background and is in charge of researching and writing our advanced scientific and medical topics, as their correctness is not something we are willing to compromise on. Dorothy specializes in top notch research, making sure no stone is left unturned.
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