Currently, 24 states, two territories, and the District of Columbia have legalized marijuana for recreational use for adults over the age of 21, and 38 states, three territories, and the District of Columbia have legalized marijuana for medical use. As more states pass legislation for legalization of marijuana either for recreational or medical use, the scientific community should prioritize understanding how driving is impacted by  tetrahydrocannabinol, also known as THC, the active ingredient in marijuana.

Although the research is limited, generally accepted results reveal that drivers with any THC in the bloodstream are two times more likely to experience a motor vehicle crash. THC is fat soluble, and the ingredient can stay in a regular user's system several days after last using marijuana. THC metabolites, compounds created as the body processes THC, can remain in a person's system for weeks after last using marijuana.  

While laboratory studies of people with THC in their bloodstream do not support significant impairment on single tasks, such as memory, addition, or subtraction, there may be more significant impact on multitasking and handling unexpected events (which are critical components of safe driving). Teen drivers should be especially wary of driving after taking any substance that may be mind-altering, including marijuana. The tasks required for driving are complex and require much higher order thinking than they realize.

A 2022 study published in JAMA Psychiatry found that smoking cannabis can impair a person's ability to drive up to 4 hours after the drug is used. For edibles, a person should probably wait at least six hours before driving, as the active components of the drug generally take longer to dissipate than when it is smoked. The drug's effects and the duration of those effects varies from person to person.

Researchers from the Center for Medicinal Cannabis Research at the University of California San Diego looked at 191 regular cannabis users and determined that smoking cannabis led to a significant decline in Composite Drive Scores (CDS) that assessed driving variables, including following a lead car at various speeds, swerving in their lane, and responding to divided attention tasks.

Detecting clinically significant levels of THC in the body can be complicated. Urine levels, which in many states are equated to blood levels (and are both illegal), can reflect past use and may not reliably detect people who are actually "high."

State Marijuana Driving Laws

Currently, 18 states have zero tolerance or non-zero per se laws for marijuana. Zero tolerance laws prohibit driving with any measurable amount of marijuana in the body. Per se laws make it illegal to drive with amounts of marijuana in the body that exceed state limits:

• 10 states have zero tolerance for THC or a metabolite.
• 4 states have zero tolerance for THC but no restriction on metabolites.
• 4 states have specific per se limits for THC. For example, Georgia provides an exception to the per se standard for people who are “legally entitled to use” marijuana and other substances other than alcohol.
• 1 state (Colorado) has a permissible inference law for THC. What this means is that the jury can infer that a person is guilty of driving under the influence (DUI) if the person has a set amount of THC in his or her blood. 

In 10 states, laws are in place related to driving with cannabis in motor vehicles. These laws are similar to "open container" statutes that limit alcohol consumption in motor vehicles. For more information on these state statutes, visit the National Conference of State Legislatures website.

To learn more about marijuana legalization laws, visit the Insurance Institute for Highway Safety website.