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Cannabis & Driving

Cannabis & Driving: By the Numbers

  • According to the latest National Roadside Survey from the National Highway Traffic Safety Administration (NHTSA), drug use among nighttime weekend drivers has risen by 25 percent since the previous survey in 2007, and driver use of cannabis has spiked by 46% in that same period of time.
  • NHTSA traffic arrest and fatality data indicate that after alcohol, marijuana is the most frequently detected impairing substance among drivers.

Marijuana Impacts Driving Ability

  • Research shows that marijuana can impair drivers in a variety of ways. It can affect psychomotor functions such as attention, reaction time and coordination, but generally it appears to affect automated or routine driving more than tasks requiring conscious effort.
  • Further, numerous laboratory-based studies have demonstrated that marijuana use impairs many aspects of cognitive and physical function that are necessary for safe driving.
  • Marijuana can decrease car handling, can impair performance and attention while increasing reaction times, following distance and lane deviation.
  • Mixing alcohol and marijuana may produce effects greater than either drug on its own.

There is no level of marijuana safe for driving

  • Marijuana can affect people differently, making it difficult to develop consistent and fair guidelines like how long someone should wait to drive after using the drug. 
  • There is no science to show that drivers reliably become impaired at specific levels of marijuana (i.e., active THC) in the body.
  • Depending on the person, drivers with relatively high levels of marijuana in their system might not be impaired, while others with low levels may be unsafe behind the wheel.
  • It is also important to consider that people who use marijuana cannot accurately determine how much marijuana is in their blood or of their brain (where impairment occurs).

What Can You Do?

  • Impaired driving is a criminal behavior regardless of whether a drug is prescribed, obtained over-the counter, bought in a retail setting, or considered an illicit substance.
  • Just because a drug is legal does not mean it is safe to use while operating a motor vehicle.
  • If you get behind the wheel while you’re impaired by any substance, you put yourself, your passengers and others on the road at risk.
  • Simply put, if you consume cannabis, don’t drive. If you plan to drive, don’t consume cannabis.

Alcohol vs. Cannabis—What We Know

Cannabis is the second most common drug linked to substance-impaired driving. A large body of research demonstrates that cannabis impairs psychomotor and physiological functions critical to driving. The table below outlines what we know about cannabis in comparison to alcohol.

Learn more about new research from AAA Foundation for Traffic Safety.

Effects on driving-related functions Impairs psychomotor functions, pursuit tracking, divided attention, signal detection, hazard perception, reaction time, attention, concentration, & hand-eye coordination. Recent use is known to diminish virtually every driving-related capacity: psychomotor functions, cognition, attention, vigilance, tracking, reaction time & coordination. The effects depend on dose, absorption, time since peak blood level, & skill/task.
Self-perception Users tend to underestimate impairment and risk, even at low doses. Users tend to accurately estimate impairment.
Effects on driving behavior Driver tends to increase variation in speed, average speed, lane positioning (weaving), passing attempts, and decrease following distance. Users tend to employ slower speed, less passing, larger following distance. The largest and most consistent effect is on lane position (weaving). Compensation may be less effective for responding to unexpected events (e.g., other driver behavior, or an animal running across the road).
Tolerance Heavy drinkers tend to be less sedated at a given BAC due to tolerance. This contributes to them being more likely to underestimate impairment and drive. Heavy drinkers show tolerance, particularly with respect to impairment of sensory and motor functions. There is inconsistent evidence on tolerance effects.
Other drug use Users in social settings often only drink alcohol (or perhaps smoke tobacco). Use with other drugs with sedative effects (opiates, barbiturates, etc.) may increase risk of crashes, but may be more likely to have taken place at home. Some research suggests chronic (but not concurrent) marijuana use decreases effects of alcohol. Users often drink alcohol while ingesting marijuana. While research generally suggests additive or even multiplicative effects with alcohol, some studies suggest effects are no different than alcohol alone. Alcohol likely reduces compensation skills marijuana users otherwise may employ. There is little research on marijuana use with other drugs other than alcohol.