Let us state right up front, driving on either marijuana or alcohol is illegal and a completely boneheaded idea that could potentially kill you, your passengers, and any innocent people driving, riding their bikes, or walking around you. Even driving on prescription medicines is illegal. Just don’t do it.

Yet, it happens, and there are people with scientific credentials doing trials to measure the effects of driving while people are high on delta-9-tetrahydrocannabinol (THC), the active ingredient of the cannabis plant otherwise known as marijuana.

Studies have shown THC helps fight glaucoma, boost appetites in cancer patients, and reduce nausea, aches, and pains for some disease sufferers. One thing no study will tell you is that marijuana cures bad driving, so anyone who tells you that marijuana makes them a better driver should have their personal observations questioned from then on.

However, like we said, there are studies being conducted that shed some light on some misconceptions of the effects of marijuana on a person’s ability to drive.

“Given the ongoing epidemic of drug-impaired driving and the increased permissibility and accessibility of marijuana for medical use in the U.S., it is urgent that we better understand the role of marijuana in causing car accidents,” Dr. Guohua Li, professor of epidemiology at Columbia University, said about of his marijuana driving study (see below).

Alcohol Effects on Driving

Anyone who has had a drink knows the effects of alcohol. One of those effects is impaired judgment, so the more a person drinks, the more they think they can drive. This is merely the first of many problems that come along with the consumption of alcohol that makes people who drive drunk a massive danger on the road.

  • Impaired eyesight
  • Impaired equilibrium
  • Impaired reaction
  • Low attention span
  • Confusion
  • Enhanced mood (good or bad)
  • Fatigue
  • Chance of blackout

Marijuana Effects on Driving

According to NORML, the marijuana advocacy site, “Subjects under the influence of marijuana are aware of their impairment and compensate for it accordingly, such as by slowing down and by focusing their attention when they know a response will be required. This reaction is just the opposite of that exhibited by drivers under the influence of alcohol, who tend to drive in a more risky manner proportional to their intoxication.”

Some of the effects include:

  • Impaired reaction
  • Low attention span
  • Enhanced mood (good or bad)
  • Fatigue
  • Confusion


Beyond what advocates say and the lighter list of impairments, a new study published in the October, 2011 issue of the American Journal of Epidemiology found that nearly 30% of fatally injured drivers tested positive for drugs other than alcohol, with marijuana being the main culprit.

Epidemiology – the branch of medicine that deals with the incidence, distribution, and possible control of diseases and other factors relating to health.

Researchers in the study used data gathered from 9 epidemiologic studies published in English in the past 2 decades to assess the association between marijuana use and crash risk. The researchers found that drivers who tested positive for marijuana within three hours of using were more than twice as likely to be involved in a motor vehicle crash.

Drug assessment methods were important in the study also. Researchers found that the crash risk was higher if the marijuana concentration levels found in the urine were higher. The reason for this is that high concentrations in urine denote recent use and the amount, whereas longer-term use (such as in habitual or medicinal users) is better assessed in hair follicle tests.

The study’s authors suggested that marijuana use may worsen reaction and coordination abilities, but weren’t sure whether the amount of marijuana used, or how it was ingested, contributed to the crash rates.

“The results of this meta-analysis suggest that marijuana use by drivers is associated with a significantly increased risk of being involved in motor vehicle crashes,” the study abstract stated.


The study also suggests that, like alcohol, the more marijuana a driver consumes before getting behind the wheel, the more likely they are to crash. This assessment and the actual danger behind the wheel has drawn some skepticism. One of those skeptics is from a very surprising place, the Insurance Institute of Highway Safety (IIHS) a long time advocate of stricter enforcement and penalties for drunk and drugged driving.

“We can’t really say yet that marijuana increases the risk by two or three times,” Chuck Farmer, director of statistics of the IIHS said about the October study cited above. “Most of their studies pointed to a very strong bad effect of marijuana on driving, but there are other studies out there that actually go the other way.”

Of course there are also advocates from NORML that will give an obvious biased, but will also cite many of the same studies that other skeptics do. Specifically on their website they cite a 2002 review of seven separate studies involving 7,934 drivers, which reported, “Crash culpability studies have failed to demonstrate that drivers with cannabinoids in the blood are significantly more likely than drug-free drivers to be culpable in road crashes.”

DUI Victims Attorney

Like our opening disclaimer, we don’t advocate anyone getting into their vehicle after ingesting any type of substance that has the remote chance of impairment, be it alcohol, marijuana, or an over the counter cough medicine. Thought those are very different in their power and effects, we urge people to please review alternative options such as mass transit or taxis to get where they are going after ingesting drugs, pain medication or alcohol.

Phillips Law Firm represents families of victims who have been hurt or died in car accidents in Washington state. They are skilled attorneys in wrongful death lawsuits and personal injury lawsuits with years of experience that they will use to assure the you get the compensation you deserve. Call today for a free consultation.