California NORML Research Report

Driving Studies Show Limited Accident Risk from Marijuana

by Dale Gieringer, Ph.D. - Coordinator California NORML

Scientific studies on marijuana and driving fail to support the notion that marijuana poses a significant public highway safety hazard, according to evidence compiled by NORML. 

In particular, the evidence fails to support  proposals by marijuana opponents to impose tough new "zero-tolerance" standards for driving under the influence of marijuana or to disallow on-site use in medical cannabis clubs out of fear for driving safety.

In general, the evidence shows that marijuana is a lesser traffic hazard than alcohol or other drugs.  Marijuana appears to be most dangerous in high doses, or when combined with alcohol.  On the other hand, drivers with lower levels of marijuana have been found to be no more dangerous, and in some cases arguably safer, than other drivers.

NORML has recently issued a comprehensive report on drugged driving,  drug testing, and driving under the influence laws, by its senior policy analyst, Paul Armentano  [1]. The report disputes the rationale for proposed "zero tolerance" laws, in which the presence of any trace of marijuana in blood or urine is taken as per se proof of driving under the influence of drugs (DUID). Because marijuana can be detected in the system long after any impairment has passed, such laws wrongly misclassify many sober drivers as "intoxicated."  Urine tests detect only non-psychoactive metabolites of marijuana, which linger in the system for days or even weeks after use.  Blood tests can measure the major psychoactive ingredient of marijuana, THC,  which provides a better but still inexact indicator of recent intoxication.  Blood THC  peaks within the first hour of intoxication, but can be detected at lower levels for  a day or more in chronic users.

A growing body of scientific evidence shows that drivers with modest amounts of THC in their system are no more dangerous than other drivers. A 2002 review of seven separate crash culpability studies involving 7,934 drivers reported, "Crash culpability studies [which attempt to correlate the responsibility of a driver for an accident to his or her consumption of a drug and the level of drug compound in his or her system] 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"[2].

       Two other, new accident studies have failed to find any hazard from marijuana.  A study of 1500+ patients admitted to a Midwest trauma center published in the Journal of Trauma Injury, Infection, and Critical Care found correlations between use of alcohol, cocaine and opiates with injuries [3 ]. However, their data did not show "any statistically significant independent associations between injury and cannabis," researchers told NORML.  "(This) offers a strong rebuttal to the Drug Czar's misleading allegations implying that marijuana is a leading cause of ER admissions," says Armentano.

         Another study of road trauma from the Netherlands, which detected the presence of drugs through urine as well as blood tests, found significantly higher accident risks for alcohol and benzodiazepines (prescription tranquilizers) and less certain risks for amphetamines, cocaine and opiates, but no increased risk for cannabis [4].

         In the largest U.S. survey of drug use and driving accidents to date, the National Highway Transportation Safety Administration found that alcohol was by far the "dominant problem." At the same time it found "no indication that marijuana by itself was a cause of fatal accidents"[5].  The report was delayed and not publicized because it failed to confirm the expectations of administration drug warriors. 

         The NHTSA report did find that the combination of marijuana with alcohol and other drugs was highly dangerous.  Similar results have been reported in other studies.  For this reason, California NORML does not recommend permitting liquor sales on premises where marijuana is allowed.

         On the other hand, studies have found that marijuana by itself tends to be significantly less dangerous than alcohol.  A second NHTSA study of marijuana on actual driving performance found that the effects of THC appeared "relatively small" and less than those of drunken driving [6].  It found that marijuana appeared to produce greater caution in drivers, apparently because users were more aware of their state and ready to compensate for it, whereas alcohol tended to encourage speeding and risky behavior.  However, it also noted that marijuana could be dangerous in emergency situations that put high demands on drivers, or in combination with other drugs, especially alcohol.

Other studies have shown that at sufficiently high doses marijuana does impair driving safety.  Lab studies have demonstrated noticeable adverse effects for the first couple hours of intoxication, including impaired attention, unsteady lane control and following distance, and slower reaction time.  

Most recently, a large-scale Australian accident survey found that drivers with higher THC blood levels -- particularly those above 5 nanograms per milliliter (ng/ml) in plasma, indicating that the cannabis use had likely occurred within the past couple of hours ­ were correlated with a higher accident risk.  However, THC levels below 5 ng/ml were associated with a lower risk than drug-free drivers [7].

Similar findings have been reported by an expert panel of the International Association of Cannabis Medicine charged with developing scientific per-se standard for driving under the influence of marijuana.  In its report, the panel concluded that THC blood plasma levels above 7 -10 ng/ml might reasonably be taken as per se standards of impairment.  It rejected lower blood level standards and standards based on urine metabolites as unsupported by the scientific evidence.   However, it did suggest that lower THC blood limits might be appropriate where alcohol is also present [8].

In sum, current scientific evidence shows that the hazards of marijuana are dose-dependent, but generally less than those of alcohol.   There is no evidence that marijuana has contributed signficantly to overall accident mortality.  In fact, traffic accident mortality  rates  declined over the time when marijuana was first popularized in the 60s and 70s, and have continued to decline since.

 In light of the evidence, California NORML supports regulations to allow on-site use of marijuana in designated public  outlets, as presently allowed in the Netherlands.  NORML likewise strongly opposes "zero tolerance" standards for DUID.

For a comprehensive summary of the literature on marijuana and driving by the U.K. Department of Transport see: Cannabis and Driving: A Review of the Literature and Commentary.

- May, 2005


1) Paul Armentano, "DUID Legislation: What It Means, Who¹s Behind It, and Strategies to Prevent it"

2) Greg Chesher et al, "Cannabis and alcohol in motor vehicle accidents," in Grotenhermen and Russo (Eds) Cannabis and Cannabinoids: Pharmacology, Toxicology and Therapeutic Potential, Haworth Press (NY, 2002) pp 313-323.

3)  RD Blondell et al, " Toxicology screening results: injury associations among hospitalized trauma patients," Journal of Trauma 58(3) Mar 2005: 561-70.

4) K.L.L. Movig et al, "Psychoactive substance use and the risk of motor vehicle accidents," Accident Analysis and Prevention 36 (2004) 631-6

5) KW Terhune et al, "The Incidence and Role of Drugs in Fatally Injured Drivers," NTHSA Report #DOT-HS-808-065 (1994).

6)  HWJ Robbe and J. O'Hanlon, "Marijuana and actual driving performance," DOT report HS 808-078 (1994).

7) OH Drummer et al, "The involvement of drugs in drivers of motor vehicles killed in Australian road traffic crashes," Accident Analysis and Prevention 36(2) Mar 2004: 239-48.

8) Franjo Grotenhermen et al, Developing Science-Based Per Se Limits for Driving under the Influence of Cannabis:  Findings and Recommendations by an Expert Panel" (International Association for Cannabis as Medicine, 2005).