Meta-analysis: cannabis use nearly doubles motor vehicle crash risk

The Study

Asbridge, Hayden, and Cartwright conducted a systematic review and meta-analysis — the gold standard for synthesizing research evidence — published in the BMJ, one of the world's most prestigious medical journals.

The search was exhaustive: 19 electronic databases with no restrictions on year or language, plus manual reference searches, unpublished study searches, and the research team's personal libraries. They included only observational studies of actual motor vehicle collisions — not simulator studies or experimental driving tests — that had appropriate control groups and measured recent cannabis use through blood toxicology or self-report.

Nine studies met the inclusion criteria. Each was assessed for risk of bias using the Newcastle-Ottawa scale, and results were combined using random effects meta-analysis.

The Finding

The result was consistent and statistically robust: acute cannabis use roughly doubled the risk of a motor vehicle collision. But the subgroup analyses revealed important nuances.

The high heterogeneity (I² = 81%) is important. It means the nine studies produced substantially different estimates, and the pooled 1.92 figure is an average across studies with real methodological differences. Some studies used blood THC to confirm recent use; others relied on self-report. Some looked at fatal crashes; others included fender-benders. The true risk likely varies with dose, tolerance, time since use, and whether alcohol is also involved.

Cannabis vs. Alcohol: A Fundamentally Different Impairment

The comparison to alcohol is the single most important piece of context for understanding cannabis impairment.

The impairment profiles are qualitatively different as well. Alcohol impairs judgment, risk perception, and inhibition — drunk drivers drive faster, tailgate more, and take more risks. Cannabis impairs reaction time, attention, and tracking — but cannabis users tend to be aware of their impairment and compensate by driving slower and increasing following distance. This compensatory behavior partially offsets the psychomotor impairment, which is why the crash risk increase is much smaller than for alcohol.

This does not make cannabis-impaired driving safe. A nearly doubled crash risk is meaningful, particularly for fatal collisions. But it does mean that equating cannabis impairment with alcohol impairment — as some DUI laws effectively do — misrepresents the relative risk.

The THC Blood Level Problem

One of the most consequential policy implications of this research is the failure of THC blood levels to correlate with actual impairment.

Myth vs. Reality

✕Myth

A THC blood level of 5 ng/mL is equivalent to a blood alcohol concentration of 0.08 — both indicate impairment

✓Reality

THC pharmacokinetics are fundamentally different from alcohol. Peak impairment occurs roughly 90 minutes after smoking, by which time blood THC has already declined more than 80% from its peak. A regular user may have 5 ng/mL with no impairment at all, while an occasional user may be significantly impaired at 2 ng/mL. The research director of the Washington Traffic Safety Commission has called the 5 ng/mL threshold 'completely arbitrary' and 'not backed by scientific theories.' AAA concluded that per se THC limits are 'unsupported by science.'

The Evidence

AAA Foundation for Traffic Safety; NHTSA Report to Congress (2017)

This creates an impossible policy dilemma. Five states have adopted per se limits (typically 5 ng/mL), but these limits miss an estimated 70% of cannabis-impaired drivers while potentially criminalizing sober regular users who happen to have residual THC in their blood. The Asbridge meta-analysis provided the crash risk data that policymakers needed, but the detection technology has not kept pace with the science.

What the Study Could Not Tell Us

The Policy Impact

The Asbridge meta-analysis was cited over 100 times in the two years following publication and directly influenced drug-impaired driving policies in Canada and beyond. It informed changes to graduated driver licensing laws in Nova Scotia, the Canadian Centre on Substance Abuse's anti-drug driving campaign, and ultimately contributed to the evidence base for Canada's federal cannabis impairment laws when legalization occurred in 2018.

The study's conclusion was careful and policy-oriented: "This information could be used as the basis for campaigns against drug impaired driving, developing regional or national policies to control acute drug use while driving, and raising public awareness." It did not advocate for specific legal limits — a wise choice given the THC blood level measurement problems that continue to vex legislators.

The Researcher

Mark Asbridge is a professor of community health and epidemiology at Dalhousie University in Halifax, Nova Scotia. His research program focuses on injury epidemiology, substance use, and traffic safety — an intersection that has made him one of the most influential voices in the cannabis-impaired driving evidence base. The Dalhousie research context — a Canadian university in a country that would legalize cannabis six years after this publication — adds particular relevance to this work.

Frequently Asked Questions

How dangerous is it to drive after using cannabis?

Cannabis roughly doubles the risk of a motor vehicle collision based on this meta-analysis. For fatal crashes, the risk is about 2.1 times higher. This is a meaningful increase but dramatically lower than alcohol impairment, which increases risk 7-14 fold at the legal limit. The combination of cannabis and alcohol is far more dangerous than either alone.

How long should I wait after using cannabis before driving?

There is no scientifically validated safe waiting period. Impairment varies dramatically by individual, dose, tolerance, and consumption method. Peak impairment from smoking typically occurs around 60-90 minutes and can last 3-4 hours. Edibles can impair for 6-8 hours or longer. If you feel any cognitive effects, you should not drive. Our driving impairment guide covers this in more detail.

Can I get a DUI for cannabis?

Yes, in every US state and Canadian province. The specifics vary: some jurisdictions use per se THC blood limits, others require demonstrated impairment, and some use both. The legal landscape is complex and the measurement science is deeply problematic. The safest approach is simple: if you have used cannabis, do not drive.

Is cannabis-impaired driving as dangerous as drunk driving?

No. The data consistently shows that cannabis impairment produces a much smaller increase in crash risk than alcohol. Cannabis roughly doubles risk; alcohol at the legal limit increases it 7-14 fold. However, cannabis impairment is still real and still dangerous — especially for fatal crashes, and especially when cannabis and alcohol are combined.