Meta-analysis of 57 studies confirmed cannabis impairs driving, comparable to low blood alcohol | Research

The takeaway

A meta-analysis of 57 driving studies with 1,725 participants found cannabis impairs lateral vehicle control and that combining cannabis with alcohol produces greater impairment than either substance alone.

Policymakers setting impaired driving laws, cannabis users who drive, and traffic safety researchers.

Cannabis+alcohol worse than either substance alone

What the researchers found

Cannabis alone impaired lateral control (lane weaving) and decreased driving speed. Cannabis combined with alcohol produced greater driving impairment than either substance alone. Cannabis effects on driving were similar to low blood alcohol concentrations.

Why it matters

This is the largest meta-analysis of experimental driving studies on cannabis to date, providing definitive evidence that cannabis impairs driving performance and that the combination with alcohol is particularly dangerous.

The numbers in context

57 studies, 1,725 participants. Cannabis lateral position variability: g=0.331. Lane excursions: g=0.198. Cannabis+alcohol vs alcohol alone: g=0.480 for lateral variability, g=0.525 for time out of lane. Cannabis+alcohol vs cannabis alone: g=0.336 for lateral variability.

How the study worked

Systematic review and meta-analysis of 57 experimental driving studies (simulator, closed-course, on-road) involving cannabis and/or alcohol administration with 1,725 participants. Searched 9 databases. Used Hedges' g effect sizes.

What this study cannot tell us

Most studies used driving simulators rather than on-road conditions. Study heterogeneity limited interpretation of some measures. THC doses and timing varied across studies.

How to read the evidence

Strong: large meta-analysis of 57 experimental studies with direct driving performance measures.

When this study was published

Published in 2022.

The bigger picture

As cannabis legalization expands, quantifying exactly how much it impairs driving, both alone and combined with alcohol, is essential for setting evidence-based impaired driving policies.

Questions still open

At what THC blood concentration does driving impairment become equivalent to the legal alcohol limit? How long after cannabis use does driving impairment persist? Should combined substance use carry enhanced legal penalties?

Common questions

How does cannabis driving impairment compare to alcohol?

Cannabis effects on driving performance were similar to low blood alcohol concentrations, primarily affecting lateral vehicle control (lane weaving) and causing drivers to slow down.

Is combining cannabis and alcohol especially dangerous?

Yes. The combination produced significantly greater driving impairment than either substance alone, with larger effect sizes for lateral position variability and time spent out of lane.

Did cannabis affect all aspects of driving equally?

No. Cannabis primarily impaired lateral control (staying in lane) and caused decreased speed. Effects on hazard response time and other measures were less consistent across studies.

Read the original research

The effects of cannabis and alcohol on driving performance and driver behaviour: a systematic review and meta-analysis.

Addiction (Abingdon, England), 117(7), 1843-1856

View on PubMed ↗Publisher’s paper ↗

Citation

Simmons, Sarah M; Caird, Jeff K; Sterzer, Frances; Asbridge, Mark. (2022). The effects of cannabis and alcohol on driving performance and driver behaviour: a systematic review and meta-analysis.. Addiction (Abingdon, England), 117(7), 1843-1856. https://doi.org/10.1111/add.15770