When Cannabis Laws Change, So Do Crash Risks

Picture this: It’s 1:45 a.m. on a quiet stretch of highway. A state trooper responds to a crash that looks like a tangle of metal and glass. Toxicology reports will later show what the eye cannot — both alcohol and cannabis were involved. It’s a scene that’s become increasingly familiar across the United States, where cannabis legalization has raced ahead of data on its safety implications.

A new study published in the American Journal of Preventive Medicine takes one of the most comprehensive looks yet at how cannabis and alcohol policies jointly affect motor vehicle fatalities. The results may surprise policymakers and public health professionals alike: both alcohol and cannabis laws matter, but in different—and sometimes overlapping—ways.

What the Study Examined

Researchers led by Dr. Timothy Naimi of the Canadian Institute for Substance Use Research analyzed nearly 350,000 motor vehicle deaths across all 50 states and Washington, D.C., from 2010 to 2019. Using data from the Fatality Analysis Reporting System (FARS), they examined whether victims’ crashes involved alcohol (at two blood alcohol concentration levels) and/or cannabis.

To capture policy environments, the team used two composite measures:

• Alcohol Policy Score (APS): A 0–100 scale measuring how strict and comprehensive each state’s alcohol laws are — things like outlet density, taxes, and drink-driving enforcement.
• Cannabis Policy Score (CPS): A parallel scale for cannabis, reflecting how permissive or restrictive each state’s laws are on sales, use, and marketing.

These weren’t simple “legal/illegal” comparisons. They represented entire policy ecosystems, recognizing that states differ dramatically in how they regulate — or fail to regulate — substance use.

What They Found

1. Strong Alcohol Policies Save Lives

Every 10-point increase in a state’s alcohol policy score — meaning stronger regulation — was linked to roughly a 6% lower risk of alcohol involvement in fatal crashes. This held true across both lower (≥0.01%) and higher (>0.08%) blood alcohol levels.

“Restrictive alcohol environments are consistently protective,” the authors note, “and these effects persist even when accounting for cannabis laws.”

2. Cannabis Policies Work—But Differently

In contrast, stronger cannabis policies were associated with a lower likelihood of cannabis involvement in fatal crashes, as well as lower odds of crashes involving both alcohol and cannabis.

A 10-point tightening of cannabis policy corresponded to about a 4–5% reduction in cannabis or co-involved fatalities.

But—critically—cannabis laws didn’t change how alcohol laws performed. Alcohol policy strength still predicted alcohol-related crash outcomes, regardless of cannabis legalization status.

3. Liberalization Carries Risks

As of March 2025, 24 states and D.C. have legalized recreational cannabis. Yet, the study suggests that loosening cannabis restrictions without corresponding alcohol or impaired-driving safeguards may increase risk for polysubstance crashes — those involving both alcohol and cannabis.

That’s not because cannabis automatically impairs driving the same way alcohol does, but because co-use magnifies impairment, even at moderate doses. And unlike alcohol, THC levels in drivers can’t be easily linked to impairment due to how the drug metabolizes and lingers in the body.

What This Means in Practice

For Public Health and State Officials

• Invest in data systems. Expand testing for cannabis metabolites in crash investigations to close the surveillance gap.
• Monitor co-use patterns. Alcohol and cannabis are increasingly used together; public health messaging and interventions must reflect that.
• Maintain strong alcohol controls. Even as cannabis laws evolve, alcohol remains the more potent and predictable crash risk factor.
• Design integrated impaired-driving campaigns. Treat alcohol and cannabis together in prevention messages rather than as separate silos.

For Policymakers

• Use policy levers, not just penalties. States with comprehensive alcohol control frameworks—such as higher excise taxes, fewer sales outlets, and strict enforcement—see measurable reductions in fatalities.
• Anticipate unintended consequences of legalization. Liberal cannabis environments may offset some of the gains achieved by alcohol policy reforms.
• Incorporate cannabis policy into traffic safety planning. Infrastructure funding (like that from the federal Infrastructure and Investment Jobs Act) can support integrated “Safe Systems” approaches.

Why This Matters

Motor vehicle crashes remain a leading cause of preventable death in the U.S., claiming more than 40,000 lives annually. Alcohol is still detected in about one-third of all fatal crashes, while cannabis detection has more than doubled since 2000. As both substances become more socially acceptable and available, policy coherence becomes critical.

This study provides evidence-based reassurance that policies matter — and that the public health community can influence outcomes through design, not just enforcement. Strong alcohol and cannabis frameworks act as guardrails that prevent substance-impaired driving deaths.

Limitations and Next Steps

Because cannabis testing remains inconsistent across states, the authors caution that the findings likely underestimate the true burden of cannabis involvement. Current toxicology systems don’t measure THC levels or timing of use, making it hard to distinguish recent use from past exposure.

Still, the signal is clear: policy context predicts harm. As cannabis legalization expands, future work should focus on:

• Developing standardized THC impairment thresholds
• Integrating substance use surveillance across states
• Assessing how legalization timing interacts with alcohol control systems

The Bottom Line

We tend to debate cannabis legalization as a binary issue — legal or not. But the reality is far more nuanced. The laws that govern how, where, and how much people can use these substances shape public health outcomes every day.

The new evidence is simple but powerful:

When states get their policies right, fewer people die.

Questions to Consider

• How might your state’s alcohol and cannabis policies interact to affect road safety?
• Are your local prevention programs treating alcohol and cannabis co-use as a single issue?
• What barriers prevent your agency from implementing stronger policy controls?