Washington State saw more traffic crashes after marijuana legalization and retail sales | Research

The takeaway

Using county-level crash data and interrupted time series analysis, marijuana legalization and especially the start of retail sales in Washington State were associated with increased traffic collisions.

Traffic safety researchers, state legislators considering legalization, cannabis policy analysts, and public health officials.

Retail cannabis sales associated with greater crash increases than legalization alone

What the researchers found

Traffic collisions increased following both the legalization of recreational cannabis and the opening of retail stores in Washington State. The commercialization phase (retail sales) showed a stronger association with crash increases than legalization alone.

Why it matters

As more states legalize cannabis, understanding the traffic safety implications is critical for policy design. Washington was among the first to legalize, providing the longest available data window.

The numbers in context

Washington legalized recreational cannabis December 6, 2012. Retail stores opened approximately 19 months later. County-level monthly crash data analyzed. Both events associated with increased collisions.

How the study worked

Interrupted time series analysis using county-level monthly vehicle crash data from the Washington State Department of Transportation. Measured the impact of two events: legalization (December 2012) and retail commercialization (~19 months later).

What this study cannot tell us

Ecological study cannot link crashes to individual cannabis use. Other factors changing simultaneously (e.g., economic conditions, tourism, smartphone use) may confound results. Pre-legalization cannabis use was already common. Cannot distinguish cannabis-impaired crashes from general crash increases.

How to read the evidence

Quasi-experimental design with appropriate time series methods. Ecological data limits individual-level causal inference.

When this study was published

Published 2023. Data from Washington State, legalization in 2012.

The bigger picture

Traffic safety is one of the most concrete measurable public health outcomes of cannabis legalization. If retail access (not just legal status) drives crash increases, it suggests availability and access patterns matter more than legal status alone.

Questions still open

Do states with different regulatory models (e.g., delivery-only, limited retail) see smaller crash increases? Did crash rates stabilize or continue rising after the initial commercialization period?

Common questions

Does legalizing marijuana increase car accidents?

This Washington State study found traffic collisions increased after both legalization and the opening of retail stores, with retail sales showing a stronger association. However, the study cannot prove cannabis use caused the crashes, as many factors changed during the same period.

Why did retail sales matter more than legalization itself?

Legalization changed legal status but retail commercialization changed access. When stores opened, cannabis became easier to obtain for more people, potentially increasing the population of impaired drivers. This suggests that availability and access drive traffic safety outcomes more than legal status alone.

Read the original research

Collisions and cannabis: Measuring the effect of recreational marijuana legalization on traffic crashes in Washington State.

Traffic injury prevention, 24(7), 527-535

View on PubMed ↗Publisher’s paper ↗

Citation

Voy, Annie. (2023). Collisions and cannabis: Measuring the effect of recreational marijuana legalization on traffic crashes in Washington State.. Traffic injury prevention, 24(7), 527-535. https://doi.org/10.1080/15389588.2023.2220853