Background and aims: A growing body of evidence suggests that cannabis impairs driving ability. We used mortality data to investigate whether the commercial sale of cannabis for recreational use affected traffic fatality rates both in states that legalized it and in neighbouring jurisdictions. Design: Interrupted time–series of traffic fatality rates adjusted for seasonality and autocorrelation. Changes are reported as step and trend effects against a comparator of states that had not implemented medicinal or recreational cannabis during the study period (2009–16). Sensitivity analyses added a 6-month ‘phase-in’ to account for lags in production. Meta-analyses were used to derive pooled results. Setting: Three states that legalized recreational cannabis sales [Colorado (January 2014), Washington State (June 2014) and Oregon (October 2015] and nine neighbouring jurisdictions [Kansas, Nebraska, New Mexico, Oklahoma and Utah (Colorado neighbours); British Columbia and Oregon (Washington neighbours); and California and Nevada (Oregon neighbours)]. Measurements: Monthly traffic fatalities rates per million residents using mortality data from CDC WONDER and RoadSafetyBC and census data. Findings: There was a pooled step increase of 1.08 traffic fatalities per million residents followed by a trend reduction of −0.06 per month (both P < 0.001), although with significant heterogeneity between sites (step: I2 = 73.7%, P < 0.001; trend: I2 = 68.4%; P = 0.001). Effects were similar in both legalizing (step: 0.90, P < 0.001; trend: −0.05, P = 0.007) and neighbouring sites (step: 1.15, P = 0.005; trend: −0.06, P = 0.001). The 6-month phase-in produced similar if larger effects (step: 1.36, P = 0.006; trend: −0.07, P < 0.001). Conclusions: The combination of step increases and trend reductions suggests that in the year following implementation of recreational cannabis sales, traffic fatalities temporarily increased by an average of one additional traffic fatality per million residents in both legalizing US states of Colorado, Washington and Oregon and in their neighbouring jurisdictions.
- interrupted time–series
- recreational marijuana legalization
- spillover effects
- traffic fatalities