"Changes in traffic crash rates after legalization of marijuana: results by crash severity"

The title of this post is the title of this new research authored by Charles Farmer, Samuel Monfort and Amber Woods and supported by the Insurance Institute for Highway Safety.  Here is its abstract:

Objective:  The objective of this study was to estimate the effects of marijuana legalization on injury and fatal traffic crash rates in the United States during the period 2009–2019.

Method:  State-by-state quarterly crash rates per mile of travel were modeled as a function of time, unemployment rate, maximum posted speed limit, seat belt use rate, alcohol use rate, and indicators of legalized recreational marijuana sales and use.

Results:  Legalization of the recreational use of marijuana was associated with a statistically significant 6.6% increase in injury crash rates and a nonsignificant 2.3% increase in fatal crash rates.  In contrast, the subsequent onset of retail marijuana sales ― 3 to 18 months later depending on the state ― did not elicit additional substantial increases to injury or fatal crash rates.  Thus, the combined effect of legalization and retail sales was a statistically significant 5.9% increase in injury crash rates and a nonsignificant 3.8% increase in fatal crash rates.  However, these estimates varied by state.  The effects of legal marijuana use and sales on injury crash rates ranged from a 7% decrease to an 18% increase.  The effects on fatal crash rates ranged from an 8% decrease to a 4% increase.

Conclusions:  The estimated increases in injury and fatal crash rates after marijuana legalization are consistent with earlier studies, but they were not always statistically significant, and the effects varied across states.  However, this is an early look at the time trends, and researchers and policymakers need to continue monitoring the data.  National, state, and local governments considering changes to their marijuana policies should be cautious, proceed slowly, and take note of the lessons learned from these initial experiences.