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To the Editor
We thank Harper & Palayew for replicating our analysis of traffic risks on April 20.1 2 We agree that the absolute risks on April 20 must be modest because the majority of Americans do not celebrate the ‘high holiday’ and because comparison days are not devoid of impaired driving. Similarly, secular trends in relative risks for April 20 reflect evolving driving norms, fluctuating traffic enforcement, changing baseline rates of cannabis use, variable celebration behaviours and rapid growth of the cannabis industry. We also agree that crashes are already recognised to be more frequent on traditional holidays such as Independence Day and Thanksgiving.3 Beyond this shared understanding, however, we disagree with Harper & Palayew on a key assumption in their analysis.
Distant or unmatched control days can be biased comparators because prevailing weather, road conditions, daylight hours, driver fatigue and travel patterns vary substantially across time. More crashes, for example, occur on weekends relative to weekdays, in summer than winter and during 2016 than 2010.4 This was our rationale for performing a matched …
Contributors All authors were responsible for drafting and revising this reply.
Competing interests Authors are supported by the Vancouver Coastal Health Research Institute (JAS), the Canadian Institutes of Health Research (JAS, DAR) and the Canada Research Chair in Medical Decision Science (DAR). Funding organisations were not involved in the design and conduct of the study; collection, management, analysis and interpretation of the data or preparation, review and approval of this manuscript.
Patient consent for publication Not required.
Ethics approval This study used publicly available statistical data with a waiver of approval from the University of British Columbia research ethics board.
Provenance and peer review Not commissioned; externally peer reviewed.