How high is the risk of colliding on a highway with someone who is high, on a highness celebration day? The answer, if there is one, can perhaps be teased out by studying this new study of drivers in the UK and also older study of drivers in the USA:

“The Relative Risk of Motor Vehicle Collision on Cannabis Day Celebration in Great Britain,” Sotiris Vandoros and Ichiro Kawachi, Accident Analysis and Prevention, epub 2019. The authors, at King’s College London, and Harvard University, explain:

“Cannabis celebration day, also known as ‘420 day‘, takes place at 4:20pm on April 20 every year. The objective of this paper is to study whether there is an increase in road traffic collisions in Great Britain on that day. We used daily car crash data resulting in death or injury from all 51 local police forces covering Great Britain over the period 2011–2015. We compared crashes from 4:20pm onwards on April 20 to control days on the same day of the week in the preceding and succeeding two weeks, using panel data econometric models. On the average cannabis celebration day in Britain, there were an additional 23 police-reported collisions compared to control days, corresponding to a 17.9% increase in the relative risk of collision.”

The study of high and not high Americans is:

“The April 20 Cannabis Celebration and Fatal Traffic Crashes in the United States,” John A. Staples and Donald A. Redelemeier, JAMA Internal Medicine. vol. 178, no. 4, April 2018. The authors, at the University of British Columbia and the University of Toronto, explain:

“The 25-year study interval identified 1.3 million drivers involved in 882 483 crashes causing 978 328 fatalities. In total, 1369 drivers were involved in fatal crashes after 4:20 pm on April 20 whereas 2453 drivers were in fatal crashes on control days during the same time intervals (corresponding to 7.1 and 6.4 drivers in fatal crashes per hour, respectively). The risk of a fatal crash was significantly higher on April 20.”

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