If you can avoid driving in the afternoon and evening of April 20, do. And don't ever drive stoned. Rihardzz/Shutterstock

Many people don't need an excuse to get high, but for those who do, April 20, or 4/20 day, is one. Canadian researchers wondered if all those extra joints and hash cookies might make the roads more dangerous. It looks like they do, implying increased risks from consumption on other days as well.

Although a group of high school students first started associating 4.20pm with marijuana in 1971, the name took a while to spread in the pre-Internet era but got a lift from those who used it to encourage the idea that no one should smoke before that hour.

Today, however, that combination of numbers is tightly associated with all things cannabis. American date nomenclature makes 4/20 April 20, making it a popular day for protests for legalization, celebrations where legalization has already been achieved, and general consumption by fans. In states where cannabis sales are legal, they increase in the lead-up, and interviews elsewhere report higher consumption on the day, particularly after 4.20pm.

There is plenty of debate as to how much risk cannabis consumption poses in regards to driving. Opponents of recreational marijuana laws point to its danger to drivers, but illegality and testing challenges make data hard to collect. Dr John Staples of the University of British Columbia and Professor Donald Redelmeier of the University of Toronto reasoned that if consumption is higher one (otherwise ordinary) day of the year, this would provide a good test.

In JAMA Internal Medicine, Staples and Redelmeier report on a comparison of fatal American car accidents over the period of 1992 to 2016, using publicly available data. The records include the dates of a horrifying 978,328 fatalities over that period, a reminder of the price society is paying for independent travel.

Road deaths were 12 percent higher on April 20 after 4.20pm than comparable days. The difference became statistically significant after 2004 as awareness of the code spread. Comparisons were restricted to the dates seven days before and after April 20, to remove the effect of weekends or seasonal conditions. By far the most dangerous time is between 8pm and midnight. The increased risk was insignificant for those aged over 40, but large for under 20s. Most of those killed were not tested for drugs in the autopsy, preventing confirmation of drugs' role.

Few regard THC as a performance-enhancing drug for car drivers, but overall effects have been uncertain, and some argue stoners staying home will reduce total traffic, and therefore deaths. This study suggests work needs to be done to ensure inhaling isn't quickly followed by last breaths.

April 20 is not the only dangerous day in April, however. Redelmeier previously showed that April 15 also has more accidents because it is usually the day American tax returns are due.

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