As states across the country move to legalize medical and recreational marijuana, questions about the impacts of these policy changes – including on driving and rates of traffic accidents – are cropping up, said Sam Harper, associate professor of epidemiology at McGill University and one of the study authors.
“There’s a lot of dynamic action happening in the cannabis policy arena,” Harper said.
Prior research has indicated an increase in traffic fatalities on April 20. A research letter published in JAMA Internal Medicine in April 2018 looked at the relationship between April 20 and fatal traffic crashes in the U.S. Researchers looked at data for 25 consecutive years — from 1992 to 2016 – and compared the number of fatal crashes that happened on April 20 after 4:20 pm to the number of crashes that happened in the same time frame on control days — the same day of the week — one week before and one week after April 20.
They found “a 12% increase in the relative risk of a fatal traffic crash after 4:20 pm on April 20 compared with identical time intervals on control days.”
The research letter spurred Harper’s interest. He wanted to build on the research by comparing the impact of 4/20 on fatal traffic accidents with other risky days.
“We were motivated to look at this in more detail and see what we could find if we did a little bit more testing,” he explained, adding that there is significant day-to-day variation in fatal traffic accidents.
So Harper and co-author Adam Palayew expanded the analysis. In addition to comparing fatal traffic accidents on April 20 with single control days before and after the holiday, they looked at data for control days two weeks before and after. They also compared the number of fatal traffic accidents on April 20 to other known risky traffic days, such as the Fourth of July and Labor Day. Additionally, they analyzed the variation in fatal traffic accidents for every day of the year, “to put 4/20 in the context of other daily variations in traffic crashes,” Harper explained.
The key takeaway? “There is very little evidence that the [4/20] effect was observable over time,” Harper said.
“When we decided to include two control days on either side of April the twentieth, we found a much more diminished effect,” Harper explained.
Further, when the authors compared fatal accidents on April 20 with every other day of the year, it did not stand out relative to the average variation in traffic crashes.
“This is not because daily crash rates are too noisy to detect any signal,” the authors write. “On the contrary, we find important, systematic and meaningful variation in the daily number of drivers involved in fatal traffic crashes across the period from 1975 to 2016. We find consistent evidence for increases in the number of drivers involved in fatal crashes on 4 July and the days prior to Labor Day and American Thanksgiving, as well as systematically fewer drivers involved in fatal crashes on Christmas Day and New Year’s Day.”
In other words, the evidence suggests that there are significant variations in fatal traffic crashes associated with some holidays – but not 4/20.
The authors also analyzed the impact of 4/20 over time. The holiday is a recent phenomenon, popularized in the 1990s, so they looked at data going back to 1976. “We also found little variation in the annual impact of 4/20 over time. If recent celebrations of 4/20 were generating excess fatal crashes we would expect to see a greater excess in recent years,” they write.
Harper added that in the analysis, he found over a dozen days “equally as risky” for fatal traffic accidents as 4/20 that were not linked to any celebratory holiday.
“There’s just a lot of noise in these traffic crashes from day-to-day and from week-to-week, and when you look at the bigger picture, the variation that we see on April 20 is consistent with that variation,” he said.
Harper noted a few limitations of the research. First, the traffic accident data did not contain any measurements of impaired driving or marijuana use.
“This is clearly a very population-focused and 1,000-foot view of this problem,” he said. “To really study carefully the impacts of marijuana consumption and impacts on driving, it needs a different kind of study. Our study isn’t about that question, it is just about fatal crashes on 4/20 compared to other days.”
He noted that the analysis was also constrained to only fatal crash data – analyzing injuries, for example, could potentially produce different results.
“This question about impaired driving and marijuana is an important question, and something I think deserves attention,” Harper said.
He suggested that journalists should pay attention to the topic, “but be careful in how it’s reported… especially where there’s not a lot of research on this.”
“Part of the way this kind of science works, there’s not going to be a single definitive study on this –we’re just trying to build on what we know,” he added. “I’d like to see journalists continue to follow up and report on the story and say there’s more research being done.”