This is Scientific Americans 60 Second Science. I'm Steve Mursky.

A year ago I reported on a study about an increase in fatal traffic accidents on April 20th,

a date considered somewhat of a holiday

by marijuana aficionados.

That study was in the journal Jama Internal Medicine

and it used data from the U.S. National Highway Traffic Safety Administration.

The researchers looked at fatal accidents on 420, between 420 PM and 1159 PM from 1992 to 2016.

And they compared that date's data with the day one week before and one week after.

They found a 12% increase in the relative risk

of a fatal traffic accident after 420 PM on 420.

420 PM being the time that a lot of pot smokers like to light up on 420. The paper

caught the attention of McGill University epidemiologists Sam Harper and Adam Pallew.

I should say at first I thought the paper was intriguing.

Sam Harper on the phone from Montreal. I think the most fundamental difficulty

with the Jama paper was really the large magnitude of the effect size, you know, given what we already know about impaired driving.

So in order to increase the national rate of fatal accidents by something like 12% would require, you know, either a really large segment, you know, as much as 15% of the population to be driving while high after 420 PM on 420 or really, really incredibly high relative risks of driving after the kind of

cannabis consumption that you might have had on 420.

So I think this was actually a hard case to make

substantively. Harper and Polyure decided to dig deeper. So one way to test

this is to see whether this kind of elevated risk persists if you compare 420 not just to the same day one

week before and after but for example to the same day two weeks before two weeks after or to every same day two weeks before, two weeks after,

or to every other day of the year,

...