As more states successfully legalize and decriminalize marijuana, there has been some concern about the possible consequences. At the center of the debate is the risk posed by marijuana impaired driving.

A slew of recent articles have tried to make the connection between the legalization of recreational cannabis in some states and a rise in the number of traffic fatalities. But that connection is flimsy, at best. There are a number of issues with the data behind these claims, and there are a number of other factors contributing to the rise of traffic fatalities which must be taken into account.

So let’s take a closer look, and see what the facts have to say.

Too High to Drive?

First of all, let’s consider how being high affects a person’s driving ability. It turns out that’s very difficult to measure. We know exactly how alcohol affects our motor skills and reaction times. We know at what point (.08% BAC) it becomes dangerous — and illegal — to get behind the wheel. And we can accurately measure a person’s alcohol impairment at the time of the accident through a breathalyzer test or other means.

When it comes to marijuana, we know none of that.

There have been relatively few studies on marijuana impaired driving, and the results of those studies have been inconsistent. Being high might make you weave a bit more, but not so badly that you leave your lane of traffic. It might slow your reaction time, but it also tends to make you drive more slowly and give more following distance. And according to a 2017 report from the National Highway Traffic Safety Administration (NHTSA):

“It should also be noted that many studies have not shown impairment on these psychomotor tasks, cognitive and executive functions as have shown statistically significant impairments. It is not clear why this is the case. It may stem from different THC doses, different time lags between doses and testing or driving, differences in the tasks used to assess the effects, tolerance developed through frequent use, and the different dependent measurement employed and their relative sensitivity to small effects.”

Also, there is also no good way to measure a person’s impairment at the time of a traffic stop. The main psychoactive compound in cannabis, THC, can linger in a person’s blood and fatty tissues for hours, days or even weeks after use. So the presence of THC in the system does not correspond to how high a person actually is. This makes it impossible to set a clear limit or threshold like we have with alcohol.

To further complicate matters, the effects of cannabis vary greatly based on several factors; like which strain is used, whether it’s ingested or smoked, a person’s individual tolerance, etc. All of which means that we have no idea whether or not a person is too high to drive.

Which is, you know, kind of a problem when you’re trying to figure out whether or not marijuana increases traffic fatalities.

Legalization and DUIs

After the legalization of recreational marijuana in Washington state in 2012, there was a brief spike in the number of drivers with THC in their bloodstream, and a slight increase in the number of fatal car accidents. It’s tempting to blame the increase on legalization alone, but the truth is more complicated.

For one, as I already mentioned, the presence of THC in the system doesn’t necessarily mean that a person is impaired. Cannabis screens are often testing for trace amounts of THC in the bloodstream: sometimes just 1 — 2 ng/ml. A frequent marijuana user might have that amount in their bloodstream all the time. So testing for such small amounts will obviously yield a lot of false positives.

Second, after a legalization measure passes, the frequency of testing for marijuana impairment tends to increase. Take a look at this graph from a study by reason.org:

In 2009, the state of Colorado administered about 1500 cannabis screens. In 2012, there were almost 3 times as many. Obviously, the more drivers you test, the more DUIs you get. That doesn’t necessarily mean that there are more impaired drivers on the road, it just means you are catching more of them. And the percentage of positive screens actually decreased from the year before.

The use of marijuana in combination with alcohol is another variable that must be taken into consideration. In one study, more than half of drivers (59%) who test positive for drugs were also found to have a blood alcohol content of .08 or higher. Which means that their impairment cannot be attributed to cannabis alone. In fact, the NHTSA has found the crash risk for marijuana is not statistically significant, after adjusting for risk factors, such as alcohol use, age, gender, sleepiness, etc.

But state and federal agencies don’t always make that distinction when they are recording data. So many of the “marijuana related” DUIs cited in the studies are actually cases of combined usage, and the driver’s impairment is not necessarily attributable to cannabis.

Taken all together, these factors make it impossible to say whether or not legalization actually results in more DUIs. No study has ever shown a direct relationship, let alone causality.

Does Legalization Increase Traffic Fatalities?

So how do we explain the increase in traffic fatalities seen in states which have legalized cannabis? There are a number of factors contributing to the rise in traffic fatalities, including:

Population growth. The more drivers on the road, the more traffic accidents. Economic growth. Accidents and fatalities often fluctuate along with GDP. That’s because economic growth means more jobs, more trucking and deliveries, and an overall increase in the number of drivers on the road, and miles traveled. Distracted driving. Cannabis legalization happens to coincide with the proliferation of smartphones, and the corresponding increase in drivers texting, using GPS, etc. while behind the wheel.

Due to these and other factors, the most important statistic to consider is not the total number of traffic fatalities, but rather deaths per vehicle miles traveled. And when you look at the number of traffic fatalities per VMT, you see a spike in the 2014 – 2015 — across the entire U.S.:

Obviously, marijuana legalization can’t be blamed for a nationwide increase in traffic fatalities, because each state has vastly different laws regarding cannabis regulation and enforcement. Bottom line: marijuana legalization has not been shown to increase traffic fatalities.

I don’t want to give the wrong impression here, so let me say for the record that driving under the influence of any mind-altering substance is dangerous and irresponsible. But when it comes to accidents and fatalities, alcohol is the real danger. Every night of the week you can find people going out to the bar or the club, having a few drinks, and then driving themselves home afterwards.

Pot smokers are much more likely to stay home and chill than get behind the wheel.