Taking Drugged Driving Seriously: What Does the Science Say?
David BienenstockPublished on September 21, 2017· Last updated July 28, 2020
A COVID boom in traffic deaths came from speeding—not legal weed —according to 2023 studies from Quartz and NHTSA show. (Leafly file photo)
I’ve been reporting on cannabis full-time for more than 15 years, so I like to think I’ve heard it all—pro and con—when it comes to the legalization debate. In all that time spent weighing facts and debunking disinformation, only one con argument has ever given me serious pause: What if a large number of newbie pot smokers suddenly get behind the wheel and all start riding dirty at once?
There are many other supposed cannabis dangers that would warrant being taken seriously, if a small bit of independent investigation didn’t reveal them to be overblown or baseless.
Not that cannabis is completely harmless, of course. But if smoking herb turned you into a scatterbrained, violent heroin addict with lung cancer, that would be a serious concern. As hard data makes plain, however, it’s just not what happens.
Is “stoned driving” any different?
Hot Button Issue
With Attorney General Jeff Sessions actively looking for excuses to crack down on legal cannabis, prohibition defenders are touting drugged driving as a reason to shut down the legal states.
Recently, the Denver Post published a major investigation of cannabis and driving. The story relied largely on data coming out of Colorado and Washington in the five years since those two states became the first to legalize the adult use of cannabis.
The series began with this headline: Traffic Fatalities Linked to Marijuana Are Up Sharply in Colorado. Is Legalization To Blame?
Well, Is Legalization to Blame?
Apparently nobody’s sure, because a smaller line directly below the headline stated: “Authorities say the numbers cannot be definitively linked to legalized pot.”
Authorities say the crash data can't be definitively linked to legalized cannabis.
In my experience, the authorities have never been shy about blaming a myriad of social ills on cannabis. So why the hesitance this time? And what, exactly, does the Denver Post mean when they describe traffic fatalities “linked to” marijuana? That’s an awfully vague term.
The Rocky Mountain High Intensity Drug Task Force—a federally funded law enforcement organization dedicated to suppressing illegal drugs—stated in a 2015 report that the term “marijuana-related” does not “necessarily prove that marijuana was the cause of the incident,” and applies “any time marijuana shows up in the toxicology report [of drivers]. It could be marijuana only or marijuana with other drugs and/or alcohol.”
Which means that if a drug test shows you smoked half a joint last week and drank a bottle of vodka twenty minutes ago, your car crash goes down in the books as “marijuana related.”
Deal With Facts, Not Fear
Before we fully delve into the confusing science of stoned driving, let’s start by stating the obvious. Operating a motored vehicle while dangerously impaired on any substance—whether legal or illegal—is rightfully a criminal act.
Compared to sober drivers, THC-impaired drivers have a 5% greater risk of crashing. Alcohol-impaired drivers under the legal limit (.08) have a 225% greater risk.
So when the Post asked if “legalization” was to blame for traffic fatalities, that was a skewed way of framing the question. Nobody would argue that alcohol legalization is responsible for a drunk driving accident. We rightly blame the drunk driver, both in the court of law and the court of public opinion.
Clearly, cannabis use can lead to driver impairment, which increases accident risk—but how much cannabis? And how much risk? That depends on a lot of factors.
While it’s literally impossible to fatally overdose on infused chocolates (unless you’re allergic to chocolate), you could fall asleep or space out at the wheel after eating them and cause a fatal accident. That’s a danger that imperils not just the driver, but anyone else in the car or on the road.
When Colorado legalized adult-use cannabis in 2012, it also included a per-se limit for drivers. State law specifies that “drivers with five nanograms of active tetrahydrocannabinol (THC) in their whole blood can be prosecuted for driving under the influence (DUI)… and no matter the level of THC, law enforcement officers base arrests on observed impairment.”
Look at the Full Data Set
Putting aside for a moment the notorious difficulty of measuring cannabis impairment through blood tests or officer observation, the Post’s analysis raised serious questions about cannabis use and drugged driving.
One of the key findings in the Post report was this startling statistic:
From 2013 to 2016, the number of drivers who tested positive for marijuana use jumped 145 percent — from 47 to 115.
That doesn’t sound good. But it’s a wholly misleading statistic. “Testing positive for marijuana” only means that cannabis metabolites remain in the driver’s blood, even though the driver may be completely sober. The body expunges alcohol within hours, but those non-impairing cannabis metabolites remain for days and even weeks. The test will register as metabolite-positive if the driver consumed cannabis anytime up to three weeks ago.
If Colorado officials conducted a similar test to find drivers who consumed alcohol within the past three weeks—if such a test existed—it would find 55% to 75% of the state adult population (the percentage range of people who consume alcohol at least once a month) register as alcohol-positive. But they’re no more “drunk” than a metabolite-positive driver is “stoned.”
In fact, the Post data is doubly misleading, because the statistics on cannabis metabolites actually predate 2013. Annual reports by the state’s Interagency Task Force on Drunk Driving published data on metabolite-positive drivers involved in fatal crashes during 2011 and 2012. Those numbers reveal 2013 not as a normal, pre-legalization baseline, but rather as a bit of an outlier–an unusually low year for metabolite-positive drivers in crashes.
What the full data set reveals is that the percentage of Colorado drivers who are involved in fatal crashes, and have consumed cannabis sometime in the past three weeks, pretty much mirrors the percentage of adults who consume cannabis in the population at large. Which is to say, around 12% to 13%. It was a little under 14% prior to legalization, it was a little over 14% after legalization. In between it fluctuated between 8% and 12%.
Colorado: Drivers in Fatal Crashes (click to enlarge)
Consider the Odds
While the Post report included pushback quotes from two representatives of prominent cannabis industry trade groups, they didn’t talk to anybody like Paul Armentano, Deputy Director of NORML. He’s a longtime expert on these matters, with the peer-reviewed papers to prove it.
Armentano argues—I believe convincingly—that “increased prevalence of THC detection in drivers tells us little about accident risk,” as it could simply be evidence of increased use among the general public, increased testing by law enforcement, or both.
For Armentano, there’s only one metric that really matters: Odds ratios.
“To determine what role, if any, a drug plays in motor vehicle accident culpability we need to looks at odds ratios, which estimate the probability of an event occurring (e.g., motor vehicle crash) over the probability that such an event does not occur,” he says. “Odds ratios greater than 1 indicate a positive relationship, with stronger relationships reflected by higher numbers.”
And guess what? Drivers who test positive for active THC—not merely inactive metabolites—do increase their risk of crashing. But that increased risk is small compared to alcohol—or compared to opioids, texting, phone use, or even the distracting company of two other passengers in the car. When Colorado saw an upsurge in traffic fatalities last year, this was the headline in the Denver Post: CDOT Director Blames Surge in Colorado Roadway Fatalities on an ‘Epidemic of Distracted Driving.’
The largest domestic case-control study to assess drugs and accident risk—published in a 2015 research note by the National Highway Traffic Safety Administration (NHTSA), a federal agency—found that the odds ratios for THC-positive drivers and crashes, when adjusted for drivers’ age and gender, came out to 1.05. That means THC-positive drivers have a 5% greater crash risk than drivers with no drugs or alcohol in their system.
Context and Relative Risk
It’s worth taking a closer look at that 2015 NHTSA study, because federal officials put a lot of stock in it as “the first large-scale [case control crash risk] study in the United States to include drugs other than alcohol.” Data was collected from more than 3,000 crash-involved drivers and 6,000 control drivers (not involved in crashes) over a 20-month period in Virginia Beach, Virginia. The data was fresh and solid: Research teams responded to crashes 24 hours a day, 7 days a week. Drivers were considered THC-positive if they tested for active THC, not for non-impairing metabolites still in their blood days or weeks after consumption.
While THC-positive drivers were 5% more likely to be involved in a crash, the researchers found that drivers who’d taken an opioid painkiller had a 14% greater risk of crashing. Here’s a chart from that NHTSA study comparing THC (marijuana) with opioids (narcotic analgesics) and other drugs:
Source: “Drug and Alcohol Crash Risk,” Compton and Berning, NHTSA Traffic Safety Facts Research Note, Feb. 2015
Those levels of increased risk were tiny, however, compared to the risk involved with alcohol. Drivers within the legal range of blood alcohol level as registered by a breathalyzer (BrAC) were found to be 20% to 222% more likely to be involved in a crash. At .08 BrAC, the legal limit, the risk increased to 293%. At 0.15 BrAC, drivers were more than 12 times (+1118%) more likely to be involved in a crash than a sober person. Here’s a chart from that same study, calculating the increased risk of crashing at rising blood alcohol levels:
Source: “Drug and Alcohol Crash Risk,” Compton and Berning, NHTSA Traffic Safety Facts Research Note, Feb. 2015
By comparison, a driver who has taken penicillin is 25% more likely to be involved in a crash. Drivers carrying two or more passengers are 120% more likely to crash. Drivers using mobile phones to talk or text are 310% more likely to crash.
A separate NHSTA study (“Marijuana And Actual Driving Performance”) further conceded it’s “difficult to establish a relationship between a person’s THC blood or plasma concentration and performance impairing effects … Drivers with high concentrations showed substantial [impairment], but also no impairment, or even some improvement.” In other words, cannabis affects different drivers in different ways, depending on a number of factors.
Favoring One Set of Data Over Another
Strangely, the Denver Post’s analysis relies heavily on data compiled by the NHSTA, yet they never mention these striking findings from the very same federal agency.
My own theory on that wide range of responses (from “substantial impairment” to “some improvement”): Cannabis affects inexperienced users very differently than seasoned consumers. A 2010 study published in the journal Psychopharmacology concluded that “heavy cannabis users develop tolerance to the impairing effects of THC on neurocognitive task performance.” And a 2012 study in the Journal of Analytical Toxicologyconfirmed finding “minimal impairment in driving-related psychomotor tasks in chronic daily cannabis users.”
So while the correlation between blood alcohol concentration and impairment is relatively consistent for most people, it may be impossible to establish a THC test that can truly gauge impairment the way a breathalyzer can for booze. Though not for a lack of trying, which creates the danger of severely punishing drivers simply for being cannabis consumers, not for driving while impaired.
A New Form of Prohibition
Because if it becomes essentially illegal to drive to work the morning after smoking a joint, then it becomes essentially illegal to smoke a joint—at least for the vast majority of us who are far more addicted to our cars than we ever could be to cannabis.
Speaking of automobile addiction: What’s with those car-junkies over at AAA (a.k.a. “Triple A”) lobbying against legalization and pushing “grossly distorted” data . According to a Leafly report, “the organization’s newly embraced anti-legalization stance is a hard turn from AAA’s previous position—which is to say, no position at all.”
Maybe it’s time for all AAA members who care about this issue to contact them and demand they start telling the truth.
The (Positive) Substitution Effect
For instance, what about the idea—hidden in all the data we’ve examined so far—that increased cannabis use could actually be making our roads safer by serving as a substitute for more dangerous behavior.
Increased cannabis use could be making our roads safer by decreasing alcohol intake.
Meaning that while cannabis use in and of itself does increase crash risk, in a zero sum game where someone’s either drinking beer, popping pills or smoking weed, then cannabis is most certainly the safest of those risk factors. A dynamic that, writ large, can have a sizable positive effect.
For example, one 2011 study found that widespread use of legally accessible medical marijuana actually produces a major improvement in public safety because of a correlated reduction in drinking and driving, and an overall reduction in opioid use.
“Specifically, we find that traffic fatalities fall by nearly 9 percent after the legalization of medical marijuana,” concluded University of Colorado Professor Daniel Rees and Montana State University Assistant Professor D. Mark Anderson.
So, to sum up—no, you definitely don’t drive better stoned, especially in high doses. And double-especially if you’re not used to being stoned, or to driving, or to both. Infrequent users of cannabis incur a higher risk of crashing based on the increased motor impairment that comes along with having less experience with THC and its effects.
Most experts recommend waiting at least three hours after your last inhale of cannabis before driving, and waiting far longer if you’ve eaten edibles, since they can sometimes take two hours before the onset of effects, which can then last six hours or longer. Also, please be aware that mixing alcohol and cannabis is more dangerous than using either alone. And don’t ever smoke in a moving vehicle, as it’s irresponsible and also the easiest way to get busted.
Oh, and if you happen to be a passenger in a car heading out for a long road trip, then I highly recommend getting really, really blazed before getting into the car, and then bumping some killer driving music once you hit the highway.
