Driving Under the Influence: The Dangers of Marijuana-impaired Driving

marijuana plant Over the past couple weeks, we’ve examined the issue of driving under the influence, discussing the role of strict laws in reducing the number of drunk drivers on the road and looking at some new approaches to keeping people from driving when they’re intoxicated . But as we consider some potential solutions, we also have to acknowledge that the problem itself is changing, as drug impairment is becoming nearly as common as alcohol impairment . For instance, one study of NHTSA crash data has found that 40% of fatally injured drivers tested positive for drugs in 2013–nearly as many as tested positive for alcohol! In addition, in a recent roadside survey, 22% of drivers tested positive for the presence of some drug (medications included).

In particular, the effect of marijuana on drivers has come under special scrutiny as states around the country continue to relax their prohibitions of it. Already, 23 states have legalized medical marijuana and four states (Alaska, Colorado, Oregon, and Washington), as well as Washington, D.C., have also decriminalized the recreational use of marijuana. Yet even before these legalization efforts began, marijuana was the drug (other than alcohol) most frequently detected in drivers; now, with marijuana legalization efforts continuing to gain steam, it’s worth wondering whether the drastic drop in alcohol-related crash fatalities that we’ve seen over the past 30 years is about to be reversed by a drastic rise in marijuana-impaired driving.

Unfortunately, we don’t yet have enough reliable and consistent data to draw any broad conclusions about marijuana-impaired driving. Part of the reason for this is that even though many states have relaxed their own policies, marijuana remains classified as an extremely dangerous Schedule 1 drug by the federal government, a decision that makes it much more difficult to conduct scientific tests to determine its effects. Nevertheless, one recent study seems to demonstrate that, while marijuana does impair driving skills to some degree, the nature of the problem is fundamentally different from that of drunk driving.

driving test In the study, funded by the National Institute on Drug Abuse, the Office of National Drug Control Policy, and the NHTSA, participants operated a driving simulator for 45 minutes, during which they had to steer and adjust their speed while maneuvering over different road conditions and hearing realistic highway sounds. The study tested drivers with a peak BAC of 0.065%, as well as with a peak blood THC blood concentration of 13.1 µg/L (THC is the main psychoactive chemical in marijuana); one important aspect of this study is that it looked at THC levels at the time of driving, rather than hours after the fact (as is often the case with police-conducted tests on drivers).

The researchers found that both alcohol-impaired drivers and marijuana-impaired drivers weaved within their lane more often . However, the speed of weaving–as well as the number of actual instances of drivers leaving their lane– increased only with alcohol impairment . And it’s worth noting that while alcohol impairment was tested at a BAC of 0.065%, an amount lower than the legal limit in the U.S., marijuana impairment was tested at a blood concentration significantly higher than the legal limit of 5 µg/L THC that’s been established in states where the drug has been legalized, like Washington and Colorado.

Another significant finding reported in this study was that while drivers seem less endangered by the effects of marijuana than by the effects of alcohol, if taken together the effects of both were amplified: when alcohol was present, the marijuana high was more intense, and when marijuana was present, the peak of alcohol intoxication was delayed, making it harder for drivers to gauge just how drunk they really were.


Marijuana-impaired Driving: Some Important Facts

h-male-driving-city According to the NHTSA, drivers remain impaired by marijuana for up to three hours after taking it; common effects associated with marijuana include increased reaction time, reduced coordination and car handling skills, and greater susceptibility to fatigue and distraction. Moreover, a recent review of existing research into marijuana-impaired driving found that the practice doubled the risk of being involved in a collision resulting in serious injury or death.

Yet despite these facts, there is a growing body of evidence that, when it comes to driving, marijuana is not nearly as dangerous as alcohol. For instance, the findings of one U.S. Department of Transportation study mirrored the results of the driving simulator test discussed above, suggesting that while alcohol increased the risk of a crash by 7.4%, marijuana use increased the risk by only 0.7%; taken together, though, they were associated with an 8.4% increase in risk.

It’s also worth noting that, even if marijuana does double the risk of a serious collision, it’s still significantly safer than alcohol , as a BAC of 0.08% can increase a 20-year-old driver’s risk of death by almost 20 times , and a 34-year-old driver’s risk of death by nine times . In fact, after taking demographics and the presence of alcohol into account, researcher Eduardo Romano has even suggested that marijuana does not statistically increase the risk of a crash.

drunk man There’s no debate that marijuana increases impairment. The only question is why, given its effects, marijuana does not seem to have a greater impact on drivers. The explanation seems to have a lot to do with alcohol’s effect on judgment. For instance, while drunk drivers tend to drive faster and overestimate their skills, stoned drivers are typically able to recognize and compensate for their impairment. Moreover, among participants taking simple memory and arithmetic tests in a laboratory environment, alcohol use was associated with a much higher rate of failure than marijuana use.

In addition, because people are more likely to consume alcohol at bars and restaurants and more likely to consume marijuana at home, marijuana users were less likely to get behind the wheel when they were most impaired. And as the effects of marijuana are much more variable than the effects of alcohol, significant impairment is less likely to be demonstrated consistently across any given blood concentration of THC.


Dealing with Marijuana Impairment on the Road

female driver in emergency None of this should be taken to mean that it’s safe to drive under the influence of marijuana. For one thing, it’s been demonstrated that marijuana does significantly impair the ability to multitask, especially when the driver was confronted with something unexpected. Given that most driving emergencies are the result of something unexpected happening, this suggests that stoned drivers will be unable to respond effectively when they most need to. And some statistics do suggest that marijuana-impairment is making the roads more dangerous: for instance, the Washington Traffic Safety Commission found that between 2013 and 2014, when the state legalized marijuana, the number of fatal collisions involving a driver who tested positive for marijuana increased by nearly 50% .

Nevertheless, even as they attempt to keep marijuana-impaired drivers off the road, law enforcement officers are finding themselves faced by unique problems that aren’t associated with DUIs. Most notably, while breathalyzers make it easy for officers to detect a person’s level of alcohol impairment fairly accurately at the moment of the arrest, no such technology exists for marijuana. Instead, officers can test a driver’s saliva, though the results of these tests can vary so widely that they’re probably not reliable, or they can take the driver back to the station for a blood test, though their THC levels will have dropped dramatically by the time the test is taken. On the other hand, because THC is stored in the body’s fatty tissues, some chemical tests may indicate that a driver is high even if they haven’t consumed marijuana in days.

field sobriety test To figure out how to deal with this issue, it may be instructive to briefly look back at the history of drunk driving laws and get more perspective on how the current BAC limit of 0.08% was established in the first place. Initially, drunk driving laws relied on an officer’s testimony that the driver was showing signs of impairment on the road and later failed a field sobriety test (such as walking in a straight line or performing a simple coordination test).

At the same time, breathalyzer technology was developed that made it possible to determine the concentration of alcohol in the driver’s blood (a measurement known as BAC) with a simple breath test; over time, researchers were able to determine that, at a certain BAC level, drivers consistently demonstrated a dangerous degree of impairment. Only after years of research were the first laws using BAC to identify drunk drivers established; today, the BAC limit for adult drivers is 0.08% around the country. Legally, these limits are known as per se limits ( per se is Latin for “by itself”) because a BAC of 0.08% is enough, by itself , to demonstrate that the driver was drunk.

This history suggests that efforts to base the first marijuana-impairment laws on THC concentration levels may be misguided, as no reliable technology yet exists to determine such information and the effects of marijuana are so variable that there’s no specific THC level that has been consistently associated with impairment for every driver. In the words of Emily Wilfong, spokesperson for the Colorado Department of Transportation, “It’s not like alcohol. People metabolize it differently. There are different potencies. So there’s really no solution in terms of saying ‘you’re now at the limit.’”

Thus, while efforts   to create a “marijuana breathalyzer” have begun, state governments and law enforcement agencies shouldn’t rely on such technology until its accuracy has been confirmed and a consistent link between THC concentration and impairment can be established. Instead, these groups may be better off pursuing policies more like the earliest drunk driving laws, relying on officers to pull people over for driving that looks impaired and examine them for specific signs of marijuana use.

eye test The good news is that many law enforcement officers are already receiving the kind of training they need to spot marijuana impairment. Through training programs like Advanced Roadside Impaired Driving Enforcement, more and more officers are learning how to observe and document the effects of alcohol and different drugs on the body, allowing them to more reliably identify impairment among drivers. Some signs of marijuana use include a green tongue, dilated pupils, and an inability to cross one’s eyes, as well as evidence of impairment that’s associated with alcohol use as well, such as poor balance, slurred speech, and erratic driving. It’s important to note, though, that specific training in marijuana impairment is essential: while a standard field sobriety test will detect 88% of drivers under the influence of alcohol, it’s only effective on about 30% of stoned drivers.


Our Highest Priority: Safety

police pull over vehicle That marijuana use makes drivers less safe is undeniable, though experts aren’t sure exactly what the implications of increased marijuana use on road safety will be. What’s more clear from a law enforcement perspective, though, is that pursuing marijuana-impaired driving shouldn’t be our top priority. In fact, the Drug Enforcement Administration’s “2015 National Drug Threat Assessment Summary” found that : “America’s cops overwhelmingly do not see marijuana as a major threat to their communities.”

This attitude is supported by a number of facts. For instance, while the number of drivers testing positive for marijuana went up after Washington legalized marijuana, the total number of marijuana-related DUI arrests in the state actually went down! And in Colorado , where the drug was also legalized in 2013, traffic deaths have remained at or near their lowest levels since 2004, matching trends in 31 other states. While this doesn’t mean that marijuana is making the roads safer, it does suggest that the roads may not be becoming any more lethal as a consequence of marijuana legalization.

It’s also worth noting that in Colorado, drunk drivers are still being arrested far more frequently than stoned ones; for instance, in 2014 Denver police arrested 2,619 people for drunk driving–and only 66 for marijuana-impaired driving. Accordingly, many experts believe that our resources would be better spent on further combating drunk driving than on developing a more comprehensive system for addressing people driving while high. Dr. Eduardo Romano, for one, thinks reducing the per se BAC limit from 0.08% to 0.05% or lower would have a much more significant effect on road safety. “I’m not saying marijuana is safe,” he explained . “But to me it’s clear that lowering the BAC should be our top priority. That policy would save more lives.”

couple wearing seat belts In fact, the biggest reason that marijuana is a danger to drivers may be the effect it has on people who have also been drinking. As we’ve seen, marijuana use significantly amplifies the effect of alcohol and is associated with an increased risk of death even at lower BAC levels. According to Dr. Gary Reisfield, professor psychiatry at the University of Florida, “Alcohol is the deadliest drug we have by practically any metric…and alcohol in combination with [marijuana] is particularly malignant.”

Make no mistake: getting behind the wheel after consuming marijuana is a dumb idea. Marijuana is dangerous. It’s associated with serious health risks and makes you less able to drive safely. If it’s illegal where you live, don’t do it, and if it’s not illegal, never drive if you do. But as dangerous marijuana-impaired driving is, if you come away from this discussion with one conclusion, it shouldn’t be that stoned driving is safe. Rather, it should be that drunk driving is so dangerous that nothing else impairs driving, or increases the risk of death on the road, at anywhere close to the same degree. Always drive sober, and do your part to make driving a little safer for everyone on the road.

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