Driving while high is not so risky

Hey man, how's my driving?

Driving while high may not be as dangerous as you think. In fact, a survey of the available research indicates that moderately high drivers are safer on the road than many sober drivers.

Traffic safety cited as major concern

Many cannabis prohibitionists cite traffic concerns as a reason for opposing the re-legalization of cannabis. For example, during a Congressional hearing entitled “Planes, Trains and Automobiles: Operating While Stoned,” Rep. John Mica (R-Fla.), chairman of the House Subcommittee on Government Operations, said stronger federal regulations on marijuana were needed in light of the wave of states legalizing the drug for medical and recreational uses.

“In the last dozen years,” the Florida Republican said, “we’ve had [half] a million Americans slaughtered on the highways…and half of those fatalities are related to people who are impaired through alcohol or drugs.” Legal pot will compound this “phenomenal devastation,” he said, since “we are going to have a lot more people stoned on the highway.” (Source: Forbes)

Congressional Republicans are not the only ones worried about stoned drivers.  For example, the Rocky Mountain High Intensity Drug Trafficking Area lists as its first finding:

Overall, traffic fatalities in Colorado decreased 14.8 percent, from 2007 to 2012. During the same five years in Colorado, traffic fatalities involving operators testing positive for marijuana increased 100 percent. (Source: The Legalization of Marijuana in Colorado: The Impact Volume 2 Aug 2014)

Statistics are misleading

As worded the finding quoted above sounds horrific, implying that the number of traffic fatalities attributable to stoned driving has doubled. However, carefully digging through the report reveals some very important flaws. Data prior to 2012 were incomplete; only about half of all drivers involved in traffic fatalities were tested. Of course, you would expect totals to double once you have gone from measuring half of the drivers to measuring them all. And this is precisely what the report found. However, the problems inherent in using blood tests to identify cannabis-impaired drivers makes the entire metric moot anyway.

The RMHIDTA report is based on data from the National Highway Traffic Safety Association’s (NHTSA) Fatality Analysis Reporting System (FARS). This dataset provides a wealth of information on fatal crashes, the roadways, vehicles, and drivers involved. However, the dataset has its limitations, as NHTSA itself warns:

An important distinction to make when evaluating impaired driving data is the mere presence of a drug in a person’s system, as compared to the person being impaired by a drug in his/her system. FARS drug data provides information about drug presence, rather than whether the driver was impaired by a drug at the time of a crash […] The presence of some drugs in the body can be detected long after any impairment. For example, traces of cannabinoids (marijuana) can be detected in blood samples weeks after use. Thus, knowing that a driver tested positive for cannabinoids does not necessarily indicate that the person was impaired by the drug at the time of the crash. (Source: DOT HS 812 027)

Currently if the police suspect you are driving under the influence of alcohol they can administer a breathalyzer test, a simple procedure that can be performed roadside and which gives instant and accurate results. Currently no such instrument is available to test for cannabis impairment, so if an officer believes a driver is impaired then the driver must be taken to a medical facility for blood testing. Generally, blood tests will identify cannabis use within a several-hour window after use, but there are limitations:

  1. Chronic users can have constant residual traces lasting up to a week or more after the last use.

    In one study of chronic users, residual THC was detected for 24 to 48 hours or longer at levels of 0.5 – 3.2 ng/ml in whole blood (1.0 – 6.4 ng/ml in serum) [ … ] In another study of 25 frequent users, 36% showed no measurable blood THC throughout 7 days of abstinence, while the rest had at least one positive, though not necessarily on the first day. Six subjects (24%) had detectable blood THC after seven days at levels ranging from 0.2 to 1.5 ng/ml (that is, 0.4 to 3.0 ng/ml in serum) [ ,,, ]  (Source: California NORML)

  2. Inexperienced users who have no buildup of cannabis metabolites may test negative by the time blood is drawn even if they were impaired at the time they were driving, owing  to the inherent delays in the blood collection process.

Ironically, most studies agree that experienced users exhibit little to no impairment while inexperienced cannabis users are most likely to exhibit signs of impairment while driving, so the testing protocols will identify the less dangerous drivers while exonerating those most likely to pose a traffic hazard.

Per se limits

Per se limits are simply hard-coded legal limits; a driver testing at or above a per se limit is presumed to be guilty of driving under the influence regardless of the presence or lack of observable symptoms of impairment. The per se limit for alcohol is a blood-alcohol concentration (BAC) above .08 in all states.This level is considered high by some experts, as driving impairment can occur after just one drink. However, one advantage of a per se limit for alcohol is that it does at least represent a consistent level of impairment across individuals.

The story with cannabis is more muddled. In eleven states, the per se limit for cannabis is 0 ng/mL which effectively means that the smallest detectable amount is enough to result in a DUI. In Nevada, it is 2 ng/mL. In Colorado and Washington, the limit is 5ng/mL. Meanwhile, it is not clear that any per se limit is appropriate for cannabis use, since the level of metabolites is not closely correlated with impairment levels. Experienced cannabis users will often have amounts in excess of the 5 ng/mL limit even if they are not high on cannabis at the time of testing, owing to residual levels. Meanwhile, an inexperienced user can experience significant impairment on as little as 2 ng/mL.

Seattle television station KIRO 7 ran an interesting test in which three individuals were given measured amounts of cannabis and then asked to drive a test course under controlled conditions. The sample drivers include a very experienced medical cannabis patient, who arrived already medicated, a casual week-end recreational cannabis user, and an occasional user who used cannabis a few times a year. Each was able to successfully navigate the course at the legal limit of 5 ng/mL. They began exhibiting impairment in order of least experienced user to the most experienced, beginning at 4 times the legal limit for the occasional user, 6 times the limit for the week-end user, and a jaw-dropping 20 times the limit for the medical cannabis patient. Although this test was not constructed with the rigor of a true scientific study, its results are in line with the findings of most studies on the topic, which have found that experience cannabis users exhibit little or no driving impairment.

These studies suggest that per se limits are not appropriate for determining cannabis impairment, and that an outcomes-based metric might be more appropriate to detect impaired drivers, such as the field sobriety test still in use today to screen for drunk drivers.

Medical cannabis laws lead to less drunk driving

Although legal recreational cannabis is too new for definitive traffic data, medical cannabis has been around since the 90’s and we have accumulated enough data to draw some interesting conclusions. A University of Colorado study has found that medical cannabis states see 9% reduction in traffic fatalities. The study attributes this to an associated decrease in alcohol consumption, thus leading to a corresponding decrease in drunk driving.

A groundbreaking new study shows that laws legalizing medical marijuana have resulted in a nearly nine percent drop in traffic deaths and a five percent reduction in beer sales.

“Our research suggests that the legalization of medical marijuana reduces traffic fatalities through reducing alcohol consumption by young adults,” said Daniel Rees, professor of economics at the University of Colorado Denver who co-authored the study with D. Mark Anderson, assistant professor of economics at Montana State University.

(Source: http://www.eurekalert.org/pub_releases/2011-11/uocd-ssm112911.php)

Cannabis use leads to safer driving practices

Besides the reduction in drunk driving, cannabis use itself leads to safer driving habits. An NIH study finds that

marijuana smokers tend to compensate effectively for their impairment by utilizing a variety of behavioral strategies such as driving more slowly, passing less, and leaving
more space between themselves and cars in front of them.

Meanwhile, a study by the auto insurance industry concluded that:

20 years of study has concluded that marijuana smokers may actually be getting a bad rap and that they may actually have fewer accidents than other drivers.

This is supported by a National Highway Traffic Safety Association (NHTSA) study which shows that cannabis use results in slower driving while alcohol use results in faster driving speeds:

After THC administration, subjects drove significantly slower than in the control condition, while after alcohol ingestion, subjects drove significantly faster than in the control condition. No THC effects were observed after 24 h on any of the measures.

A UK study found that [d]rivers under the influence of cannabis seem aware that they are impaired, and attempt to compensate for this impairment by reducing the difficulty of the driving task, for example by driving more slowly. And a German study suggests that in persons with ADHD THC may have atypical and even performance-enhancing effects on driving.

These compensatory measures appear to be effective. According to a federal government study, cannabis users exhibit no actual impairment at mild to moderate levels of use:

Surprisingly, given the alarming results of cognitive studies, most marijuana-intoxicated drivers show only modest impairments on actual road tests. Experienced smokers who drive on a set course show almost no functional impairment under the influence of marijuana, except when it is combined with alcohol.

Meanwhile, according to that same study, alcohol-impaired drivers are responsible for 25% of all traffic deaths. The risk that cannabis users pose, however, is about the same as people who send text messages (and less than people who make cell phone calls).


Clearly, the level of concern expressed about cannabis use by drivers is out of proportion to the risks, as are the penalties for driving under the influence of cannabis. Since the risks that alcohol-impaired drivers pose to highway safety are about eight times the risks posed by cannabis users, it is patently unfair that both offenses be treated as having the same severity. Driving under the influence of cannabis should carry penalties more in line with distracted driving penalties, since it carries a similar risk profile. The public needs to be better educated on this issue as a means of encouraging support for more science-based policies.

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