Pushing the Limit: How THC Levels Affect (Or Don’t?) Driving Abilities

By: Bridget Clerkin December 19, 2015
As marijuana legalization expands, officials and experts are pondering the proper measurements for driving while stoned.
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A debate over going green has broken out in Maine, and it has nothing to do with clean energy.

The Pine Tree State is seriously contemplating putting the legalization of recreational marijuana on the 2016 ballot, a move that would not only allow the state to appease an expanding list of residents who support the initiative but, if executed, give Maine the chance to tap into the fastest growing industry in the country – at an estimated profit of $26.7 million annually.

In addressing the possibility of such a radical change, the state’s legislature has begun the arduous task of setting boundaries in a vague environment and found itself zeroing in on one question in particular: How much marijuana is too much to drive?

It may seem like a simple query, but it’s one that’s caused heated discussions in every state that has so far adopted a recreational marijuana system.

In Maine, the Secretary of State’s office recently submitted a bill with its official answer: 5 nanograms or more of tetrahydrocannabinol (THC), the active element in marijuana, in a person’s blood. According to the legislation, which has not yet been up for a vote, tests showing positive for this measurement would be legally tantamount to DUI.

Numbers cultivated in Colorado since the state adopted legalization show a spike in drivers found with THC in their system, but a relatively flat growth rate of traffic fatalities.

Adopting the measure—should recreational marijuana be legalized in Maine—would put the state in line with the movement’s two pioneers, Colorado and Washington State, which both use the same THC limit for drivers. (Oregon, Alaska, and Washington, D.C., the other areas which have legalized recreational marijuana, have yet to set a limit and currently rely on other factors of detection.)

But a task force created by the Maine legislature to analyze the figure is divided over whether it should stand. They join a growing chorus of advocates and scientists who believe there’s no easy solution to the issue.

And with nearly a dozen more states poised to put legalization on the ballot in the coming years, Maine’s decision could create a trend that may be followed throughout the country.


The image of marijuana-addled drivers in pop culture hasn’t changed much since Cheech and Chong jumped into their jalopy with the shaggy interior: billowing smoke pouring from the windows as bleary-eyed drivers attempt to stay between the lines.

The reality of the situation is also hazy. While the National Highway Traffic Safety Administration recently found that smoking marijuana does increase the risk of crashing, they added that marijuana smokers were typically bigger risk-takers in general, making it difficult to parcel out exactly how much more dangerous the subjects became while driving high.

Other studies have found that marijuana smokers tend to drive more slowly than those who drive drunk – giving scientific bolstering to a parody made famous by Cheech and Chong themselves: the stoned driver who’s pulled over for going 20 miles per hour on the highway.

Where drivers under the influence of marijuana have difficulty, these studies suggest, is when multitasking becomes necessary or when they’re forced to react to something unexpected.

In a typical example of how complicated the debate has become, one study has found that stoned driving could be related to an increase in traffic fatalities, while another study claims that its effect on the number of driving deaths is negotiable at best.

Numbers cultivated in Colorado since the state adopted legalization show a spike in drivers found with THC in their system, but a relatively flat growth rate of traffic fatalities. Still, experts advise that it hasn’t been long enough for such numbers to show any significant patterns.

The most reliable figure in the realms of research on the subject is a twofold increase in the likelihood that anyone with any THC in their blood will cause an accident, which has been found independently by several studies, according to Dr. Marilyn Huestis of the National Institute on Drug Abuse. It’s a significant factor, she told the New York Times, but not as significant as researchers once imagined, and not nearly as significant as the effects of alcohol on driving, which can increase the risk of crashing by up to twentyfold.

Experts are split on how to deal with the findings and disagree over whether driving while high is a problem that should be exclusively focused on or if resources would be better spent fighting the seemingly more perilous act of driving drunk.

Institutionalize It

Despite the debate over how much of a risk it is to drive while stoned, scientists and policymakers alike agree that driving under the influence of anything is a bad idea, which makes the creation of some sort of regulation a necessity.

The 5 nanogram figure has given Colorado and Washington a watermark to utilize, but the reading may be misleading due to several factors, researchers and advocates say.

One issue is the absorption pattern of the drug—especially for heavy users, such as those who utilize medicinal marijuana, which is currently legal in 23 states.

It takes much longer for THC to leave the blood stream than alcohol, and regular smokers can test positive for the substance several days after their last inhale – “well beyond the duration of impairment,” according to Paul Armentano, Deputy Director of the National Organization for the Reform of Marijuana Laws (NORML).

This effect is typically not seen in casual users, he added, which “rais[es] concerns that proposed estimates are unlikely to be consistently applicable to individual subjects.”

Heavy smokers are also more likely to build a tolerance to the drug, and those with a 5-nanogram reading may have little-to-no problem operating a vehicle, some researchers say.

On the other hand, studies done in Europe suggest that the 5-nanogram limit is far too high, and would miss 90% of impaired-driving cases in Sweden, according to Huestis, of the National Institute on Drug Abuse.

Further compounding the issue is the cumbersome process of determining someone’s THC level, which is found through blood or urine tests, often taken hours after an arrest, as opposed to a Breathalyzer test, which can be easily administered at the scene.

In response to these concerns, law enforcement in all 5 states and territories where marijuana has been legalized have received training on detecting signs of cannabis impairment – often funded by pot smokers themselves, through taxes collected from sales of marijuana.

But inconsistencies arise in these types of analyses, as well. A 2012 study by the journal Psychopharmacology showed that just 30% of drivers under the influence of marijuana would fail a common field sobriety test, consisting of feats like following a pen with one’s eyes, walking nine steps from heel to toe, and standing on one leg for 30 seconds.

Looking Ahead

So what, if anything, can be done to solve the problem?

One California company, Hound Labs Inc., is hoping to provide the solution in the form of a THC Breathalyzer test. While the device has yet to endure clinical trials, the company promises its technology is sophisticated enough to detect even tiny amounts of THC through the breath.

Still, it does little to settle the debate over the 5-nanogram limit itself. To work around that grey area, other states poised to legalize recreational marijuana may turn to more nuanced laws surrounding the drug.

In fact, many states already have laws concerning “drugged driving,” which take different stances on tolerance and punishment.

While some, like Colorado and Washington, set a standard limit that will trigger a DUI—called a “per se” law, others employ so-called “effect-based” laws, which rely on detection of visible impairment. A handful of states, such as Arizona and Oklahoma, adhere to zero-tolerance policies, which call for DUIs to be issued to anyone showing any trace of an illegal substance in their system.

Armentano, of NORML, even suggests that having drivers use apps intended to track mental performance may be a helpful way to separate those who are truly impaired from those with higher tolerances.

But despite the ultimate decision each state makes on the subject, at least two things remain clear: If you are convicted of drugged driving it will impact your car insurance rate the same way as a DUI conviction. And if Google gets its way to have driverless cars take over the roads, none of these laws may soon matter much.

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