Few subjects spark more heated debate than legalization ofmarijuana, whether for medical or recreational use.

Proponents argue that it's no more dangerous than alcohol, andthe medical benefits outweigh any dangerous side effects.

A related issue for drivers and law enforcement professionals isthe question of at what point is someone using cannabis impaired?It's not as easy to determine, and there isn't a Breathalyzer test— yet — that provides the same guidance as blood alcohollevels.

To put some science around the anecdotal evidence, theWashington, D.C.-based AAA Foundation for Traffic Safety conductedresearch on the subject, focusing on fatal crashes among drivers inWashington state from 2012 to 2014. Released May 10, thefoundation's report, “Prevalence of Marijuana Involvement in FatalCrashes: Washington, 2010–2014,” found that fatal crashesinvolving drivers who recently used marijuana doubled after thestate legalized the drug in December 2012.

Related: Drug-impaireddriving on the increase while drunk driving declines in theU.S.

Related research also showed that legal limits for marijuana anddriving are arbitrary and unsupported by science, the foundationsaid, which could result in unsafe motorists being allowed to driveand others being wrongfully convicted for impaired driving.

The research report, “AnEvaluation of Data from Drivers Arrested for Driving Under theInfluence in Relation to Per se Limits for Cannabis,”also released May 10, noted the increased concern shared by allsides regarding increased cannabis use, its impact on driverperformance and relationship to adverse effects on trafficsafety.

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(Source: AAA)

Washington was 'early adopter'

Washington was one of the first two states to legalize therecreational use of marijuana, and at least 20 states areconsidering marijuana legalization in 2016. The foundation examineddrug tests and fatal crashes among drivers in Washington, and theresearchers found:

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  • The percentage of drivers involved in fatal crashes whorecently used marijuana more than doubled from 8% to 17% between2013 and 2014.
  • One in six drivers involved in fatal crashes in 2014 hadrecently used marijuana, which is the most recent dataavailable.

“The significant increase in fatal crashes involving marijuanais alarming,” said Peter Kissinger, president and CEO of the AAAFoundation for Traffic Safety. “Washington serves as an eye-openingcase study for what other states may experience with road safetyafter legalizing the drug.”

Drawing on their experience with alcohol-related accidents, somestates have created legal limits, also known as per selimits, which specify the maximum amount of active THC that driverscan have in their system based on a blood test, similar to the 0.08legal limit for blood alcohol content, or BAC, that most people arefamiliar with. As the researchers note in both reports, THC is themain chemical component in marijuana that can impair driverperformance and affect the mind, and the presence of active THC isgenerally suggestive of recent marijuana use.

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(Photo: iStock)

THC levels don't equal blood alcoholcontent

The researchers examined the lab results of drivers arrested forimpaired driving, finding that the results suggest that legallimits for marijuana and driving are problematic for the followingreasons:

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  • There is no science showing that drivers reliablybecome impaired at a specific level of marijuana in theblood. Depending on the individual, drivers withrelatively high levels of marijuana in their system might not beimpaired, while others with low levels may be unsafe behind thewheel. This finding is very different from alcohol, where it isclear that crash risk increases significantly at higher BAClevels.
  • High THC levels may drop below legal thresholds beforea test is administered to a suspected impaired driver. Theaverage time to collect blood from a suspected driver is often morethan two hours because taking a blood sample typically requires awarrant and transport to a facility. Active THC blood levels maydecline significantly and could drop below legal limits during thattime.
  • Marijuana can affect people differently, making itchallenging to develop consistent and fair guidelines. Forexample, frequent users of marijuana can exhibit persistent levelsof the drug long after use, while drug levels can decline morerapidly among occasional users.

“There is understandably a strong desire by both lawmakers andthe public to create legal limits for marijuana impairment, in thesame manner as we do with alcohol,” said Marshall Doney, AAA'spresident and CEO. “In the case of marijuana, this approach isflawed and not supported by scientific research. It's simply notpossible today to determine whether a driver is impaired basedsolely on the amount of the drug in their body.”

Four states — Alaska, Colorado, Oregon andWashington — as well as the District of Columbiahave legalized the recreational use of marijuana, and 20 stateshave legalized it for therapeutic and medicinal use. Montana andWashington have implemented a limit for marijuana at 5 ng/mL;Nevada and Ohio have set a limit at 2 ng/mL; and Pennsylvania's isset at 1 ng/mL. Twelve states forbid the presence of any levels ofmarijuana. In Colorado, a blood concentration of 5 ng/mL or moregives rise to permissible inference that a person was driving underthe influence of the drug.

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(Photo: iStock)

Enforcement measures

To improve road safety, AAA is urging states to usemore-comprehensive enforcement measures. Rather than relying onarbitrary legal limits, the organization suggests that states use atwo-component system that requires

• A positive test for recent marijuana use.
• Behavioral and physiological evidence of driverimpairment.

As suggested by AAA, the proposed system would rely heavily ontwo current law-enforcement training programs: Advanced RoadsideImpaired Driving Enforcement and the 50-state Drug Evaluation andClassification program, which train law enforcementofficers to more effectively recognize drug-impaired driving.

“Marijuana can affect driver safety by impairing vehicle controland judgment,” continued Doney. “States need consistent, strong andfair enforcement measures to ensure that the increased use ofmarijuana does not impact road safety.”

Related: A tale of 2 studies on the effects of drugs andalcohol on driving