Related: Drug-impaired driving on the increase while drunk driving declines in the U.S.

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

The research report, “An Evaluation of Data from Drivers Arrested for Driving Under the Influence in Relation to Per se Limits for Cannabis,” also released May 10, noted the increased concern shared by all sides regarding increased cannabis use, its impact on driver performance and relationship to adverse effects on traffic safety.

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(Source: AAA)

Washington was 'early adopter'

Washington was one of the first two states to legalize the recreational use of marijuana, and at least 20 states are considering marijuana legalization in 2016. The foundation examined drug tests and fatal crashes among drivers in Washington, and the researchers found:

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  • The percentage of drivers involved in fatal crashes who recently used marijuana more than doubled from 8% to 17% between 2013 and 2014.
  • One in six drivers involved in fatal crashes in 2014 had recently used marijuana, which is the most recent data available.

“The significant increase in fatal crashes involving marijuana is alarming,” said Peter Kissinger, president and CEO of the AAA Foundation for Traffic Safety. “Washington serves as an eye-opening case study for what other states may experience with road safety after legalizing the drug.”

Drawing on their experience with alcohol-related accidents, some states have created legal limits, also known as per se limits, which specify the maximum amount of active THC that drivers can have in their system based on a blood test, similar to the 0.08 legal limit for blood alcohol content, or BAC, that most people are familiar with. As the researchers note in both reports, THC is the main chemical component in marijuana that can impair driver performance and affect the mind, and the presence of active THC is generally suggestive of recent marijuana use.

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(Photo: iStock)

THC levels don't equal blood alcohol content

The researchers examined the lab results of drivers arrested for impaired driving, finding that the results suggest that legal limits for marijuana and driving are problematic for the following reasons:

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  • There is no science showing that drivers reliably become impaired at a specific level of marijuana in the blood. Depending on the individual, drivers with relatively high levels of marijuana in their system might not be impaired, while others with low levels may be unsafe behind the wheel. This finding is very different from alcohol, where it is clear that crash risk increases significantly at higher BAC levels.
  • High THC levels may drop below legal thresholds before a test is administered to a suspected impaired driver. The average time to collect blood from a suspected driver is often more than two hours because taking a blood sample typically requires a warrant and transport to a facility. Active THC blood levels may decline significantly and could drop below legal limits during that time.
  • Marijuana can affect people differently, making it challenging to develop consistent and fair guidelines. For example, frequent users of marijuana can exhibit persistent levels of the drug long after use, while drug levels can decline more rapidly among occasional users.

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

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

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(Photo: iStock)

Enforcement measures

To improve road safety, AAA is urging states to use more-comprehensive enforcement measures. Rather than relying on arbitrary legal limits, the organization suggests that states use a two-component system that requires

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

As suggested by AAA, the proposed system would rely heavily on two current law-enforcement training programs: Advanced Roadside Impaired Driving Enforcement and the 50-state Drug Evaluation and Classification program, which train law enforcement officers to more effectively recognize drug-impaired driving.

“Marijuana can affect driver safety by impairing vehicle control and judgment,” continued Doney. “States need consistent, strong and fair enforcement measures to ensure that the increased use of marijuana does not impact road safety.”

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