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  • By: Kevin Leckerman, Esq.
  • Published: June 30, 2015
A man sleeping on a table with a bottle of alcohol nearby - Leckerman Law, LLC

According to a recent study conducted and funded partly by the federal government, alcohol has a greater impact on drivers as compared to marijuana.

Researchers carried out this study to understand and find the impact of alcohol and cannabis on drivers and concluded that alcohol had a significant impact as compared to cannabis. Drivers who had consumed alcohol had “significantly increased lane departures/minimum as well as maximum lateral acceleration; these measures were not sensitive to cannabis.” The research also concluded that cannabis-influenced drivers attempted to drive more cautiously “to compensate for impairing effects, whereas alcohol-influenced drivers often underestimate their impairment and take more risk.”

The study was funded by the National Institute on Drug Abuse, the Office on National Drug Control Policy, and federal safety regulator, the National Highway Traffic Safety Administration, along with the federal government.

A number of methodologies and technologies were used to conduct the study, including the most sophisticated driving simulator of its kind to mirror real-life situations, used by the National Institute on Drug Abuse.

19 adults participated in the study, including 13 men, and were divided into three groups. The first group of participants drank alcohol to reach a breath alcohol concentration of 0.065% on the breath analyzer. The second group inhaled vaporized marijuana while the third group had a placebo. Most of the participants consumed cannabis more than two times a month but not more than 3 times a week.

Participants had to take part in a 45 minute driving session inside the driving simulator. During these sessions, researchers noted how many times the car left the lane, how many times the driver weaved and how many times the driver sped their car. The study was conducted in Iowa city.

The driving simulator consisted of a 1996 Malibu sedan mounted in a dome with a motion system. Drivers participating in the driving sessions could experience acceleration, braking, steering cues, road conditions like gravel and more. Researchers observed that drivers who had consumed alcohol left the lane and sped and weaved an increased number of times as compared to those who took marijuana.

Researchers found that although marijuana has an effect on drivers, this effect was less dramatic than the effect of alcohol. Marijuana reduced the drivers peripheral vision, giving them tunnel vision. A concentration of 13.1 µg/L THC in the bloodstream of drivers caused increased weaving within lanes. This effect is similar to the effect shown by drivers with a BAC of 0.08%, which is the legal threshold in many states.

A combination of both alcohol and marijuana resulted in an enhanced effect. These drivers weaved within lanes even with THC and alcohol concentrations below the impairment thresholds for each substance.

Medical marijuana is legal and approved in 23 states as well as in the District of Columbia. Washington DC and four states have legalized recreational cannabis for adults.

As restrictions on the use of marijuana loosen in different states, law enforcement and safety regulators struggle to figure out how a legal limit for marijuana can be established for drivers, similar to the 0.08% limit for alcohol in most states.

Apart from observing the effects of alcohol and marijuana on drivers, the study also presented challenges faced in accurately testing drivers after they have consumed an intoxicant as well as developing a threshold which will be considered “legal” to drive.

According to observations, THC levels in the blood drop rapidly within two to four hours after consumption which is the time to collect a blood sample. Oral tests which use the drivers’ saliva can be done at the roadside, but they are not reliable and do not provide a precise measure of the level of impairment. Similarly, THC may be detected in blood of individuals for up to a month after consumption in chronic frequent smokers even after they have abstained during this time.

The effects of marijuana on driving impairment remain elusive after the study.

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