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Marijuana testing flawed, may not detect impaired driving

The legalization of marijuana use has posed new criminal defense issues. A 2016 study indicates that testing for marijuana, particularly marijuana-impaired driving, is flawed. This report may challenge the validity of tests used by Nevada law enforcement for prosecuting motorists under the state's operating under the influence laws.

Recent legalization of recreational marijuana use in Nevada and other states has prompted scrutiny of its effect on driving and impairment of motorist skills. Nevada, like other states, tests suspected drivers for levels of the cannabis' active component, tetrahydrocannabinol. Unlike breathalyzer tests for suspected drunk drivers, more intrusive blood tests are used. In Nevada, the presence of THC in the blood exceeding a legal limit constitutes a criminal offense, even without proof of impaired driving.

The study, conducted by the AAA Foundation for Traffic Safety, found several flaws after evaluating data from drivers arrested on a DUI charge involving the presence of THC. These arrests include the suspect's performance on the Drug Recognition Expert exam. The DRE is an assessment of physiological standards and psychophysical examinations including the Standardized Field Sobriety Test. The performances of motorists suspected of marijuana-impaired driving were compared against a control group of drug-free drivers.

Drivers who undertook the DRE showed poorer performance in tests involving walking and turning, one-legged stand and placing finger to nose. They also had bloodshot and watery eyes and eyelid tremors that was much less prevalent in the control group.

The drivers suspected of drug-impaired driving and the drug-free drivers underwent a blood test. However, these blood test results misclassified a substantial number of motorists as being impaired even though they did not demonstrate any impairments on the SFST.

The research concluded that there is no set level of THC in the blood stream that could, alone, constitute evidence of impaired driving. In other words, laws setting a specific level of THC as constituting a criminal offense are unscientific.

These offenses may have long-term consequences for those who are convicted. An attorney can help a suspected impaired driver defend against these charges and attack the validity of testing, where appropriate. Being aggressive when facing such allegations could result in an acquittal, dismissed charges, or lesser penalties. No outcome can be guaranteed, though, so criminal defendants need to do everything they can to ensure their legal rights are protected.

Source: AAA Foundation for Traffic Safety, "Fact sheet: An evaluation of data from drivers arrested for driving under the influence in relation to per se limits for cannabis," Accessed March 26, 2017

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