SWOV Catalogus


Cannabis use among drivers suspected of driving under the influence or involved in collisions : analyses of Washington State patrol data.
20160283 ST [electronic version only]
Banta-Green, C. Rowhani-Rahbar, A. Ebel, B.E. Andris, L.M. & Qiu, Q.
Washington, D.C., American Automobile Association AAA Foundation for Traffic Safety, 2016, 33 p., 28 ref.

Samenvatting In 1999, Washington State voters approved the legalization of medical marijuana. In November 2012, Washington voters passed Initiative-502 (I-502), legalizing retail cannabis sales and recreational cannabis use for adults 21 years and older. As with alcohol, the law provides two options for prosecuting suspected impaired drivers: 1) demonstrating impairment through detailed observation notes, field test results, witness observations, or Drug Recognition Expert assessments; and 2) determining the suspect’s blood level for the drug is above the legal "per se" limit. I-502 established a per se level of 5ng/mL of active delta-9-tetrahydrocannabinol (hereafter THC) in blood for cannabis-impaired driving. THC is a psychoactive compound in cannabis. The objectives of this study were to examine drivers involved in collisions and/or arrested for suspected driving under the influence (DUI), who were investigated by the Washington State Patrol and for which blood evidence was collected in order to: describe the trends in THC involvement over time and in relation to the passage of I-502; to describe the prevalence of THC alone and in combination with alcohol and other potentially intoxicating drugs; and to describe the estimated time to blood draw under real world conditions, and examine the relationship between estimated time to blood draw and the level of THC detected. Additionally, to provide necessary context, law enforcement and toxicology testing procedures as well as arrests, state patrol staffing levels, and training over the study period were documented. Semi-structured interviews were conducted with law enforcement, prosecutors, and toxicology laboratory staff. Documents were reviewed to determine DUI arrests, law enforcement staffing and training over time. Data from the Washington State Patrol’s toxicology laboratory, dispatch, and officer activity log were linked. Longitudinal analyses were conducted to test trends over time from 2005 to 2014 for the presence and level of THC (excluding alcohol-involved cases due to changes in laboratory procedures in 2013). We also explored whether there was a change in the presence of THC following the passage of I-502. Driver characteristics including drugs detected were explored for collisions and/or for those with suspected DUIs that did not involve a collision. An estimated time to blood draw (ETBD) variable was created from data in the computer automated dispatch system. The relationship between the estimated time to blood draw and measured THC level was tested. Law enforcement staffing and training, arrests, policies and procedures: * From 2009-2014, the overall number of Washington State Patrol (WSP) troopers assigned to traffic enforcement was relatively unchanged. However, there was an increase in the number of state patrol officers with specialized training in Advanced Roadside Impaired Driving Enforcement (109 in 2009, compared with 669 in 2013) with the potential impact of increased sensitivity and ability to identify THC-impaired driving. In April of 2013, the Missouri v. McNeely court opinion was issued which essentially required a warrant for DUI-related blood tests. Washington State Patrol DUI-related arrests declined from 2012 onwards. Trends in THC-involved driving: * Between 2005 and 2014, the proportion of Washington State DUI and collision cases tested by toxicology, excluding those positive for alcohol, that involved THC increased significantly, from 20 percent to 30 percent. Among these cases, the prevalence of THC continued to grow after passage of I-502 in 2012, but at a significantly slower pace. * The median blood level of THC increased significantly from 4.0ng/mL in 2005 to 5.6ng/mL in 2014 (p for trend = 0.015). Prevalence of THC in collisions and suspected DUIs: * Among drivers for whom blood evidence was submitted following a collision, 11 percent were positive for THC in conjunction with another potentially impairing substance (alcohol or other drugs). An additional 4 percent were positive for THC only. The majority (53%) of collision involved drivers were under the influence of alcohol at a level of 0.08 g/dL or higher, and 7 percent met or exceeded the per se level of THC, 5ng/mL. * Among drivers suspected of DUI in the absence of a collision, 11 percent were positive for THC in conjunction with another potentially impairing substance. An additional 26 percent tested positive for THC only. Non-collision-involved drivers arrested for DUI were most commonly under the influence of alcohol at 0.08 g/dL or above (30%). Among these drivers, 20 percent had a THC level of 5ng/mL or above. Estimated time to blood draw: * he median time to blood draw for all cases was 165 minutes. *The median estimated time to blood draw for THC-positive drivers (among collisions and non-collisions) was 139 minutes. Estimated time to blood draw was significantly longer for those positive for the inactive metabolite carboxy-THC, but not THC, at the time of testing (175 minutes). * The measured THC blood level for the population studied declined 5ng/mL on average during the first 120 minutes from contact with police. *The proportion of those with an estimated time to blood draw of less than 2 hours who had a THC blood level greater than or equal to 5ng/mL was 26 percent compared to 10 percent for those with an estimated time to blood draw of 2 hours or more. Evaluating the impacts of cannabis legalization on the prevalence of THC detection in blood evidence from collisions and suspected DUI cases is complicated by historical factors related to other laws, policies liberalizing cannabis access and use as well as likely improved capacity to detect drug-impaired driving. It is likely that prolonged delays in blood testing routinely resulted in those who were above the 5ng/mL THC per se limit at the time of a collision or driving violation being below this level by the time blood was drawn. In the context of historical changes and data limitations, we documented an increase in the proportion of DUI cases involving THC and an increase in the level of THC in cases from 2005-2014 among cases tested by toxicology, excluding those positive for alcohol, however there was no additional increase related to the passage of I-502 in 2012. Among drivers in collisions, the majority (53%) were alcohol-impaired at a level of 0.08 g/dL or higher and 7 percent met or exceeded the per se level of THC, 5ng/mL. Drivers arrested for suspected DUI in the absence of a collision were most commonly under the influence of alcohol, with 30 percent at 0.08 g/dL or above, and 20 percent had a THC level of 5ng/mL or above. Overall the average estimated time to blood draw was 165 minutes. These findings indicate that THC—involved driving is relatively common, appears to be increasing and is likely underestimated given the generally protracted time until a blood specimen is obtained. Evaluating the impact of protracted time until blood testing is complicated by the lack of available standardized law enforcement data on the time of testing. These findings highlight the challenges in enforcing drugged driving laws, particularly with a per se component, in the absence of point-of-contact testing modalities and in the presence of logistical delays in obtaining blood specimens. (Author/publisher)
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