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As a Georgia DUI attorney with over 25 years of experience, I’ve defended numerous clients facing charges of driving under the influence of marijuana. With marijuana legalization spreading across the U.S., understanding the science and legal landscape of marijuana-impaired driving is crucial. The 2017 National Highway Traffic Safety Administration (NHTSA) report, Marijuana-Impaired Driving: A Report to Congress (DOT HS 812 440), offers a detailed examination of the challenges in detecting and prosecuting marijuana-related DUIs, focusing on delta-9-tetrahydrocannabinol (THC), the primary psychoactive substance in marijuana. This blog post breaks down the NHTSA study’s findings, explores their implications for Georgia drivers—especially in light of the 1999 Georgia Supreme Court decision in Love v. State (271 Ga. 398, 517 S.E.2d 53)—and provides insights into defending marijuana DUI Cases.
The NHTSA report explains the pharmacokinetics of THC—its absorption, distribution, and elimination in the body—contrasting it with alcohol, which is far better understood. When you drink alcohol, it’s absorbed through the gastrointestinal tract, peaks in your blood within about 20 minutes after your last drink, and is metabolized at a steady rate (about 0.015 BAC per hour). This predictable process allows law enforcement to correlate blood alcohol concentration (BAC) with impairment, forming the basis for per se laws (e.g., 0.08 BAC in Georgia under O.C.G.A. § 40-6-391(a)(5)).
THC behaves differently. When marijuana is smoked, THC is detectable in the blood within minutes, peaking at the end of smoking or shortly after (see Figure 2, p. 5 of the report). Unlike alcohol, THC is fat-soluble, meaning it’s stored in the body’s fatty tissues and can be released back into the blood long after use—sometimes up to 30 days later. The report notes that THC levels drop rapidly after smoking, declining 80-90% within 30 minutes (p. 5), and are often low or undetectable within a few hours. This rapid decline poses a significant challenge for law enforcement, as blood samples in DUI cases are typically collected hours after a stop or crash, when THC levels may no longer reflect peak impairment (p. 6).
A key finding of the NHTSA study is the poor correlation between THC blood levels and impairment. Unlike alcohol, where higher BAC levels reliably predict greater impairment, THC levels in the blood do not consistently indicate the degree of impairment. The report states that “peak impairment does not occur when THC concentration in the blood is at or near peak levels” (p. 7). In fact, peak impairment often occurs around 90 minutes after smoking, when THC levels have already dropped significantly (see Figure 4, p. 7). This means a driver with a low THC level might still be impaired, while another with a high level might show little impairment, depending on factors like tolerance, frequency of use, and individual physiology.
This disconnect has significant legal implications, especially in Georgia. Until 1999, Georgia had a zero-tolerance per se law for marijuana under O.C.G.A. § 40-6-391(a)(6), which made it illegal to drive with any detectable amount of THC or its metabolites in your system, regardless of impairment. However, in Love v. State (271 Ga. 398, 517 S.E.2d 53 (1999)), the Georgia Supreme Court struck down this statute as unconstitutional, ruling that it violated equal protection. The court found that the law unfairly targeted marijuana users by criminalizing the presence of inactive metabolites—sometimes detectable weeks after use—while not applying similar per se standards to other drugs, like prescription medications, that could also impair driving. The court noted that this disparity lacked a rational basis, as the presence of metabolites did not necessarily indicate impairment at the time of driving.
As a result, Georgia no longer has a per se law for marijuana DUIs. Instead, prosecutors must prove actual impairment under O.C.G.A. § 40-6-391(a)(2), which prohibits driving “under the influence of any drug to the extent that it is less safe for the person to drive.” This aligns with the NHTSA study’s critique of per se laws, which it deems not evidence-based. A Washington State study cited in the report found that 62.8% of drivers suspected of marijuana-impaired driving had THC levels below the 5 ng/ml per se threshold set by states like Colorado and Washington (p. 28). In Georgia, where the focus is now on proving impairment rather than mere presence, the NHTSA’s findings underscore the importance of officer observations and other evidence over THC blood levels.
The NHTSA study reviews research on how marijuana affects driving-related skills, confirming that smoking marijuana can impair several critical abilities (p. 11):
A study using the National Advanced Driving Simulator found that marijuana increased lane position variability, though drivers dosed with marijuana tended to drive slower, maintain greater following distances, and take fewer risks compared to when sober (p. 11). This suggests some compensatory behavior, but the report cautions that these adjustments are unlikely to fully mitigate the detrimental effects on cognitive performance and judgment (p. 12).
When marijuana is combined with alcohol, the effects can be additive. The same simulator study found that both substances together increased lane position variability more than either alone, though marijuana mitigated some of alcohol’s effects, like reducing the time spent speeding (p. 12). However, the report concludes there are currently “no evidence-based methods to detect marijuana-impaired driving” or to differentiate impairment caused by alcohol versus marijuana (p. 13, 15).
The NHTSA report outlines the impaired driving detection process, which typically begins with an officer observing inappropriate driving behavior, followed by pre-arrest screening tests like the Standardized Field Sobriety Test (SFST) (p. 8). If alcohol is suspected, a breath test is usually conducted first. If the BAC is below the legal limit or inconsistent with observed impairment, officers may investigate drug use, often involving a Drug Recognition Expert (DRE) from the Drug Evaluation and Classification (DEC) program (p. 9).
Detecting marijuana impairment is far more complex than alcohol. Blood testing, considered the “gold standard,” is invasive, often requires a search warrant, and doesn’t correlate well with impairment (p. 10). Oral fluid testing is less invasive and can detect recent marijuana use, but devices like the Alere DDS2 and Dräger DrugTest 5000 lack proven accuracy and reliability (p. 14). Urine, sweat, and hair testing are even less useful, as they can detect marijuana use days or weeks prior but not current impairment (p. 10). In Georgia, post-Love v. State, the focus on proving actual impairment means officers must rely heavily on their observations and DRE evaluations, rather than just a positive THC test.
The NHTSA study examines the prevalence of marijuana use among drivers and its role in crashes. The 2013-2014 National Roadside Survey found that 12.6% of weekend nighttime drivers tested positive for THC, up from 8.6% in 2007 (Table 6, p. 22). Marijuana is the most frequently detected drug (other than alcohol) in crash-involved drivers (p. 22). However, estimating crash risk is challenging due to the lack of reliable data and methodological issues.
The NHTSA’s case-control study in Virginia Beach (2010-2012) found that THC-positive drivers had an unadjusted odds ratio of 1.25 for crash involvement—a 25% increased risk (p. 25). But after adjusting for demographics (age, gender, race/ethnicity) and alcohol presence, the odds ratio dropped to 1.00, indicating no increased risk over drug- and alcohol-free drivers (p. 26). This suggests that factors like age and alcohol use may account for much of the perceived risk associated with marijuana.
Other studies cited in the report show mixed results. A meta-analysis by Li (2012) estimated a pooled crash risk of 2.66 times for marijuana users, while Asbridge (2012) found a pooled odds ratio of 1.92 (p. 23). The European DRUID study reported a combined risk of 1.33-1.39 times for THC-positive drivers, but these findings were not statistically significant (p. 24). The variability in these estimates highlights the difficulty of establishing a clear link between marijuana use and crash risk, especially compared to alcohol, where drivers with a BAC above 0.12 have a crash risk 20-200 times that of sober drivers (p. 24).
The Love v. State decision in 1999 fundamentally changed how Georgia handles marijuana DUIs. By striking down O.C.G.A. § 40-6-391(a)(6), the Georgia Supreme Court rejected the zero-tolerance per se approach, which had made it illegal to drive with any detectable amount of THC or its metabolites. The court ruled that this law violated equal protection because it treated marijuana users differently from users of other drugs (e.g., prescription medications) without a rational basis. The presence of inactive metabolites, which can linger for weeks, did not necessarily indicate impairment, yet the law punished marijuana users more harshly.
Now, Georgia prosecutors must prove actual impairment under O.C.G.A. § 40-6-391(a)(2), which requires showing that the driver was under the influence of marijuana “to the extent that it is less safe for the person to drive.” This aligns with the NHTSA study’s findings that THC levels are not a reliable indicator of impairment. The report notes that per se laws, like those in other states (e.g., 5 ng/ml in Colorado), often fail to capture impaired drivers while punishing non-impaired ones (p. 28). In Georgia, the shift to an impairment-based standard means that a positive THC test alone isn’t enough for a conviction—prosecutors must rely on evidence like officer observations, field sobriety tests, and DRE evaluations.
The NHTSA study offers several recommendations that could improve how Georgia addresses marijuana-impaired driving:
If you’re charged with a marijuana DUI in Georgia, the NHTSA study and the Love v. State decision provide several defenses that I, as your attorney, can leverage:
The NHTSA study and Love v. State highlight the complexities of prosecuting marijuana DUIs in Georgia. The lack of a reliable impairment standard and the shift to proving actual
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