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Use: Use for drug per se laws Medium
Use for drug per se laws
Time: Short

It is illegal to drive under the influence of drugs in all 50 States, Puerto Rico, and the District of Columbia (Boddie & O’Brien, 2018). However, there is a great deal of variability in how States approach this issue. In some States, impairment-based statutes stipulate that prosecution must prove the driver was impaired (for example, by driving recklessly or erratically). Some States have per se laws in which it is illegal to operate a motor vehicle if there are specific detectable levels of a prohibited drug in a driver’s system. Other States have “zero-tolerance” laws, which make it illegal to drive if there is any quantity of illegal substance detected.

Lacey et al. (2010) conducted interviews with law enforcement officers, prosecutors, and other traffic safety professionals in States with per se laws. Most were supportive of such laws. Although they did not believe per se laws made enforcement easier, they reported these laws had a positive effect on the prosecution and conviction of drug-impaired drivers. Moreover, discussions with officers and prosecutors in States without per se laws also revealed relatively high conviction rates, with few cases reaching trial. 

NHTSA’s 2009 Drug Impaired Driving: Understanding the Problem and Ways to Reduce It: A Report to Congress included a model drug-impaired-driving law (Compton et al., 2009). Because the relationship between blood levels of drugs and driving impairment has not been established for drugs other than alcohol, the model law does not include a per se provision. NHTSA recommended enhanced penalties for drivers who are under the influence of several drugs (which could include alcohol) above the sanction for only one substance. In addition, NHTSA recommended State statutes provide separate and distinct offenses and sanctions for alcohol- and drug-impaired driving (Compton et al., 2009; Compton, 2017). NHTSA’s 2017 Marijuana-Impaired Driving: A Report To Congress (Compton, 2017) also recommended measures for improved data and records maintenance at the State level, including the distinction between alcohol-use, drug-use, or both in impaired driving cases, and the distinction between the types of drugs. See Compton (2017) for a detailed list of recommendations.

For a detailed discussion of issues related to drug-impaired-driving laws, see DuPont et al. (2012). The authors make several recommendations including the improvement of drug testing technology, enactment of laws requiring drug testing of all drivers in injury crashes, and the addition of drug use to underage zero-tolerance laws. See also Reisfeld et al. (2012) for arguments in favor of per se laws for drug-impaired driving and a discussion of the challenges of establishing impaired drug thresholds equivalent to a .08 g/dL BAC. Finally, see Robertson et al. (2016) and Gourdet et al. (2020) for recommendations to improve the prosecution of drug-impaired-driving cases. 


The recent trend in legalizing cannabis for medical or recreational use has affected the adoption and enforcement of zero-tolerance and per se laws in the United States, since the metabolites of THC can be detected in the blood and urine for days to weeks after consumption, long after any impairing effects of the drug have concluded (Axel, 2020; Wong et al., 2014). In addition, although there have been recent advances in drug screening tools such as on-site oral fluid testing, this is still an emerging area of study (see Emerging Issues). As of February 3, 2022, there were 37 States, the District of Columbia, and 3 territories that allow for the medical use of cannabis. In addition, 19 States, the District of Columbia, and 2 Territories have enacted legislation permitting adult recreational use of cannabis (NCSL, 2022). Currently, 11 States (Arizona, Delaware, Georgia, Indiana, Iowa, Michigan, Oklahoma, Rhode Island, South Dakota, Utah, Wisconsin) have zero-tolerance laws for one or more drugs, including cannabis. Six States (Illinois, Ohio, Pennsylvania, Montana, Nevada, Washington) have specific per se limits for THC ranging from 1 to 5 ng/mL. One State, Colorado, has a “reasonable inference law” stating that if THC is detected in the blood at levels >5 ng/mL, it is reasonable to assume that the driver is impaired. The remaining States have impairment-based laws (NCSL, 2022). More information about the drug-impaired-driving laws in each State can be found in NCSL (2022), GHSA (n.d.), Boddie and O’Brien (2018), Lacey et al. (2010), and Walsh (2009). 


Lacey et al. (2010) tried to determine whether drug per se laws increased drug-impaired-driving arrests and convictions. However, they were hampered by the fact that many States do not record drug-impaired offenses separately from alcohol-impaired offenses. Similar limitations were found in a study by the AAAFTS (Smith et al., 2019). Watson and Mann (2016) performed a scan of the international literature and found a lack of evidence supporting the effectiveness of zero-tolerance and per se laws in reducing the prevalence of driving under the influence of cannabis. To date, there is insufficient evidence suggesting that State zero-tolerance and per se laws are more effective than impairment-based laws in reducing drug-impaired driving and improving safety outcomes, especially for cannabis.


The costs of drug-impaired-driving laws will depend on the number of offenders detected and the penalties applied to them. 

Time to implement:

Drug-impaired driving laws can be implemented as soon as appropriate legislation is enacted, although time will be needed to educate law enforcement officers, prosecutors, and judges about the new legislation and to inform the public. 

Other Considerations:

  • Per se laws and prescription medications: Some States with per se laws for drug-impaired driving exclude prescription medications from the list of prohibited drugs. Others require the driver to provide a valid prescription to avoid being charged or convicted for drug-impaired driving. Using a medication as prescribed, however, can still lead to impairment in driving ability. See Smith et al. (2018) and Voas et al. (2013) for a discussion of issues related to per se laws and prescription medications. 
  • Drug testing of fatally injured drivers: Drug presence is not reported in many fatal crashes. Moreover, there is inconsistent testing of drugs by laboratories, threshold differences for determining a positive test result, and variation in how results are reported. To better understand and track the drug-impaired driving problem in the United States, improved data and data collection on drug-impaired drivers is needed, see Berning and Smither (2014) and Berning et al. (2022). D’Orazio et al. (2021) describes minimum recommendations for toxicological investigation of fatal motor vehicle crashes and drug-impaired driving.
  • Equity: The enactment and enforcement of stricter State drug-impaired-driving laws, such as zero-tolerance laws, has equity implications. As mentioned elsewhere in Countermeasures That Work, there are persistent racial and ethnic disparities in the frequency of traffic stops, searches and citations among people of color (Novak & Chamlin, 2012; Epp et al., 2017). For example, in an examination of 20 million traffic stops in North Carolina, Baumgartner et al. (2018) found that Black male drivers were twice as likely to be stopped by law enforcement and, if stopped, twice as likely to be searched, as compared to White male drivers. Although not specific to drug-impaired driving, a San Diego survey of Black men found that their likelihood of receiving DUI arrest citations was 80% higher than among non-Latino/non-Hispanic White men, despite having lower mean BACs at the time of arrest (Alimohammad, 2017). In addition, a second California-based study found that Latino/Hispanic men were 1.66 times as likely to be convicted for driving under the influence of alcohol, as compared to White men (Kagawa et al., 2021). There is little literature focused specifically on racial and ethnic disparities and the enforcement of drug-impaired driving, therefore more studies are needed to examine the extent to which these disparities exist.
  • Public support: There is strong approval among the general public for laws that prohibit drug-impaired driving. A 2018 survey by the AAAFTS found 81% of drivers support per se laws for cannabis and 77% of drivers support zero-tolerance laws for drugs not legally prescribed (AAAFTS, 2019).