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Enforcement of drug-impaired driving laws can be difficult. Typically, drug-impaired driving is only investigated when a driver is obviously impaired but the driver's BAC is low. If drivers have BACs over the illegal limit, many officers and prosecutors do not probe for drugs, as in many States drug-impaired driving carries no additional penalties.

Although several devices are available that allow officers to screen suspects for illegal drug use at point-of-contact, none have been proven to be accurate and reliable (Compton et al., 2009). Many LEAs employ drug recognition experts to assist in investigating potential drug-impaired-driving cases. NHTSA recommends that DREs participate in HVE  and checkpoints, and respond to serious and fatal crashes. DREs use a standardized procedure to observe a suspect’s appearance, behavior, vital signs, and performance on psychophysical and physiological tests to determine whether and what type of drug or drug category may have been used (Talpins et al., 2018). If drug intoxication is suspected, a blood or urine sample is collected and submitted to a laboratory for confirmation. NHTSA has developed the Advanced Roadside Impaired Driving Enforcement training, which bridges the gap between the SFST and the DRE training programs. This program is available to those who are already certified to conduct the SFST and requires 16 hours of pre-classroom instruction and 56 hours of classroom instruction (International Association of Chiefs of Police, 2020b).

Use: As of August 2014 all 50 States and the District of Columbia had drug evaluation and classification (DEC) programs, which are designed to train officers to become DREs (GHSA, 2015). As of December 2019 these programs have prepared more than 1,700 instructors and trained more than 9,800 officers (IACP, 2020a). During 2019 there were over 36,000 drug enforcement evaluations conducted by DREs as part of enforcement. This suggests drug-impaired-driving arrests are not as common in comparison to arrests for alcohol-impaired driving. However, it should be noted that the number of drug-impaired-driving arrests cannot be known as many States only record “impaired-driving” arrests, and do not separate alcohol from drug arrests. Additionally, it is suspected, many arrests are a combination of drugs and alcohol.

In DRE enforcement evaluations in 2019, cannabis was the most frequently identified drug category, followed by CNS stimulants, narcotic analgesics (opioids), and CNS depressants (IACP, 2020). Porath-Waller and Beirness (2014) investigated the validity of using SFSTs in detecting drug impairment among suspected drug-impaired drivers. Results of their study indicate CNS stimulants, CNS depressants, narcotic analgesics, and cannabis are significantly associated with impairment using SFST. Specifically, users of all drug types were significantly more likely to sway while balancing and use their arms to maintain balance on the one-leg-stand. Users of CNS depressants, CNS stimulants, and narcotic analgesics were significantly less likely to keep their balance while listening to test instructions on the walk-and-turn test. Finally, users of CNS depressants were significantly more likely to experience lack of smooth pursuit and distinct nystagmus at maximum deviation on the horizontal gaze nystagmus test.

Effectiveness: Several studies have shown DRE judgments of drug impairment are corroborated by toxicological analysis in 85% or more of cases (NHTSA, 1996). However, one experimental laboratory study found DREs' ability to distinguish between impaired and non-impaired people was moderate to poor for several types of drugs including marijuana, codeine, and amphetamines (Shinar et al., 2000). This study showed DREs tended to rely on just one or two “pivotal” cues to identify specific drug impairment.

A study of the drug evaluation and classification program determined that a combination of cues could provide higher levels of true positives in DRE identification of cannabis consumption (Hartman et al., 2016). These cues included metrics from physiological tests (finger-to-nose test, one-leg stand, and the walk-and-turn) and visual indicators (eyelid tremors). The findings suggest that there are certain differentiating cues for use by DREs in discerning cannabis consumption, though the relation to driving impairment is still unclear.

To date there have been no studies examining the effectiveness of enforcement in reducing drug-impaired driving or crashes. Research has been focused on the impact of decriminalization and legalization of marijuana on several aspects of the DWI system, including prevalence and enforcement. See the joint report by NHTSA, GHSA, and the Volpe National Transportation Systems Center (2017) and Otto et al. (2016) for comparative discussions across States.

Costs: As with other enforcement strategies, the primary costs are for law enforcement time and training. The time to conduct a DRE evaluation can be 2 to 4 hours. Training includes 72 hours of classroom instruction and approximately 50 hours of field work.

Time to implement: Drug-impaired-driving enforcement can be integrated into other enforcement within 3 months; however, time will be needed to train DREs in detecting drug impairment. DRE training consists of 9 days of classroom instruction, and DRE candidates are also required to perform  supervised field evaluations to become certified (Compton et al., 2009).