Driving under the influence of drugs (DUID) appears to be a growing factor in impaired-driving crashes. NHTSA’s 2013-2014 National Roadside Survey reveals a decreasing trend in alcohol use by drivers between 1973 and 2013-2014. By contrast, the percentage of weekend nighttime drivers who tested positive for the presence of marijuana rose from 8.6% in 2007 to 12.6% in 2014. While in 2013-2014, 8.3% of weekend nighttime drivers tested positive for alcohol (a BAC of more than .005) and 1.5% had a BAC of .08 or higher, 22.5% had a positive drug oral fluid and/or drug blood test; the survey focused on drugs with the potential to impair driving skills, including over-the-counter, prescription, and illegal drugs. The study does point out that the presence of drugs does not necessarily imply impairment. According to NHTSA, drug use among fatally injured drivers who were tested for drugs rose from 25% in 2007 to 42% in 2016, and marijuana presence doubled in this time frame.
Testing for drug impairment is problematic due to the limitations of drug-detecting technology and the lack of an agreed-upon limit to determine impairment. The nationally recognized level of impairment for drunken driving is .08 g/mL blood alcohol concentration. But there is no similar national standard for drugged driving. Drugs do not affect people consistently. Drugs such as marijuana can also stay in the system for weeks, thus appearing in roadside tests while no longer causing impairment. In addition, tracking marijuana-impaired driving is difficult because drivers who may be under the influence of both marijuana and alcohol are often cited for a high blood alcohol concentration and rarely tested for additional substances.
- Zero tolerance law: Prohibits driving with any amount of THC and/or its metabolites in the body
- Per se law: Prohibits driving with a detectable amount of THC in the body that exceeds the legal limit
- Under the influence DUID: Requies the driver to be under the influence of or affected by THC
- Permissible inference law: Applies if THC is identified in a driver's blood in quanities of 5ng/ml or higher. If so, it is permissible to assume that the driver was under the influence
- South Dakota has a zero tolerance law for drivers under the age of 21.
- Nevada's per se law of 2 ng/ml for THC and 5 ng/mg for THC metabolite only applies for felony violations.
To address marijuana-impaired driving, every state has a law that, in some manner, deals with the issue (see below). Some states, including Alabama and Michigan, have permanent or active oral fluid roadside testing programs to screen drivers for marijuana and other drugs that could affect their behavior behind the wheel. Additionally, states are exploring methods to enhance driving under the influence of drugs (DUID) legislation and enforcement by better collecting crash and citation data.
- Twenty-three states—Alaska, Arizona, California, Colorado, Connecticut, Delaware, Illinois, Maine, Maryland, Massachusetts, Michigan, Minnesota, Missouri, Montana, Nevada, New Jersey, New Mexico, New York, Rhode Island, Oregon, Vermont, Virginia and Washington—Washington, D.C., Guam and Northern Mariana Islands permit recreational marijuana use. Thirty-eight states, Washington, D.C., Guam, Puerto Rico and the U.S. Virgin Islands allow for comprehensive public medical marijuana/cannabis programs.
- Oregon legalized psilocybin (psychedelic plants) products for persons age 21 and older in 2020—becoming the first state to do so.
- Five states—Illinois, Montana, Nevada, Ohio and Washington—have specific per se limits for Tetrahydrocannabinol (THC), the component that gives cannabis its psychological effects and causes impairment, ranging between 2 nanograms per milliliter (ng/ml) of blood and 5 ng/ml. Per se laws establish that once a person is shown to have reached or surpassed the legal limit, that person will be considered impaired by law.
- Colorado’s reasonable inference law states that in instances where THC is identified in a driver’s blood in quantities of 5ng/ml or higher, it is permissible to assume the driver was under the influence. Reasonable inference laws differ from per se laws in that they allow drivers who are charged to introduce an affirmative defense to show that despite having tested at or above the legal limit, they were not impaired.
- Twelve states—Arizona, Delaware, Georgia, Indiana, Iowa, Michigan, Oklahoma, Pennsylvania, Rhode Island, South Dakota, Utah and Wisconsin—have zero-tolerance laws for certain drugs, including THC.
- Research from the AAA Foundation for Traffic Safety concluded that the number of drivers in Washington who test positive for marijuana after a fatal crash has doubled—from about 9% to about 18%—since the state legalized it in 2012. However, the authors pointed out that the study found only a correlation between legalization and an increased number of drivers involved in fatal crashes who tested positive, not a causative link.
- In Colorado, where recreational marijuana was also legalized in 2012, the number of fatalities in crashes in which drivers tested positive for THC rose from 18 in 2013 to 77 in 2016.
- AAA’s 2021 Traffic Safety Culture Index reveals that only 65% of respondents consider driving shortly after using marijuana to be very or extremely dangerous. The index also found that 79% of survey participants support making it illegal to drive with a certain amount of marijuana in a person’s system.
- Currently, the most common methods to detect marijuana are through blood, urine or saliva. However, some states have launched oral fluid testing programs or pilot programs. Two states - Alabama and Indiana - have permanent or active oral fluid roadside screening programs. Michigan allows the collection of oral fluid for the state’s pilot program only. Minnesota began its pilot program in October 2023.