Alcohol and drugs


Progress on alcohol-impaired driving has stalled since the mid-1990s. Despite earlier declines in alcohol-related highway deaths, more than a quarter of all drivers who die in crashes in the U.S. have blood alcohol concentrations of 0.08% or higher.

The key to reducing alcohol-impaired driving is deterrence. People are less likely to drink and drive if they believe they'll get caught. Sustained and well-publicized enforcement is the best way to let potential violators know they won't get away with it.

Effective measures against impaired driving include:

  • administrative license suspension. This procedure, allowed in most states, lets police immediately take away the license of someone who either fails an alcohol test or refuses to be tested.
  • sobriety checkpoints. Checkpoints, which have been upheld by the U.S. Supreme Court, don't always result in a lot of arrests, but they are a good deterrent if they are visible and publicized. Not all states have them.
  • minimum drinking age of 21. Young drivers have a much higher crash risk after drinking alcohol than adults. Setting 21 as the minimum legal age for purchasing alcohol has helped reduce alcohol-impaired driving among teenagers. However, better enforcement of these laws is needed in many places.
  • alcohol interlocks. Many states require these devices for people with impaired driving convictions. People are less likely to reoffend when they're required to have an interlock, and laws requiring interlocks for all impaired-driving offenders reduce alcohol-involved crashes.

Marijuana use among drivers is a growing concern as more states legalize recreational use. Experimental studies have found that recent marijuana use can worsen driving performance. Crash rates have risen in states that have legalized retail sales for recreational use.

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Alcohol and crash risk

When people are impaired by alcohol, they may have poor judgment, impaired visual functions, declines in coordination and reduced reaction time (Moskowitz & Fiorentino, 2000; Moskowitz et al., 2000). Even when people don't appear drunk, small amounts of alcohol may impair driving skills (Moskowitz & Fiorentino, 2000). As alcohol levels rise in a driver's system, impairment also increases (Moskowitz et al., 2000).

Blood alcohol concentration (BAC) is the amount of alcohol in a person's blood, expressed as weight of alcohol per unit of volume of blood. For example, 0.08% BAC indicates 80 mg of alcohol per 100 ml of blood. However, a blood sample is not necessary to determine a person's BAC. It can be measured more simply by analyzing exhaled breath.

In general, the probability of a crash increases steadily with increasing driver BAC. Fatal crash risk increases substantially above 0.05% BAC and climbs more rapidly after 0.08% (Voas et al., 2012). The likelihood that a driver will be involved in a crash of any severity also increases steadily as BAC increases (Peck et al., 2008Compton & Berning, 2015).

At all BACs, fatal crash risk is much higher among 16-20 year-old drivers than among drivers 21 and older. At a BAC of 0.08% compared with a zero BAC, the likelihood of involvement in a fatal crash is 10 times as high among 16-20 year-old drivers, 7 times as high among drivers ages 21-34, and 6 times as high among drivers 35 and older. At the same BAC, fatal crash risk is the same for male and female drivers in a given age group (Voas et al., 2012).

Relative risk of fatal crash involvement at various BACs compared with zero BAC, passenger vehicle drivers by age

By the numbers

The most reliable information about alcohol involvement comes from fatal crashes. The proportion of fatally injured drivers with BACs at or above 0.08% was 30% in 2022.

Institute researchers calculated that 9,409 fatalities per year, or 25% of all crash deaths, could have been prevented in 2015-18 if all drivers with BACs of 0.08% or higher were kept off the roads (Farmer, 2021). An update of that analysis based on 2018-21 statistics found that 10,158 lives would have been saved.

The proportion of fatally injured drivers with BACs at or above 0.08% declined from 49% in 1982 to 33% in 1994 but has changed little since 1994.

Alcohol involvement is much lower in nonfatal crashes, but is still quite high, relative to drivers not involved in crashes.

A study conducted during 2010-12 in Virginia Beach gathered BAC measurements from drivers involved in crashes of all severities and from a comparable group of non-crash-involved drivers (Compton & Berning, 2015). Nearly 3% of crash-involved drivers had BACs of 0.08% or higher, compared with 0.4% of drivers not involved in crashes.

In 2013-14, the National Highway Traffic Safety Administration (NHTSA) conducted a national roadside breath survey in which data were collected during weekend nights. Patterned after earlier surveys, the 2013-14 survey found that 1.5% of drivers had BACs at or above 0.08% (Berning et al., 2015). This compares with 2.2% in 2007, 4.3% in 1996, 5.4% in 1986 and 7.5% in 1973 and represents an 80% drop since 1973.  The percentage of drivers with any detectable alcohol in their systems declined almost as much from 1973 to 2013-14 as the percentage of drivers with BACs at or above 0.08% (77% vs. 80%).

Alcohol laws

Forty-nine states and the District of Columbia have per se laws making it a crime to drive with a BAC at or above 0.08%. In Utah, it's a crime to drive with a BAC of 0.05% or above.

In all 50 states, drivers younger than 21 are prohibited from operating a vehicle with any detectable blood alcohol. Most states define this as a BAC at or above 0.02%; others specify BACs lower than 0.02%.

The threshold in the U.S. used to be higher than 0.08%. A review by the Centers for Disease Control and Prevention of research on the effects of lowering per se BAC thresholds from 0.10% to 0.08% in the U.S. found a median decrease of 7% in alcohol-related motor fatalities (Shults et al., 2001).

In 2018, Utah lowered its per se BAC threshold from 0.08% to 0.05%. An evaluation of this change reported a 20% reduction in the fatal crash rate from 2016 to 2019, compared with a 6% fatal crash rate reduction in the rest of the U.S. over the same time period (Thomas, et al., 2022).

In all U.S. states, a driver with a BAC under the legal limit can still be charged with impaired driving if an officer observes the person driving in a way that suggests impairment and the person exhibits signs of impairment after being stopped.

In the United States, police can't stop an individual driver without first having a reasonable suspicion that the driver has committed an offense. Anyone driving erratically can be stopped on suspicion of impaired driving. Typical signs of impairment are weaving, inability to maintain a consistent speed, making overly wide turns and stopping too close to or too far from traffic signs or signals. Impaired drivers are also found during stops for traffic offenses like speeding.

After a driver is stopped, if an officer determines that he or she may be impaired, the officer usually asks the driver to perform standard field sobriety tests (e.g., one leg stand, walk and turn). Police can also request a preliminary breath test using a hand-held device. The results are inadmissible in court but are helpful in an officer’s evaluation.

If the officer decides a driver is impaired, the driver is arrested. After arrest, the driver is asked to submit to a BAC test that is admissible in court. Refusal results in license suspension or revocation.

Drivers can refuse the alcohol test, but refusal will result in license revocation or suspension or other penalties. This is provided under implied consent laws that establish that by driving in the state a person consents to alcohol testing.

The law requires that test samples be taken shortly after arrest, typically within two hours. This is because BAC falls as alcohol is metabolized.

In some states, such as Texas and Washington, judicial warrants can be used to compel drivers to submit to tests even after the drivers refuse; in other states, such as Georgia, law enforcement officers cannot request a warrant once a driver has refused. 

The penalties for refusal vary by state (NHTSA, 2008). All states impose some form of administrative, pre-conviction license suspension or revocation for test refusal. Some states have criminal sanctions for refusals.

Alcohol enforcement

Because the police can't catch all offenders, the success of alcohol-impaired driving laws depends on deterring potential offenders by creating the public perception that apprehension and punishment are likely. Research has shown that the perceived likelihood of apprehension is more important in deterring offenders than the severity of punishment. The key to creating this perception is sustained and well publicized enforcement.

Sobriety checkpoints

Checkpoints are a highly visible enforcement method intended to deter potential offenders, as well as to catch violators. Where permitted, police can use checkpoints to stop drivers at specified locations to identify impaired drivers. Locations with a history of crashes or a high incidence of alcohol-impaired driving are often selected. All drivers, or a predetermined proportion of them, are stopped. The checkpoint stop must be brief, and it must be done following strict guidelines to ensure there is no discriminatory stopping of some people and not others. The standard for administering a breath test is the same for checkpoints as for individual stops.

The U.S. Supreme Court held in 1990 that properly conducted sobriety checkpoints are legal under the Constitution.

Sobriety checkpoints are prohibited by state constitution or statute in 10 states (Idaho, Iowa, Michigan, Minnesota, Oregon, Rhode Island, Texas, Washington, Wisconsin, and Wyoming). Law enforcement agencies in Montana and Alaska also don't conduct sobriety checkpoints, though they are not specifically prohibited from doing so.

Even in states where sobriety checkpoints are allowed, many enforcement agencies rarely conduct them.

Based on a nationally representative survey of enforcement agencies conducted by the Institute, 58% of law enforcement agencies reported that they conducted or helped conduct sobriety checkpoints during 2011-12 (Eichelberger & McCartt, 2016). Of the agencies that conducted checkpoints, 24% conducted them once a month or more frequently.

Sobriety checkpoints have been criticized for producing fewer arrests per staff-hour than dedicated patrols, but focusing on the number of arrests is misleading. If checkpoints are set up frequently over long enough periods and are well-publicized, they can increase public awareness and deter potential offenders.

A 1984 Institute study in two neighboring jurisdictions demonstrated how checkpoints can change public perceptions (Williams & Lund, 1984). Surveys of licensed drivers revealed that public awareness of enforcement programs was far greater in Montgomery County, Maryland, a county with well-publicized sobriety checkpoints, compared with nearby Fairfax County, Va., a county that at the time relied on unpublicized drinking-driver patrols.

These changed perceptions can lead to reduced alcohol-impaired driving and fewer crashes. In 1988, the Institute and the city of Binghamton, N.Y., implemented a publicized sobriety and safety belt checkpoint program that decreased the number of drivers stopped who had been drinking alcohol by about 40% during its first two years. Late-night crashes decreased 21% while checkpoints were in place, and injury-producing nighttime crashes declined 16% (Wells et al., 1992).

In 1995, North Carolina implemented a statewide intensive three-week publicized enforcement campaign focusing on alcohol-impaired driving, including statewide checkpoints and dedicated patrols. The number of drivers on the road with BACs at or above 0.08% declined from 198 per 10,000 before the program to 90 per 10,000 after (Williams et al., 1995).

A 2009 meta-analysis (a statistical analysis that aggregates the results of multiple studies) found sobriety checkpoints reduced crashes involving alcohol by 17% or more and all crashes by about 10%-15% (Erke et al., 2009). A 2014 review found a median relative percentage decrease in alcohol-involved fatal crashes of about 9% (Bergen et al, 2014).

Some police departments may hesitate to conduct checkpoints because they believe it requires a large number of officers. However, small-scale checkpoints with as few as 3-5 officers can be conducted successfully and safely (Lacey et al., 2006; Stuster & Blowers, et al., 1995) and can be effective in reducing alcohol-impaired driving (Lacey et al., 2006) and alcohol-related crashes (Stuster & Blowers, et al., 1995).

Dedicated DUI patrols

Dedicated DUI patrols, in which officers are assigned to patrols dedicated exclusively to finding alcohol-impaired drivers, may serve as general deterrence if their activities are publicized and become widely known, especially where sobriety checkpoints cannot be conducted (Fell at al., 2008).

The federal government cites saturation patrols as an effective enforcement strategy (Richard et al., 2018). Saturation patrols target a specific area to identify and arrest impaired drivers, often combining the efforts of multiple agencies to concentrate their resources.

In an Institute survey of enforcement agencies, 87% of agencies reported that they conducted dedicated DUI enforcement patrols, such as saturation or roving patrols (Eichelberger & McCartt, 2016).

Passive alcohol sensors

Many people can effectively mask the overt behavioral symptoms of alcohol impairment for short periods, but it is much more difficult to hide the evidence of impairment from a passive alcohol sensor, which identifies alcohol in the exhaled breath near a driver's mouth.

Passive alcohol sensors are used to identify potentially impaired drivers for further testing. The sensors aren't intrusive and therefore don't violate constitutional prohibitions against unreasonable search and seizure.

Studies of sobriety checkpoints in Fairfax County and Charlottesville, Va., show that police officers using sensors were able to detect more offenders compared with officers who did not use sensors (Ferguson et al., 1995Voas, 2008).

Despite the evidence that the use of passive alcohol sensors can aid officers in detecting impaired drivers, only 18% of police agencies in a national survey indicated that they used them, and 54% of these agencies used them infrequently (Eichelberger & McCartt, 2016).

Passive alcohol sensor

Administrative license suspension

Thirty-nine states and the District of Columbia have administrative license suspension (ALS) laws that apply to first offenders. These laws authorize police to confiscate the licenses of drivers who either fail or refuse to take a chemical test for alcohol. Drivers are given a notice of suspension, which also serves as a temporary permit to drive. Depending on the state, this permit may be valid for anywhere from seven to 90 days, during which time the suspension may be challenged. People have the right to a prompt administrative hearing to determine the validity of the arrest and any alcohol testing. If there is no challenge or if the suspension is upheld, the license is suspended for a prescribed period of time.

Suspensions for first offenses vary among jurisdictions but most commonly are 90 days. Longer suspensions are specified for repeat offenders and those who refuse testing. It's important to note that ALS laws do not replace criminal prosecution, which is handled separately through the courts.

Well-designed studies have found ALS laws are associated with reductions in fatal crashes likely to be alcohol-related (Klein, 1989Wagenaar & Maldonado-Molina, 2007; Zador et al., 1989). The reductions in violations and crashes associated with license suspension continue well beyond the suspension period (Peck et al., 1985; Voas et al., 2000).

Although many suspended drivers continue to drive (McCartt et al., 2003), they tend to drive less. Longer periods of license suspension may be expected to have stronger effects, while those of short duration may have very limited effects. 

Alcohol interlocks

An alcohol ignition interlock consists of a breath-testing unit connected to a vehicle's ignition. In order to start the vehicle, the driver must blow into the device and register a blood alcohol reading that is below a predetermined level. If the blood alcohol reading exceeds this level, the interlock prevents the vehicle from starting.

Studies have shown that alcohol ignition interlocks are effective in reducing recidivism, but this effect seems to last only while the devices are installed in the offenders' vehicles (Willis et al., 2004).

In most states, all alcohol-impaired driving offenders, including first-time offenders, must install interlocks to resume driving. Some states require interlocks only for offenders with high BACs (usually 0.15% or higher) and/or for repeat offenders. Seven states (Indiana, Montana, Nevada, North Dakota, Rhode Island, South Dakota and Wisconsin) have no mandatory interlock requirements.

A national study by the Institute found that laws requiring interlocks for all impaired-driving offenders reduce the number of drivers with BACs at or above 0.08% involved in fatal crashes by 26%, compared with no interlock law. Laws requiring interlocks only for repeat offenders were associated with a 9% reduction, and laws requiring them for repeat offenders and high-BAC offenders were associated a 20% reduction  (Teoh et al., 2021).

While interlocks are intended for drivers who have been arrested for impaired driving, advanced in-vehicle alcohol detection technologies that would be suitable for all drivers, are under development. The goal of the Driver Alcohol Detection System for Safety program, a cooperative venture of vehicle manufacturers and the federal government is overseeing the development, is to develop a device that can quickly, accurately and unobtrusively measure BACs and keep drivers from operating vehicles when their BACs exceed 0.08%.

The Infrastructure Investment and Jobs Act of 2021 requires the National Highway Traffic Safety Administration to create a rule mandating that new vehicles be equipped with advanced technology to prevent impaired driving, beginning no later than 2026.

Addiction treatment

Although studies have had mixed results, research has shown that treatment and rehabilitation programs may have a small, positive effect on the subsequent behavior of alcohol-impaired driving offenders. 

A 1995 examination of more than 200 studies of the effects of various treatment and rehabilitation programs found a reduction of 7%-9%, on average, in subsequent alcohol-impaired driving events, including repeat offenses and crashes (Wells-Parker et al., 1995).

A 1997 study of California programs found that treatment combined with driver license penalties was more effective than license penalties alone in reducing repeat offenses among first and repeat offenders (DeYoung, 1997).

Recently, a Florida study found that offenders in interlock programs were less likely to reoffend after the removal of the interlocks if they had been referred for treatment (Voas et al., 2016).

Treatment and rehabilitation in lieu of license sanctions or other penalties have not been shown to be effective in reducing recidivism or alcohol-involved crashes (Nichols et al., 1978; Nichols et al., 1979).


Marijuana, also known as cannabis because it is derived from the cannabis plant, is a drug with recreational and medicinal uses (National Academies, 2017). Marijuana contains hundreds of chemical compounds, including several types of cannabinoids (ElSohly et al., 2017), which act on cannabinoid receptors in cells throughout the brain and body.

The primary psychoactive (mind-altering) cannabinoid found in marijuana is delta-9-tetrahydrocannabinol (THC). Marijuana also contains other cannabinoids, such as cannabidiol (CBD).

Nationally, self-reported past-year marijuana use among people ages 12 and older was about 19% in 2021, which was higher than the percentages from 2002 to 2020 (Substance Abuse and Mental Health Services Administration, 2022). However, trends differ by age. From 2002 to 2021, past-year marijuana use declined among ages 12 to 17 (16% to 10%) but increased among ages 18 to 25 (30% to 35%) and ages 26 and older (7% to 17%).

In national roadside surveys, the proportion of nighttime, weekend drivers who were positive for marijuana increased from 9% in 2007 to 13% in 2013–14 (Kelley-Baker et al., 2017).

Meta-analyses of experimental studies find that recent marijuana use can reduce performance in simulated and on-road driving (Simmons et al., 2022) and affect cognitive functions (McCartney et al., 2021). The effects of marijuana may include greater variation in lane position, a measure of lateral control, and decreased performance on measures of attention.

A meta-analysis of 26 studies reported a 32% increase in the odds of crash involvement among drivers who used marijuana, compared with those who did not (Rogeberg et al., 2018). However, the best-controlled study did not find THC-positive drivers to be at greater risk of crashing than other drivers after controlling for alcohol, age and sex (Lacey et al., 2016). It isn’t known how many THC-positive drivers in this study were under the effects of marijuana. Unlike alcohol concentrations, THC levels in the body cannot reliably predict impairment, and low levels of THC can be detected for several hours after peak impairment (Compton, 2017).

A recent meta-analysis did not find that drivers who combined marijuana with alcohol were more likely to crash than drivers who used alcohol alone (White & Burns, 2023). Alcohol was associated with significant crash risk, regardless of the presence of marijuana. An Institute study of drivers treated at hospital emergency departments found significant increases in the odds of being injured in a crash both among drivers who used alcohol alone and among those who used alcohol in conjunction with marijuana (Choo et al., 2023).  

Marijuana laws

Marijuana is classified as a Schedule 1 controlled substance and is illegal under federal law. However, certain types of marijuana use have been legal under state law in some parts of the country since the 1990s.

Today, 14 states allow medical use of marijuana, while another 10 states (Georgia, Indiana, Iowa, Kansas, North Carolina, South Carolina, Tennessee, Texas, Wisconsin and Wyoming) permit the use of specific cannabis products for designated medical conditions. Twenty-four states and the District of Columbia permit both medical use of marijuana and recreational use for adults 21 and older.

Marijuana laws by state in detail

Medical marijuana laws have been associated with 8%-11% reductions in traffic fatality rates, compared with before the laws were enacted (Anderson et al., 2013, Santaella-Tenorio et al., 2016).

Recreational marijuana laws have been associated with increases in crashes in the U.S. A HLDI study examined insurance data before and after sales of marijuana for recreational use began in Colorado, Nevada, Oregon and Washington (HLDI, 2020) and found an overall 4% greater increase in collision claims after retail sales of marijuana took effect, compared with nearby states that didn't change their marijuana laws over the same period.

Similarly, an IIHS study found that legalization and retail sales of marijuana were associated with a 6% increase in injury crash rates in California, Colorado, Nevada, Oregon and Washington, compared with other Western states (Farmer et al., 2021). The study also found a 4% increase in fatal crash rates, but the effect was not statistically significant.

Marijuana enforcement

Driving under the influence of marijuana is illegal in all 50 states. Officers conduct a traffic stop when they observe inappropriate driving behavior. If a driver exhibits signs of impairment after being stopped, then the officer conducts pre-arrest screening tests for alcohol or drugs. In some instances, an officer with special training may be called upon to evaluate the driver.

If the driver is arrested and drug impairment is suspected, the officer may gather a biological sample, such as blood or urine, to be tested for drugs.

Some states have per se laws for marijuana, which make it illegal to drive with the presence of specified amounts of substances in the body. In states with zero-tolerance per se laws, the per se limits are any measurable amount of marijuana.

Marijuana per se laws may apply only to THC or both THC and its metabolites. States vary in the bodily fluids (e.g., blood, urine) permitted for testing, and per se limits may vary depending on the type of specimen.

One state, Colorado, has a reasonable inference law, which allows judges or juries to infer that drivers with a specified amount of THC in their blood are impaired, though drivers can rebut the inference with evidence that they weren't impaired.

In states where the law doesn't give a specified limit, prosecutors must rely exclusively on documented evidence of impairment and marijuana use for successful prosecution.

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