May 2018

  1. How does alcohol affect driving?
    Impairing effects of alcohol include poor judgment, impaired visual functions, declines in coordination and reduced reaction time.  Moskowitz, H. and Fiorentino, D. 2000. A review of the literature on the effects of low doses of alcohol on driving-related skills. Washington, DC: National Highway Traffic Safety Administration. Moskowitz, H.; Burns, M.; Fiorentino, D.; Smiley, A.; and Zador, P. 2000. Driver characteristics and impairment at various BACs. Washington, DC: National Highway Traffic Safety Administration. Even when drivers do not appear drunk, small amounts of alcohol may impair skills involved in driving.  Moskowitz, H. and Fiorentino, D. 2000. A review of the literature on the effects of low doses of alcohol on driving-related skills. Washington, DC: National Highway Traffic Safety Administration. As alcohol levels rise in a driver's system, impairment also increases.  Moskowitz, H.; Burns, M.; Fiorentino, D.; Smiley, A.; and Zador, P. 2000. Driver characteristics and impairment at various BACs. Washington, DC: National Highway Traffic Safety Administration.
  2. What does blood alcohol concentration (BAC) measure?

    A BAC describes the amount of alcohol in a person's blood, expressed as weight of alcohol per unit of volume of blood. For example, 0.08 percent 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.

  3. What BAC is considered illegal for drivers in the United States?

    All 50 states and the District of Columbia have per se laws making it a crime to drive with a BAC at or above 0.08 percent. Utah passed a law in March 2017 making it a crime to drive with a BAC of 0.05 percent or above. The law will take effect on Dec. 30, 2018. Some states also make it a noncriminal offense to drive at BACs higher than 0.05 but below 0.08 percent. 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 percent; others specify BACs lower than 0.02 percent.

    The threshold in the U.S. used to be higher than 0.08 percent. 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 percent in the U.S. found a median decrease of 7 percent in alcohol-related motor fatalities.  Shults, R.A.; Elder, R.W.; Sleet, D.A.; Nichols, J.L.; and Alao, M.O. 2001. Reviews of evidence regarding interventions to reduce alcohol-impaired driving. American Journal of Preventive Medicine 21:66-88.

    The BAC threshold is lower in many other countries. For example, the threshold is 0.05 percent in Australia and 0.02 percent in Sweden. Almost all of the European Union member states have BAC thresholds of 0.05 percent or lower.  European Transport Safety Council. 2017. Blood alcohol content (BAC) drinking driving limits across Europe. Available at http://etsc.eu/blood-alcohol-content-bac-drink-driving-limits-across-europe/

    In all U.S. states a driver may be charged with impaired driving even if the BAC is below the illegal threshold or the person refuses an alcohol test. If an officer observes a person driving in a way that suggests impairment and the person exhibits signs of impairment after being stopped, then the officer can charge the person with impaired driving.

  4. What is the effect of alcohol on crash risk?

    In general, the probability of a crash increases steadily with increasing driver BAC. Fatal crash risk increases substantially above 0.05 percent BAC and climbs more rapidly after 0.08 percent. Voas, R.B.; Torres, P.; Romano, E.; and Lacey, J.H. 2012. Alcohol-related risk of driver fatalities: an update using 2007 data. Journal of Studies on Alcohol and Drugs 73(3):341-50. The likelihood of involvement in a crash of any severity also increases steadily with increasing driver BAC.  Peck, R.C.; Gebers, M.A.; Voas, R.B.; and Romano, E. 2008. The relationship between blood alcohol concentration (BAC), age, and crash risk. Journal of Safety Research 39(3):311-9. Compton, R.P. and Berning, A. 2015. Drug and alcohol crash risk. Report no. DOT HS-812-117. Washington, DC: National Highway Traffic Safety Administration.

    At all BACs, the fatal crash risk is much higher among 16-20 year-old drivers than among drivers 21 and older. At a BAC of 0.08 percent 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.


    In a study conducted during 1996-98 of drivers involved in evening and nighttime crashes of all severities in two cities and a comparable group of non-crash-involved drivers, the risks for drivers younger than 21 were much higher than the risks for drivers 21 and older across the range of BACs. Peck, R.C.; Gebers, M.A.; Voas, R.B.; and Romano, E. 2008. The relationship between blood alcohol concentration (BAC), age, and crash risk. Journal of Safety Research 39(3):311-9. At a BAC of 0.08 percent compared with a zero BAC, 16-20 year-old drivers were more than 7 times as likely to crash, and drivers 21 and older were about 1.6 times as likely to crash.

  5. What proportion of motor vehicle crashes involves alcohol?

    The most reliable information about alcohol involvement comes from fatal crashes. The proportion of fatally injured drivers with BACs at or above 0.08 percent was 27 percent in 2016.

    Alcohol involvement is much lower in nonfatal crashes, but is still quite high relative to drivers not involved in crashes. A study conducted during 1996-98 gathered BAC measurements from drivers involved in evening and nighttime crashes of all severities in two cities and from a comparable group of non-crash-involved drivers. The proportion of crash-involved drivers with BACs of 0.09 percent or higher was 11 percent, compared with 2 percent for non-crash-involved drivers. Blomberg, R.D.; Peck, R.C.; Moskowitz, H.; Burns, M.; Fiorentino, D. 2005. Crash risk of alcohol involved driving: a case-control study. Stamford, CT: Dunlap and Associates, Inc. A similar study was conducted during 2010-12 in Virginia Beach of drivers involved in crashes of all severities that occurred at any time of the day and a comparable group of drivers not involved in crashes. Compton, R.P. and Berning, A. 2015. Drug and alcohol crash risk. Report no. DOT HS-812-117. Washington, DC: National Highway Traffic Safety Administration. Nearly 3 percent of crash-involved drivers had BACs of 0.08 or higher, compared with 0.4 percent of drivers not involved in crashes.

  6. How has the prevalence of alcohol-impaired driving changed over time?

    Alcohol-impaired driving has become less prevalent but remains a major problem. In 2013-14, the National Highway Traffic Safety Administration (NHTSA) undertook 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 percent of drivers had BACs at or above 0.08 percent. Berning, A.; Compton, R.; and Wochinger, K. 2015. Results of the 2013-14 National Roadside Survey of Alcohol and Drug Use by Drivers. Report no. DOT HS-812-118. Washington, DC: National Highway Traffic Safety Administration. This compares with 2.2 percent in 2007, 4.3 percent in 1996, 5.4 percent in 1986 and 7.5 percent in 1973 and represents an 80 percent 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 percent (77 vs. 80 percent).

    Among fatally injured drivers, the proportion with BACs at or above 0.08 percent has declined from 49 percent in 1982 to 33 percent in 1994 and has changed little since 1994. Similar declines have occurred in the proportion of fatally injured drivers with very high BACs (0.15 percent or above).

  7. Does alcohol-impaired driving differ by gender?

    Male drivers are more likely to have BACs at or above .08 percent than female drivers (1.7 percent versus 1.4 percent), according to the 2013-2014 national roadside alcohol survey. Ramirez, A.; Berning, A.; Kelley-Baker, T.; Lacey, J.H.; Yao, J.; Tippetts, A.S.; Scherer, M.; Carr, K.; Pell, K.; and Compton, R. 2016. 2013-2014 National Roadside Survey of Alcohol and Drug Use by Drivers: alcohol results. Report No. DOT HS 812 362. Washington, DC: National Highway Traffic Safety Administration. Both percentages were lower than in 1996, when 5 percent of males and 3 percent of females had BACs at or above 0.08 percent. Insurance Institute for Highway Safety. 2012. [Unpublished analysis of data from the U.S. Department of Transportation's and Insurance Institute for Highway Safety’s1996 National Roadside Survey of Alcohol Use by Drivers and the U.S. Department of Transportation’s 2007 National Roadside Survey of Alcohol and Drug Use by Drivers]. Arlington, VA.

    The extent of alcohol impairment among fatally injured drivers has fallen since 1982 for both genders and for all age groups. Among fatally injured male drivers of passenger vehicles in 2016, 32 percent had BACs at or above 0.08 percent. The corresponding proportion among female drivers was 20 percent. Alcohol impairment in fatal crashes was highest for males ages 21-30 (45 percent).

  8. When do alcohol-impaired driving crashes occur?

    They happen at all hours, but alcohol involvement in crashes peaks at night and is higher on weekends than on weekdays. Among passenger vehicle drivers who were fatally injured between 9 p.m. and 6 a.m. in 2016, 50 percent had BACs at or above 0.08 percent, compared with 17 percent during other hours. Forty percent of all fatally injured passenger vehicle drivers on weekends (from 6 p.m. Friday to 6 a.m. Monday) in 2016 had BACs at or above 0.08 percent. At other times the proportion was 22 percent.

  9. Are most alcohol-impaired driving crashes caused by repeat offenders?

    No, although research shows that people with prior convictions for alcohol-impaired driving are overrepresented among drivers in fatal crashes. Fell, JC. 2013. Update: repeat DWI offenders: their involvement in fatal crashes in 2010. Traffic Injury Prevention, Advance online publication doi: 10.1080/15389588.2013.838230. In 2016, 9 percent of drivers in fatal crashes with BACs of 0.08 percent or higher had previous alcohol-impaired driving convictions on their records. The actual incidence of previous convictions is likely higher, because information on convictions is available for only the prior five years. In addition, some alcohol offenses are not included on driver records because of court programs that allow drivers to remove or avoid a conviction if they comply with court-ordered conditions, such as attending educational or treatment programs or paying fines or restitution. Still, most alcohol-impaired drivers in fatal crashes don't have a long history of multiple alcohol convictions.

May 2018

  1. How can we deter alcohol-impaired driving?

    Most impaired drivers are never stopped. Others are stopped, but police may miss signs of impairment. In studies based on telephone surveys and official arrest records, estimates of the chance of arrest when driving impaired range from small (about 1 in 50) to miniscule (1 in 480). Hedlund, J.H. and McCartt, A.T. 2002. Drunk driving: seeking additional solutions. Washington, DC: AAA Foundation for Traffic Safety. Zador, P.; Krawchuck, S.; and Moore, B. 2001. Drinking and driving trips, stops by police, and arrests: analyses of the 1995 national survey of drinking and driving attitudes and behavior. Report no. DOT HS-809-184. Washington, DC: National Highway Traffic Safety Administration. Dowling, A.M.; MacDonald, R.; and Carpenter, K.H. 2011. Frequency of alcohol-impaired driving in New York State. Accident Analysis and Prevention 12(2):120-7. Quinlan, K.P.; Brewer, R.D.; Siegel, P.; Sleet, D.A.; Mokdad, A.H.; Shults, R.A.; and Flowers, N. 2005. Alcohol-impaired driving among U.S. adults: 1993-2002. American Journal of Preventive Medicine 28:346-50. Bergn, G.; Shults, R.A.; and Rudd, R.A. 2011. Vital signs: alcohol impaired driving among adults - United States, 2010. Morbidity and Mortality Weekly Report 60(39):1351-6.  Researchers in two cities combined arrest data with information on driver blood alcohol concentrations (BACs) gathered in roadside surveys, finding that the risk of arrest for a driver with a BAC of 0.10 percent or higher was between 1 and 6 in 1,000. Beitel, G.A.; Sharp, M.C.; and Glauz, W.D. 2000. Probability of arrest while driving under the influence of alcohol. Injury Prevention  6(2):158-61. Hause, J.M.; Voas, R.B.; and Chavez, E. 1982. Conducting voluntary roadside surveys: the Stockton experience. In M.R. Valverius (Ed.), Proceedings of the Satellite Conference to the 8th International Conference on Alcohol, Drugs and Traffic Safety, June 23-25, 1980, Umea, Sweden (pp. 104-113). Stockholm: The Swedish Council for Information on Alcohol and Other Drugs.

    Because the police cannot 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.

  2. When can a driver be stopped and tested for alcohol?

    In the United States, police cannot 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 or signal violations.

    If, after stopping a driver, an officer determines that the driver may be impaired, the officer initiates an investigation that includes asking the driver to perform standard field sobriety tests (e.g., one leg stand, walk and turn). Police can request a preliminary breath test using a hand-held device. These 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.

    Other countries have different laws for testing drivers for alcohol. For example, in Australia police officers are allowed to administer a breath test to any driver, regardless of whether or not an officer has reason to believe the person has been drinking.

  3. How often do drivers refuse to be tested and what are the consequences?

    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. National Highway Traffic Safety Administration. 2008. Blood alcohol concentration test refusal laws. Report no. DOT HS-810-884W. Washington, DC: U.S. Department of Transportation. More than 40 states and the District of Columbia impose some form of administrative, pre-conviction license suspension or revocation. Some states have criminal sanctions for refusals.

    Refusal rates vary greatly from state to state. An analysis of data on alcohol-impaired driving arrests in 2011 from 34 states and Puerto Rico found that breath test refusal rates ranged from 1 percent in Puerto Rico and 4 percent in California to 82 percent in Florida. Namuswe, E.S.; Coleman, H.L.; and Berning, A. 2014. Breath test refusal rates in the United States - 2011 update. Report no. DOT HS-811-881. Washington, DC: National Highway Traffic Safety Administration. The weighted mean of the refusal rates, based on state population, was 19 percent, compared with 21 percent in 2005 and 24 percent in 2001. 

  4. What are sobriety checkpoints and which states conduct them?

    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 don't conduct them or conduct them rarely. Based on a nationally representative survey of enforcement agencies conducted by the Institute, 58 percent of law enforcement agencies reported that they conducted or helped conduct sobriety checkpoints during 2011-12. Eichelberger, A.H. and McCartt, A.T. 2015. Impaired driving enforcement practices among state and local law enforcement agencies in the United States. Journal of Safety Research 58: 41-47. The proportion of agencies conducting checkpoints varied by agency type, with 60 percent of county agencies, 55 percent of municipal agencies, and 77 percent of state agencies reporting that they conducted or helped conduct checkpoints. Of the agencies that conducted checkpoints, 24 percent conducted them once a month or more frequently.

  5. Are sobriety checkpoints effective?

    Yes. Sobriety checkpoints have been criticized for producing fewer arrests per staff-hour than dedicated patrols, but focusing on the number of arrests is a misleading way to assess the value of checkpoints. 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, A.F. and Lund, A.K. 1984. Deterrent effects of roadblocks on drinking and driving. Traffic Safety Evaluation Research Review 3:7-18. Washington, DC: National Highway Traffic Safety Administration. Surveys of licensed drivers revealed that public awareness of enforcement programs was far greater in Montgomery County, Md., a county with well-publicized sobriety checkpoints, compared with nearby Fairfax County, Va., a county with vigorous enforcement using 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 percent during its first two years. Late-night crashes decreased 21 percent while checkpoints were in place, and injury-producing nighttime crashes declined 16 percent. Wells-Parker, E.; Bangert-Drowns, R.; McMillen, R.; and Williams, M. 1995. Final results from a meta-analysis of remedial interventions with drink/drive offenders. Addiction 90(7):907-26. In 1995, North Carolina implemented a statewide intensive three-week publicized enforcement campaign focusing on alcohol-impaired driving, including statewide checkpoints and dedicated patrols. Drivers on the road with BACs at or above 0.08 percent declined from 198 per 10,000 before the program to 90 per 10,000 after. Williams, A.F.; Wells, J.K.; and Foss, R.D. 1995. The North Carolina Governor's Highway Safety Initiative: initial results from "Booze It and Lose It." Proceedings of the 13th International Conference on Alcohol, Drugs, and Traffic Safety, 1:347-51. Adelaide, Australia: NHMRC Road Accident Research Unit, University of Adelaide.

    In 2002, the Centers for Disease Control and Prevention reviewed studies evaluating sobriety checkpoint programs. The median decline in crashes thought to involve alcohol was about 20 percent. Elder, R.W.; Schults, R.A.; Sleet, D.A.; Nichols, J.L.; Zaza, S.; and Thompson, R.A. 2002. Effectiveness of sobriety checkpoints for reducing alcohol-involved crashes. Traffic Injury Prevention 3(4):266-74. A 2014 review of 10 more recent studies found a median relative percentage decrease in alcohol-involved fatal crashes of about 9 percent. Bergen, G.; Pitan, A.; Qu, S.; Shults, R.A.; Chattopadhyay, S.K.; Elder, R.W.; Sleet, D.A.; Coleman, H.L.; Compton, R.P.; Nichols, J.L.; Clymer, J.M.; Calvert, W.B.; and the Community Preventive Services Task Force. 2014. Publicized sobriety checkpoint programs: a community guide systematic review. American Journal of Preventive Medicine 46(5): 529-539. A 2009 meta-analysis (a statistical analysis that aggregates the results of multiple studies) found sobriety checkpoints reduced crashes involving alcohol by 17 percent or more and all crashes by about 10-15 percent. Erke, A; Goldenbeld, C.; and Vaa, T. 2009. The effects of drink-driving checkpoints on crashes — a meta-analysis. Accident Analysis and Prevention 41(5):914-23.

  6. If sobriety checkpoints are so effective, why aren't they more widely used?

    When surveyed by the Institute about their use of checkpoints, enforcement agencies that did not conduct checkpoints most often reported that checkpoints were illegal in the state (31 percent) or cited a lack of staff (24 percent). Eichelberger, A.H. and McCartt, A.T. 2015. Impaired driving enforcement practices among state and local law enforcement agencies in the United States. Journal of Safety Research 58: 41-47.

    Some police departments believe a large number of officers are required, but small-scale checkpoints with as few as 3-5 officers can be conducted successfully and safely Lacey, J.H.; Ferguson, S.A.; Kelley-Baker, T.; and Rider, R.P. 2006. Low-manpower checkpoints: can they provide effective DUI enforcement for small communities? Traffic Injury Prevention 7(3):213-8. Stuster, J.W. and Blowers, M.A. 1995. Experimental evaluation of sobriety checkpoint points. Report no. DOT HS-806-989. Washington, DC: National Highway Traffic Safety Administration. and can be effective in reducing alcohol-impaired driving Lacey, J.H.; Ferguson, S.A.; Kelley-Baker, T.; and Rider, R.P. 2006. Low-manpower checkpoints: can they provide effective DUI enforcement for small communities? Traffic Injury Prevention 7(3):213-8. and alcohol-related crashes. Stuster, J.W. and Blowers, M.A. 1995. Experimental evaluation of sobriety checkpoint points. Report no. DOT HS-806-989. Washington, DC: National Highway Traffic Safety Administration. The federal government encourages states to do frequent, low-staff checkpoints with 3-5 officers. National Highway Traffic Safety Administration. 2006. Low-staffing sobriety checkpoints. Report no. DOT HS-810-590. Washington, DC: U.S. Department of Transportation.  In the Institute's national survey of enforcement agencies, the number of officers present at checkpoints varied across agencies. Overall, 75 percent of agencies typically used 7 or more officers, with 27 percent using more than 15. Eichelberger, A.H. and McCartt, A.T. 2015. Impaired driving enforcement practices among state and local law enforcement agencies in the United States. Journal of Safety Research 58: 41-47.

  7. What types of enforcement other than sobriety checkpoints are effective?

    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.  Michigan Department of State Police v. Sitz, 496 US 444 (1990). Dedicated DUI patrols have potential to reduce alcohol-involved fatal crashes.  Stuster, J.W. and Blowers, M.A. 1995. Experimental evaluation of sobriety checkpoint points. Report no. DOT HS-806-989. Washington, DC: National Highway Traffic Safety Administration. Fell, J.C.; Tippetts, A.S.; and Levy, M. 2008. Evaluation of seven publicized enforcement demonstration programs to reduce impaired driving: Georgia, Louisiana, Pennsylvania, Tennessee, Texas, Indiana, and Michigan. Annals of Advances in Automotive Medicine 52:23-38

    The federal government cites saturation patrols as an effective enforcement strategy.  National Highway Traffic Safety Administration. 2009. Saturation patrols & sobriety checkpoints guide. Report no. DOT HS-809-063. Washington, DC: U.S. Department of Transportation. 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 percent of agencies reported that they conducted dedicated DUI enforcement patrols, such as saturation or roving patrols. Eichelberger, A.H. and McCartt, A.T. 2015. Impaired driving enforcement practices among state and local law enforcement agencies in the United States. Journal of Safety Research 58: 41-47.

  8. Are there ways to enhance sobriety enforcement at checkpoints and traffic stops?

    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 screening devices that help an officer detect possible impaired drivers for further testing. The sensors are not intrusive and therefore do not violate constitutional prohibitions against unreasonable search and seizure.

    Studies of sobriety checkpoints in Fairfax County, Va., and Charlottesville, Va., show that police officers using sensors were able to detect more offenders compared with officers who did not use sensors. Ferguson, S.A.; Wells, J.K.; and Lund, A.K. 1995. The role of passive alcohol sensors in detecting alcohol-impaired drivers at sobriety checkpoints. Alcohol, Drugs, and Driving 11(1):23-30. Voas, R.B. 2008. A new look at NHTSA's evaluation of the 1984 Charlottesville sobriety checkpoint program: implications for current checkpoint issues. Traffic Injury Prevention 9(1):22-30.

    Passive alcohol sensor

    Passive alcohol sensor


    Despite the evidence that the use of passive alcohol sensors can aid officers in detecting impaired drivers, only 18 percent of police agencies in a national survey indicated that they used them, and 54 percent of these agencies used them infrequently. Eichelberger, A.H. and McCartt, A.T. 2015. Impaired driving enforcement practices among state and local law enforcement agencies in the United States. Journal of Safety Research 58: 41-47.

  9. What is an administrative license suspension law?

    Forty-one states and the District of Columbia have administrative license suspension (ALS) laws. 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 is important to note that ALS laws do not replace criminal prosecution, which is handled separately through the courts.

  10. Are license suspensions and ALS laws effective?

    Yes. Well-designed studies have found reductions in crashes and recidivism among offenders who receive ALS or judicial suspension, compared with offenders whose licenses are not suspended. Zador, P.L.; Lund, A.K.; Fields, M.; and Weinberg, K. 1989. Fatal crash involvement and laws against alcohol-impaired driving. Journal of Public Health Policy 10:467-85. The reductions in violations and crashes associated with license suspension continue well beyond the suspension period. Peck, R.C.; Sadler, D.D.; and Perrine, M.W. 1985. The comparative effectiveness of alcohol rehabilitation and licensing control actions for drunk driving offenders: a review of the literature. Alcohol, Drugs and Driving; Abstracts and Reviews 1(4):15-39. Voas, R.B.; Tippetts, A.S.; and Taylor, E. 2000. Effectiveness of the Ohio vehicle action and administrative license suspension laws. Washington, DC: National Highway Traffic Safety Administration.

    Although many suspended drivers continue to drive, McCartt, A.T.; Geary, L.L.; and Berning, A. 2003. Observational study of the extent of driving while suspended for alcohol impaired driving. Injury Prevention 9(2):133-7.  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. 

    An Institute study found ALS laws reduce the number of drivers involved in fatal crashes by about 9 percent during nighttime hours when alcohol is very likely to be involved. Zador, P.L.; Lund, A.K.; Fields, M.; and Weinberg, K. 1989. Fatal crash involvement and laws against alcohol-impaired driving. Journal of Public Health Policy 10:467-85.  Another study reported that among 17 states implementing ALS either alone or in combination with other laws, the median effect was a 6 percent decrease in fatal crashes likely to be alcohol-related. Klein, T.M. 1989. Changes in alcohol-involved fatal crashes associated with tougher state alcohol legislation. Report no. DOT HS-809-511. Washington, DC: National Highway Traffic Safety Administration.  A long-term study (1976-2002) of the effects of pre-conviction license suspension laws in 38 states found a 5 percent reduction among drivers with positive BACs involved in fatal crashes. Wagenaar, A.C. and Maldonado-Molina, M.M. 2007. Effects of drivers' license suspension policies on alcohol-related crash involvement: long-term follow-up in forty-six states. Alcoholism: Clinical and Experimental Research 31(8):1399-1406.

  11. What are alcohol ignition interlocks? Do they have a role in deterrence?

    An alcohol ignition interlock has a breath-testing unit that is 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. An estimated 318,714 interlocks were in use in 2014 in the United States. Roth, R. 2014. 2014 survey of currently-installed interlocks in the U.S. Available: http://www.rothinterlock.org/2014_survey_of_currently_installed_interlocks_in_the_us.pdf. Accessed March 25, 2015.

    Studies have shown that alcohol ignition interlocks are effective in reducing recidivism. In a 1999 Institute study, multiple offenders eligible for license reinstatement were randomly assigned interlock-restricted licenses or unrestricted licenses coupled with the conventional post licensing treatment program. The interlock restriction reduced the risk of committing an alcohol-related traffic violation within the first year following conviction by 64 percent. Beck, K.H.; Rauch, W.J.; Baker, E.A.; and Williams, A.F. 1999. Effects of ignition interlock license restrictions on drivers with multiple alcohol offenses: a randomized trial in Maryland. American Journal of Public Health 89:1696-1700. A systematic review and meta-analysis of studies of ignition interlock programs found the programs seemed to reduce recidivism while the devices were installed in offenders' vehicles, but found no evidence of effectiveness once the devices were removed. Willis, C.; Lybrand, S.; and Bellamy, N. 2004. Alcohol ignition interlock programmes for reducing drink driving recidivism. Cochrane Database of Systematic Reviews 2004, Issue 4. Art. no.: CD004168. Oxfordshire, England: The Cochrane Collaboration.

    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 percent or higher) and/or for repeat offenders. Five states (Indiana, Montana, North Dakota, 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 alcohol-impaired drivers in fatal crashes by 16 percent, compared with no interlock law. Laws requiring interlocks only for repeat offenders had a small and nonsignificant effect on the number of alcohol-impaired drivers in fatal crashes.  Teoh, E.R.; Fell, J.C.; Scherer, M.; and Wolfe, D.E.R. 2018. State alcohol ignition interlock laws and fatal crashes. Arlington, VA: Insurance Institute for Highway Safety.  The study estimated that if all states without all-offender interlock laws in 2016 adopted them, more than 500 additional lives could have been saved.

    An Institute study showed that when Washington expanded its interlock requirement to cover everyone convicted of DUI, not just those with multiple offenses, alcohol test refusals, or high BACs, the recidivism rate declined among first-time offenders with BACs under 0.15 percent. McCartt, A.T.; Leaf, W.A.; Farmer, C.M.; and Eichelberger, A.H. 2013. Washington state’s alcohol ignition interlock law: effects on recidivism among first-time DUI offenders. Traffic Injury Prevention 14(3):215-29.  However, even when laws require offenders to have interlocks if they drive, many people don't go through with the installation. An updated analysis of Washington data found that even after the state further strengthened its interlock law, the installation rate was only 38 percent.  McCartt, A.T.; Leaf, W.A.; and Farmer, C.M. 2018. Effects of Washington state’s alcohol ignition interlock laws on DUI recidivism: an update. Arlington, VA: Insurance Institute for Highway Safety. If all first-time offenders had obtained interlocks, their recidivism rate would have fallen from 5.6 percent to an estimated 2 percent. In a pilot program in Santa Fe County, N.M., when judges imposed house arrest via electronic monitoring as the alternative to an interlock, 70 percent of offenders installed interlocks, compared with only 17 percent in other counties. Roth, R.; Marques, P.R.; Voas, R.B. 2009. A note on the effectiveness of the house-arrest alternative for motivating DWI offenders to install ignition interlocks. Journal of Safety Research 40: 437-41. When the program ended, the Santa Fe installation rate declined.

  12. Are treatment and rehabilitation programs effective?

    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 percent, on average, in subsequent alcohol-impaired driving events, including repeat offenses and crashes. Wells-Parker, E.; Bangert-Drowns, R.; McMillen, R.; and Williams, M. 1995. Final results from a meta-analysis of remedial interventions with drink/drive offenders. Addiction 90(7):907-26. 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, D.J. 1997. An evaluation of the effectiveness of alcohol treatment, driver license actions, and jail terms in reducing drunken driving recidivism in California. Addiction 92(8):989-97. 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, R.B.; Tippetts, A.S.; Bergen, G.; Grosz, M.; and Marques, P. 2016. Mandating treatment based on interlock performance: evidence for effectiveness. Alcoholism: Clinical & Experimental Research 40(9):1953-60. 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, J.; Weinstein, E.; Ellingstad, V. and Struckman-Johnson, D. 1978. The specific deterrence effect of ASAP education and rehabilitation programs. Presented at the National Safety Congress. Chicago, Illinois. Nichols, J.; Ellingstad, V. and Struckman-Johnston, D. 1978. An experimental evaluation of the effectiveness of short term education and rehabilitation programs for convicted drinking drivers. Presented at the National Council on Alcoholism Annual Forum, St. Louis, Missouri.

  13. Are new technologies being developed to prevent drivers from operating vehicles with illegal BACs?

    Yes. A cooperative venture of motor vehicle manufacturers and the federal government is overseeing the development of advanced in-vehicle alcohol detection technologies that would be suitable for all drivers, not just convicted offenders. The goal of the Driver Alcohol Detection System for Safety program 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 percent.

    An Institute study estimated that 10,600 deaths in 2010 were directly attributable to alcohol. Lund, A.K.; McCartt, A.T.; and Farmer, C.M. 2012. Contribution of alcohol-impaired driving to motor vehicle crash deaths in 2010. Arlington, VA.  These lives could have been saved if all drivers had BACs of zero. An estimated 7,082 deaths would have been prevented in 2010 if all drivers with BACs of 0.08 percent or higher had been kept off the roads. Applying the same methods yields an estimate of 7,152 preventable deaths if all drivers with BACs of 0.08 percent or higher were kept off the roads in 2016. A 2009 survey showed that 2 out of 3 members of the general public support universal alcohol detection technology to prevent any driver from operating a vehicle after having too much to drink. McCartt, A.T; Wells, J.K.; and Teoh, E.R. 2010. Attitudes toward in-vehicle advanced alcohol detection technology. Traffic Injury Prevention 11(2):156-64.

October 2018

  1. What is marijuana?

    Marijuana, also known as cannabis because it is derived from the cannabis plant, is a drug with recreational and medicinal uses. National Academies of Sciences, Engineering, and Medicine (2017). The Health Effects of Cannabis and Cannabinoids: The Current State of Evidence and Recommendations for Research. Washington, DC: The National Academies Press. Marijuana contains hundreds of chemical compounds, including several types of cannabinoids, ElSohly, M. A., Radwan, M. M., Gul, W., Chandra, S., & Galal, A. (2017). Phytochemistry of Cannabis sativa L. In A. D. Kinghorn, H. Falk, S. Gibbons, & J. Kobayashi (Eds.), Phytocannabinoids: Progress in the Chemistry of Organic Natural Products. Springer. 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 nonpsychoactive cannabinoids, such as cannabidiol (CBD).

  2. What proportion of drivers use marijuana?

    A 2015 national phone survey by the Institute found 9 percent of U.S. drivers ages 18 and older reported using marijuana within the past year. Eichelberger, A. H. (2016). Survey of U.S. drivers about marijuana, alcohol, and driving. Arlington, VA: Insurance Institute for Highway Safety Young drivers ages 18-29 were more likely to report past-year marijuana use (19 percent), compared with drivers ages 30-59 (8 percent) and 60 and older (4 percent). Men were more likely to report past-year marijuana use (11 percent), compared with women (6 percent).

    The most recent national roadside survey of drivers found that 13 percent of nighttime, weekend drivers and 9 percent of daytime (Friday) drivers tested positive for marijuana in blood or saliva during 2013–14. Kelley-Baker, T., Berning, A., Ramirez, A., Lacey, J. H., Carr, K., Waehrer, G., & Compton, R. (2017). 2013–2014 National Roadside Study of alcohol and drug use by drivers: Drug results. (DOT HS 812 411). Washington, DC: National Highway Traffic Safety Administration.

  3. How does marijuana affect driving performance?

    Reviews of experimental studies report that recent marijuana use can reduce performance in both simulated and on-road driving, Bondallaz, P., Favrat, B., Chtioui, H., Fornari, E., Maeder, P., & Giroud, C. (2016). Cannabis and its effects on driving skills. Forensic Science International, 268, 92-102. Hartman, R. L. & Huestis, M. A. (2013). Cannabis effects on driving skills. Clinical Chemistry, 59(3), 1-25. Sewell, R. A., Poling, J., & Sofuoglu, M. (2009). The effect of cannabis compared with alcohol on driving. The American Journal on Addictions.18(3), 185-193. but the effects of marijuana are inconsistent. In some studies, drivers who smoked marijuana had slower reaction times Lenné, M.G., Dietze, P. M., Triggs, T.J., Walmsley, S., Murphy, B., & Redman, J. R. (2010). The effects of cannabis and alcohol on simulated arterial driving: Influences of driving experience and task demand. Accident Analysis and Prevention, 42, 859-866. Ronen, A., Gershon, P., Drobiner, H., Rabinovich, A., Bar-Hamburger, R., Mechoulam, R., Cassuto, Y., & Shinar, D. (2008). Effects of THC on driving performance, physiological state and subjective feelings relative to alcohol. Accident Analysis and Prevention, 40, 926-34. and greater lane position variation, Hartman, R.L., Brown, T.L., Milavetz, G., Spurgin, A., Pierce, R.S., Gorelick, D.A., Gaffney, G., & Huestis, M.A. (2015). Cannabis effects on driving lateral control with and without alcohol. Drug and Alcohol Dependence, 154, 25-37. Lenné, M.G., Dietze, P. M., Triggs, T.J., Walmsley, S., Murphy, B., & Redman, J. R. (2010). The effects of cannabis and alcohol on simulated arterial driving: Influences of driving experience and task demand. Accident Analysis and Prevention, 42, 859-866. Ronen, A., Gershon, P., Drobiner, H., Rabinovich, A., Bar-Hamburger, R., Mechoulam, R., Cassuto, Y., & Shinar, D. (2008). Effects of THC on driving performance, physiological state and subjective feelings relative to alcohol. Accident Analysis and Prevention, 40, 926-34. compared with drivers in placebo conditions. Other studies failed to find such differences in reaction time Anderson, B. M., Rizzo, M., Block, R. I., Pearlson, G. D., & O'Leary, D. S. (2010). Sex differences in the effects of marijuana on simulated driving performance. Journal of Psychoactive Drugs, 42(1), 19-30. Downey, L.A., King, R., Papafotiou, K., Swann, P., Ogden, E., Boorman, M., & Stough, C. (2013). The effects of cannabis and alcohol on simulated driving: Influences of dose and experience. Accident Analysis and Prevention, 50, 879-886. and lane position variation. Ronen, A., Chassidim, H.A., Gershon, P., Parmet,Y., Rabinovich, A., Bar-Hamburger, R., Cassuto, Y., & Shinar, D. (2010). The effect of alcohol, THC and their combination on perceived effects, willingness to drive and performance of driving and non-driving tasks. Accident Analysis and Prevention, 42, 1855-65. In addition, drivers who recently smoked marijuana drove more slowly Anderson, B. M., Rizzo, M., Block, R. I., Pearlson, G. D., & O'Leary, D. S. (2010). Sex differences in the effects of marijuana on simulated driving performance. Journal of Psychoactive Drugs, 42(1), 19-30. Hartman, R.L., Brown, T.L., Milavetz, G., Spurgin, A., Pierce, R.S., Gorelick, D.A., Gaffney, G., & Huestis, M.A. (2016). Cannabis effects on driving longitudinal control with and without alcohol. Journal of Applied Toxicology, 36(11), 1418-29. Lenné, M.G., Dietze, P. M., Triggs, T.J., Walmsley, S., Murphy, B., & Redman, J. R. (2010). The effects of cannabis and alcohol on simulated arterial driving: Influences of driving experience and task demand. Accident Analysis and Prevention, 42, 859-866. Ronen, A., Gershon, P., Drobiner, H., Rabinovich, A., Bar-Hamburger, R., Mechoulam, R., Cassuto, Y., & Shinar, D. (2008). Effects of THC on driving performance, physiological state and subjective feelings relative to alcohol. Accident Analysis and Prevention, 40, 926-34. Ronen, A., Chassidim, H.A., Gershon, P., Parmet,Y., Rabinovich, A., Bar-Hamburger, R., Cassuto, Y., & Shinar, D. (2010). The effect of alcohol, THC and their combination on perceived effects, willingness to drive and performance of driving and non-driving tasks. Accident Analysis and Prevention, 42, 1855-65.  and allowed more headway when following other vehicles, Hartman, R.L., Brown, T.L., Milavetz, G., Spurgin, A., Pierce, R.S., Gorelick, D.A., Gaffney, G., & Huestis, M.A. (2016). Cannabis effects on driving longitudinal control with and without alcohol. Journal of Applied Toxicology, 36(11), 1418-29. Downey, L.A., King, R., Papafotiou, K., Swann, P., Ogden, E., Boorman, M., & Stough, C. (2013). The effects of cannabis and alcohol on simulated driving: Influences of dose and experience. Accident Analysis and Prevention, 50, 879-886. Lenné, M.G., Dietze, P. M., Triggs, T.J., Walmsley, S., Murphy, B., & Redman, J. R. (2010). The effects of cannabis and alcohol on simulated arterial driving: Influences of driving experience and task demand. Accident Analysis and Prevention, 42, 859-866.  compared with drivers in placebo conditions.

    Combining marijuana with alcohol or other drugs may make driving worse than using marijuana alone. A recent study that examined the effects of both alcohol and marijuana found that the combined substances caused greater lane position variation, compared with either substance alone. Hartman, R.L., Brown, T.L., Milavetz, G., Spurgin, A., Pierce, R.S., Gorelick, D.A., Gaffney, G., & Huestis, M.A. (2015). Cannabis effects on driving lateral control with and without alcohol. Drug and Alcohol Dependence, 154, 25-37.

  4. Does marijuana increase crash risk?

    A recent meta-analysis of 26 studies reported a 32 percent increase in the odds of crash involvement among drivers who used marijuana, compared with those who did not. Rogeberg, O., Elvik, R., & White, M. (2018). Correction to: ‘The effects of cannabis intoxication on motor vehicle collision revisited and revised (2016).’ Letter to the editor. Addiction. Available at https://onlinelibrary.wiley.com/doi/full/10.1111/add.14140 However, there are many challenges in conducting these studies, and estimates based on them may be biased. For example, many studies did not select crash-involved drivers and controls from equivalent sources, and many failed to control for alcohol use. 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, J. H., Kelley-Baker, T., Berning, A., Romano, E., Ramirez, A., Yao, J., Moore, C., Brainard, K., Carr, K., Pell, K., & Compton, R. (2016). Drug and alcohol crash risk: A case-control study. (DOT HS 812 355). Washington, DC: National Highway Traffic Safety Administration. but it is not 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, R. (2017). Marijuana-impaired driving: A report to Congress. (DOT HS 812 440). Washington, DC: National Highway Traffic Safety Administration. Available at https://www.nhtsa.gov/sites/nhtsa.dot.gov/files/documents/812440-marijuana-impaired-driving-report-to-congress.pdf Therefore, estimates of crash risk from such studies could underestimate the acute effects of marijuana on crash risk.

  5. Does legalizing marijuana have an effect on crashes?

    Medical marijuana laws have been associated with 8-11 percent reductions in traffic fatality rates, compared with before the laws were enacted. Anderson, D. M., Hansen, B., & Rees, D. I. (2013). Medical marijuana laws, traffic fatalities, and alcohol consumption. Journal of Law and Economics, 56, 333-369. However, the results varied across individual states. Santaella-Tenorio, J., Mauro, C. M., Wall, M. M., Kim, J. H., Cerda, M., Keyes, K. M., Hasin, D. S., Galea, S., & Martins, S. S. (2016). US traffic fatalities, 1985–2014, and their relationship to medical marijuana laws. American Journal of Public Health, 107(2), 336-342. Medical marijuana laws were associated with reductions in fatalities in Arizona, California, Colorado, Nevada, New Mexico, Oregon and Washington but increases in fatalities in Connecticut and Rhode Island. The laws were not significantly associated with changes in fatality rates in 10 states.

    Recreational marijuana laws have been associated with increases in crashes in the U.S. A study by the Highway Loss Data Institute examined insurance data before and after sales of marijuana for recreational use began in Colorado, Nevada, Oregon and Washington Highway Loss Data Institute. (2018). Recreational marijuana and collision claim frequencies. HLDI Bulletin, 35(8), 1-14.  and found an overall 6 percent greater increase in collision claims after retail sales of marijuana took effect, compared with nearby states that did not change their marijuana laws over the same period. Similarly, an Institute study found that retail sales of marijuana were associated with a 5 percent higher rate of police-reported crashes in Colorado, Oregon and Washington, relative to neighboring states. Monfort, S.S. (2018). Effect of recreational marijuana sales on police-reported crashes in Colorado, Oregon, and Washington. Arlington, VA: Insurance Institute for Highway Safety. Another study examined changes in fatal crash rates in Colorado and Washington before and after recreational marijuana laws were passed, relative to several southern states that did not legalize marijuana. Aydelotte, J. D., Brown, L. H., Luftman, K. M., Mardock, A. L., Teixeira, P. G. R., Coopwood, B., & Brown, C. V. R. (2017). Crash fatality rates after recreational marijuana legalization in Washington and Colorado. American Journal of Public Health, 107(8), 1329-1331. Fatal crashes increased in Colorado and Washington relative to the control states. This increase was not statistically significant.

  6. How do state laws on marijuana use vary in the U.S.?

    Twenty-two states allow only medical use of marijuana. Nine states and the District of Columbia allow both medical use of marijuana and recreational use of marijuana for adults 21 and older. An additional 15 states allow only specific types of cannabis products, such as CBD oil for medical use.

    Marijuana is classified as a Schedule 1 controlled substance and is illegal under federal law.

  7. Is marijuana use changing in the U.S.?

    Yes, marijuana use has been increasing. Center for Behavioral Health Statistics and Quality. (2016). 2014-2015 National Survey on Drug Use and Health: Model-Based Prevalence Estimates (50 States and the District of Columbia). Rockville, MD: Substance Use and Mental Health Services Administration. Kelley-Baker, T., Berning, A., Ramirez, A., Lacey, J. H., Carr, K., Waehrer, G., & Compton, R. (2017). 2013–2014 National Roadside Study of alcohol and drug use by drivers: Drug results. (DOT HS 812 411). Washington, DC: National Highway Traffic Safety Administration. Nationally, self-reported past-month marijuana use among people ages 12 and older was about 6 percent from 2002 to 2008 and then gradually increased to 9 percent in 2016. Center for Behavioral Health Statistics and Quality. (2016). 2014-2015 National Survey on Drug Use and Health: Model-Based Prevalence Estimates (50 States and the District of Columbia). Rockville, MD: Substance Use and Mental Health Services Administration. Center for Behavioral Health Statistics and Quality. (2017). 2016 National Survey on Drug Use and Health: Detailed Tables. Rockville, MD: Substance Use and Mental Health Services Administration. However, trends differ by age. From 2002 to 2016, past-month marijuana use declined among ages 12–17 (8 percent to 6 percent) but increased among ages 18–25 (17 percent to 23 percent) and ages 26 and older (4 percent to 7 percent).

    In national roadside surveys, the proportion of nighttime, weekend drivers who were positive for marijuana increased from 9 percent in 2007 to 13 percent in 2013–14. Kelley-Baker, T., Berning, A., Ramirez, A., Lacey, J. H., Carr, K., Waehrer, G., & Compton, R. (2017). 2013–2014 National Roadside Study of alcohol and drug use by drivers: Drug results. (DOT HS 812 411). Washington, DC: National Highway Traffic Safety Administration.

  8. How does marijuana legalization affect use of the drug?

    Marijuana use has recently increased in Washington, where recreational use of the drug is legal, though it is not clear to what extent trends in use are attributable to law changes.

    Washington legalized possession of marijuana in December 2012 and retail sales in July 2014. The proportion of marijuana-positive drivers in fatal crashes changed little in the state from 2010 to late 2013, but an upward trend in marijuana-positive drivers began in late 2013, about 9 months after possession became legal. Tefft, B. C., Arnold, L. S., & Grabowski, J. G. (2016). Prevalence of marijuana involvement in fatal crashes: Washington, 2010-2014. Washington, DC: AAA Foundation for Traffic Safety. The proportion of marijuana-positive drivers involved in fatal crashes increased from 8 percent in 2013 to 17 percent in 2014.

    A roadside survey of drivers who did not crash examined marijuana prevalence in drivers before and after retail sales went into effect in Washington. Ramirez, A., Berning, A., Carr, K., Scherer, M., Lacey, J. H., Kelley-Baker, T., & Fisher, D. A. (2016). Marijuana, other drugs, and alcohol use by drivers in Washington State. (DOT HS 812 299). Washington, DC: National Highway Traffic Safety Administration. Among weekend, nighttime drivers, marijuana use differed little: 18 percent were marijuana-positive in June 2014, before retail marijuana sales were legal, and 22 percent were marijuana-positive one year later. However, the percentage of daytime (Friday) drivers who were marijuana-positive increased from 8 percent to 19 percent over the same period.

    An Institute study showed that the proportion of Washington drivers who self-reported marijuana use within the past 24 hours was similar before and after retail sales were legal (9-10 percent). Eichelberger, A. H. (in press). Marijuana use and driving in Washington State: Risk perceptions and behaviors before and after implementation of retail sales. Traffic Injury Prevention.

    A study of medical marijuana laws found that these law changes were not generally associated with higher marijuana use, but a detailed analysis of specific policies found that allowing dispensaries was associated with a 2-percentage-point increase in marijuana use within the past 30 days relative to states that did not allow dispensaries. Pacula, R. L., Powell, D., Heaton, P., & Sevigny, E. L. (2015). Assessing the effects of medical marijuana laws on marijuana use: The devil is in the details. Journal of Policy Analysis and Management, 34(1), 7-31. Home cultivation was associated with a 1.8-percentage-point increase in the probability of marijuana use within 30 days.

  9. Is it illegal to drive after using marijuana?

    In all U.S. states, it is illegal to drive impaired by marijuana. 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 and/or drug impairment. In some instances, an officer with special training such as a drug recognition expert (DRE) 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.

    Eleven states (Arizona, Delaware, Georgia, Indiana, Iowa, Michigan, Oklahoma, Pennsylvania, Rhode Island, Utah and Wisconsin) have zero-tolerance per se laws for marijuana, which make it illegal to drive with any amount of marijuana in a person's body. South Dakota has a zero-tolerance law for drivers under age 21. Five states (Illinois, Montana, Nevada, Ohio and Washington) have per se laws that make it illegal to drive with specified amounts of marijuana in a person's body.

    Marijuana per se laws, which make it illegal to drive with the presence of specified amounts of substances in the body, 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 were not 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.

  10. Are countermeasures to address marijuana-impaired driving effective?

    Little research has focused on the effectiveness of various countermeasures for marijuana-impaired driving. A recent review noted a need for rigorous research on this topic. Watson, T. M. & Mann, R. E. (2016). International approaches to driving under the influence of cannabis: A review of evidence on impact. Drug and Alcohol Dependence, 169, 148-155. Although several states have adopted per se drugged driving laws, an analysis of 17 states with these laws found no evidence that the laws were effective at reducing traffic fatalities. Anderson, D. M. & Rees, D. I. (2015). Per se drugged driving laws and traffic fatalities. International Review of Law and Economics, 42, 122-134. In states with and without DRE programs, per se laws were not associated with differences in fatalities.