State laws making it illegal to drive with high blood alcohol concentrations (BACs) serve as the cornerstone of all efforts to reduce alcohol-impaired driving. Many people think the principal goal of such laws is to arrest and punish the drivers who put everyone else at risk. But arrest and punishment of offenders is a secondary objective. The most important objective is for the law to be a deterrent so that police find no alcohol-impaired drivers to arrest. This is commonly referred to as "general deterrence" because it targets and influences the general public. "Specific deterrence" refers to measures that seek to prevent re-offenses among individuals who have already committed an offense.

Most impaired drivers are never stopped. Others are stopped, but police may miss signs of impairment. It was estimated that in 1999 the chance of arrest when driving with a BAC at or above 0.08 percent was less than 1 in 50.¹ 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 is 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 enforcement. Merely putting strong laws on the books is not enough. Enforcement efforts must be sustained and well publicized to create a realistic threat of apprehension.

A variety of enforcement efforts are used to enforce alcohol-impaired driving laws. In some jurisdictions, specialized patrols work exclusively on alcohol-impaired driving enforcement. Police in the majority of states can stop drivers at sobriety checkpoints. Sobriety checkpoints are one of the most effective approaches to deterring impaired driving.^2,3,4,5,6

Because there aren't enough police officers to apprehend all drivers impaired by alcohol, efforts to go beyond traditional enforcement and succeed in deterring potential offenders before they drive are ongoing. In almost all states some alcohol impaired driving offenders are permitted to drive only if their vehicles have been equipped with alcohol ignition interlocks. These devices analyze a driver's breath and disable the ignition if the driver has been drinking. With funding from the federal government and motor vehicle manufacturers, an expert panel is overseeing the development of advanced in-vehicle alcohol detection technologies that would be suitable for all drivers, not just those convicted of alcohol-impaired driving. Challenges for the panel include addressing not only the devices' accuracy but also how quickly and unobtrusively they measure BACs. Institute research indicated that almost 9,000 deaths would have been prevented in 2005 if all drivers' BACs had been reduced to less than 0.08 percent.⁷

The first state laws prohibited driving while intoxicated or while under the influence of alcohol. In practical terms, this meant that only obviously impaired drivers (so-called drunks) were likely to be arrested and, even then, it was difficult to obtain a conviction because no objective standard existed to prove intoxication. When the relationship between BACs and impairment of skills was established, it became possible to define offenses in terms of a BAC above a specific threshold.

Every state now uses BAC results to prosecute offenders. Initially, this was done through what are known as presumptive laws that establish a presumption of impairment at or above a specified BAC (defendants could try to rebut the presumption). All states and the District of Columbia have per se laws defining the offense as driving with a BAC above a proscribed limit, rather like a speed limit. Defendants can no longer try to prove they were not impaired, although they can challenge the validity of the BAC tests.

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. For most legal purposes, however, a blood sample is not necessary to determine a person's BAC. It can be measured more simply by analyzing exhaled breath.

All 50 states and the District of Columbia have laws defining the BAC threshold as 0.08 percent.

Alcohol-impaired driving is a serious problem throughout the industrialized world. BAC thresholds range from more than 0.08 percent in the United Kingdom to more than 0.05 percent in Australia and 0.02 percent or higher in Sweden. The European Commission in 2001 recommended a blood alcohol limit not exceeding 0.05 percent for all drivers but novice and professional drivers who cannot exceed 0.02 percent. Among the 27 European Union member states, only Great Britain, Ireland, and Malta have BAC thresholds above 0.05 percent.⁸

In the United States, breath testing is considered a search, and the US Constitution prohibits the searches unless the police have some reasonable basis for suspecting that a driver may be impaired by alcohol. US police can carry out sobriety checkpoints that briefly stop some or all drivers, if the checkpoint is done in accordance with strict guidelines to ensure there is no discriminatory stopping of some people and not others. Even at a checkpoint, police must show they have a reasonable basis to administer a breath test to a driver. Depending on state law, police may be able to ask all drivers legally stopped to voluntarily consent to a breath test, but they cannot require a test without reasonable suspicion of an alcohol offense.

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.

Although police in the United States cannot stop and test individual drivers without cause, police can investigate any driver who, based on established criteria, appears to be driving while impaired by alcohol. Based on the totality of circumstances, they determine whether there is sufficient reason to detain the driver for an investigation of alcohol impairment. Although it is not required, many jurisdictions use a preliminary breath test (PBT), which is administered using a hand-held breath tester. After the officer makes an arrest, a court-admissible test is requested. In most states a driver can refuse this test, but refusal will result in license revocation or suspension.

Refusal rates vary greatly from state to state. A recent study found that refusal rates nationwide have remained stable at about one-quarter of all drivers arrested for alcohol-impaired driving from 1996 to 2001. California has the lowest refusal rate (5 percent) and Rhode Island has the highest (85 percent). The consequences of refusal also vary by state. In some states, the administrative sanctions for a refusal are more stringent than those for failing a breath test. Some states have provisions to force a BAC test after a refusal.⁹

Police can use checkpoints to stop drivers at specified locations to identify impaired drivers. All drivers, or a predetermined proportion of them, are stopped based on rules that prevent police from arbitrarily selecting drivers to stop. Checkpoints are a visible enforcement method intended to deter potential offenders as well as to catch violators. If checkpoints are set up frequently over long enough periods and are well publicized, they can establish a convincing threat in people's minds that impaired drivers will be apprehended. This threat is key to general deterrence.

Annotated bibliography of research reports on the effectiveness of sobriety checkpoints

In a survey of US states conducted in 2002, 37 states and the District of Columbia reported conducting checkpoints; 13 reported not conducting them because of legal or policy reasons.¹⁰ Only 11 states reported conducting checkpoints as often as once a week.

The US Supreme Court held in 1990 that properly conducted sobriety checkpoints are legal under the federal constitution. Most state courts that have addressed the issue have upheld checkpoints, too, but some have interpreted state law to prohibit checkpoints.¹¹

State statutes and court decisions on sobriety checkpoints

Measured in arrests per man-hour, a dedicated police patrol is the most effective method of apprehending offenders.² Such patrols also may serve general deterrence if their activities are publicized and become widely known, but this usually is not the case. Sobriety checkpoints have been criticized for producing fewer arrests per man-hour than dedicated patrols, but some studies show arrest rates can be increased greatly when passive alcohol sensors are used to help police detect drinking drivers.² In any case, focusing on the number of arrests is a misleading way to assess the value of checkpoints. The primary purpose of frequent checkpoints is to increase public awareness and deter potential offenders, resulting in the ideal situation of few offenders left to apprehend. For example, in the Australian state of Victoria, which has had an extensive roadside breath testing program for several years, only 1 in 555 drivers stopped in 1993 tested positive for alcohol to the point of breaking the law. In 1978, before the roadside breath testing program, the ratio was 1 positive to 45 negatives.¹²

A 1984 Institute study in two neighboring jurisdictions demonstrated how checkpoints can change public perceptions.¹³ Fairfax County, Virginia, had a long history of vigorous enforcement of alcohol-impaired driving laws and used unpublicized drinking-driver patrols to achieve relatively high arrest rates. Nearby Montgomery County, Maryland, had historically lower arrest rates but used well-publicized sobriety checkpoints during the study period. Surveys of licensed drivers revealed that public awareness of enforcement programs was far greater in Montgomery County than in Fairfax County. Respondents in both counties incorrectly believed the probability of arrest was higher in Montgomery County, where checkpoints were conducted.

These changed perceptions can lead to fewer crashes. Checkpoint programs in Florida,¹⁴ New Jersey,¹⁵ and Virginia² have led to significant reductions in alcohol-related crashes. In 1988, the Institute and the city of Binghamton, New York, implemented an integrated enforcement program that emphasized the publicized use of sobriety and safety belt checkpoints.¹⁶ During the program's first 2 years, the number of drivers stopped who had been drinking decreased about 40 percent. Late-night crashes decreased 21 percent while checkpoints were in place, and injury-producing nighttime crashes declined 16 percent. Results from North Carolina's 1995 statewide intensive 3-week alcohol-impaired driving enforcement and publicity campaign indicated drivers with BACs at or above 0.08 percent declined from 198 per 10,000 before the program to 90 per 10,000 after.¹⁷

In 2002, the Centers for Disease Control and Prevention reviewed studies in a variety of settings (urban, rural, and mixed) evaluating sobriety checkpoint programs. The median decline in fatal crashes thought to involve alcohol was about 20 percent.¹⁸

Low-manpower checkpoints with as few as 3-5 officers can be effective in reducing the percentage of drivers operating at higher BACs. Institute research found that police agencies in two rural counties in West Virginia were able to sustain a year-long program of weekly low-manpower checkpoints. The proportion of nighttime drivers with BACs at or above 0.05 percent was 70 percent lower following implementation of the checkpoint program, compared with drivers in other counties where additional checkpoints were not conducted.¹⁹

Annotated bibliography of research reports on the effectiveness of sobriety checkpoints

In 10 states, sobriety checkpoints cannot be conducted because they are illegal under these states' constitutions or statutes. In other states, sobriety checkpoints may be underutilized because police departments believe a large number of officers are required, placing checkpoints beyond the resources of small police agencies and draining the personnel and financial resources of larger agencies. However, research by the Institute and others has shown that small-scale checkpoints with only a few officers can be conducted successfully and safely^3,19 and can be effective in reducing alcohol-impaired driving¹⁹ and alcohol-related crashes.³ The federal government encourages states to do frequent, low-manpower checkpoints, which can be conducted with 3-5 officers.

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. In a 1993 Institute study of sobriety checkpoints in Fairfax County, Virginia, police officers using sensors were able to detect more offenders compared with officers who did not use sensors.²⁰ Police without sensors detected 55 percent of drivers whose BACs were at or above 0.10 percent. With sensors, police successfully detected 71 percent of the drivers with BACs at or above 0.10 percent. Results of the Fairfax study parallel previous Institute evaluations.

Annotated bibliography of research on passive alcohol sensors

Passive alcohol sensor

Laws providing for the suspension or revocation of licenses have been shown to reduce the subsequent crash involvement of drivers convicted of alcohol offenses.^21,22,23 Even after the suspension, the effects last. Although many suspended drivers continue to drive,²⁴ they tend to drive less. License suspension also has led to a general reduction in fatal crashes in states where the threat of this sanction has been made more certain through laws that provide for administrative license suspension.

See Q&A Alcohol: administrative license suspension

The surprising answer is, probably not. Sanctions range from fines and license suspension to jail terms or community service. Historically, the potential punishment has been severe. However, the probability of apprehension has been low and some penalties infrequently applied even if mandatory by statute. In fact, a sanction may be more likely to be applied when judges and juries consider it to be appropriate but not too severe, such as license suspension.²⁵

The world's first per se drinking driving laws, enacted in Norway in 1936 and Sweden in 1941, were regarded as strict because imprisonment routinely was imposed on drivers apprehended with high BACs. However, a 1975 evaluation of the effects of these laws showed no significant change in the number or rate of fatal motor vehicle crashes.²⁶ The notion that the sanctions were effective has thus been characterized as "the Scandinavian myth."

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. Several evaluations of 35 alcohol and treatment programs (Alcohol Safety Action Projects) implemented in the 1970s found lower re-offense rates for first offenders compared with repeat offenders, but no effect on crash rates.²⁷ 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 both alcohol-impaired driving re-offenses and alcohol-involved crashes.²⁸ 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.²⁹ Treatment and rehabilitation in lieu of license sanctions or other penalties have not been shown effective in reducing recidivism or alcohol-involved crashes.^30,31

In a 1989 study, Institute researchers evaluated the effectiveness of administrative license suspension laws, per se BAC laws, and laws requiring jail or community service after a first alcohol-impaired driving offense.²² During the mostly late-night hours when at least half of all fatally injured drivers had BACs at or above 0.10 percent, administrative license suspension laws were estimated to reduce driver involvement in fatal crashes by about 9 percent. Laws requiring jail or community service for a first offense were estimated to reduce driver involvement in fatal crashes by about 6 percent. The effect of per se BAC laws was an estimated 6 percent reduction in crash involvement during daytime hours when fatal crashes typically are less likely to involve alcohol. A review by the Centers for Disease Control and Prevention of the effects of lowering per se BAC thresholds from 0.10 to 0.08 percent found a median decrease of 7 percent in alcohol-related motor vehicle fatalities.⁴ Laws establishing a minimum age for purchasing alcohol also have proved effective in reducing nighttime fatal crashes among young drivers.³² An Institute study conducted in 26 states that raised the minimum legal purchase ages during 1975-84 estimated a 13 percent reduction in nighttime driver fatal crash involvement.³³ Zero tolerance laws that set zero BAC thresholds for drivers younger than 21 also have been shown to reduce crashes likely to involve alcohol impairment among drivers ages 15-20.³⁴

In many countries around the world, including the United States, substantial progress against alcohol-impaired driving was achieved from the early 1980s through the mid 1990s. In recent years, substantial reductions in deaths related to driving under the influence of alcohol have been achieved in some European countries. From 1996-1998 to 2005, Germany, the Netherlands, and the Czech Republic reduced drunk-driving deaths by 50 percent. However, this is not the case for all European countries. In Great Britain, Spain, Finland, Lithuania, and Hungary, the average yearly percentage change in alcohol-related crash deaths increased slightly and some other European countries do not even keep track of alcohol-related crashes.³⁵

Australia has the most successful documented programs to reduce alcohol-impaired driving. All programs have in common highly visible, sustained, widespread enforcement. The state of Victoria maintains an aggressive program of random breath testing of drivers. The percentage of drivers and motorcyclists killed in crashes with BACs above 0.05 percent dropped from 50 percent in 1977 to about 25 percent in 1995, indicating the deterrent result of a long-term, high-visibility enforcement program.¹²

Yes, studies have shown alcohol ignition interlocks to be effective in reducing recidivism among persons convicted of alcohol-impaired driving.³⁶

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 pre-determined level. If the blood alcohol reading exceeds this level, the interlock prevents the vehicle from starting.

In a 1999 Institute study, multiple offenders eligible for license reinstatement were randomly assigned to participate in an ignition interlock program or the conventional postlicensing treatment program. Participation in the interlock program reduced the risk of committing an alcohol-related traffic violation within the first year following conviction by nearly 65 percent.³⁷ Other studies also have found lower recidivism rates when an interlock device is installed on a vehicle,^{38,39,40,41,42} but any benefits disappear when the device is removed.^37,41,43

Two states (New Mexico and Arizona) require as a condition of relicensing all eligible first offenders to equip their vehicles with ignition interlocks following conviction. Most states allow or require ignition interlocks for at least some repeat offenders, and some states' laws also apply to first offenders with very high BACs (e.g., at or above 0.15 percent). However, studies of these laws have found very low participation rates. In a 2007 study in New Mexico, researchers found that of the over 21,000 repeat offenders eligible for interlocks, less than 5 percent installed one.⁴⁴ Similarly, a 2002 study of California's interlock program found that despite a mandatory requirement of interlocks for drivers convicted of alcohol-impaired driving while on a DUI-suspended license, less than 10 percent had a device installed.⁴⁵ As of 2006, there were an estimated 100,000 interlocks in use in the United States⁴⁶ representing only a tiny portion of the approximately 1.4 million annual DWI convictions for alcohol-impaired driving.⁴⁷

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 is to develop a device that can quickly, accurately, and unobtrusively measure BACs.