Alcohol Interlocks

An alcohol ignition interlock prevents a vehicle from starting unless the driver provides a breath sample with a BAC lower than a pre-set level, usually .02. Interlocks typically are used as a condition of probation for DWI offenders, to prevent them from driving while impaired by alcohol after their driver’s licenses have been reinstated. (UNC Highway Safety Research Center, 2011, p. 1-32)

Interlocks are highly effective in allowing a vehicle to be started by sober drivers but not by alcohol-impaired drivers. A post-start retest requires the driver to remain sober while driving. A data recorder logs the driver’s BAC at each test and can be used by probation officers to monitor the offender’s drinking and driving behavior. Marques and Voas (2010) provide an overview of interlock use, effectiveness, operational considerations, and program management issues. Marques (2005), Beirness and Robertson (2005), and Robertson, Vanlaar, and Beirness (2006) summarize interlock programs in the United States and other countries and discuss typical problems and solutions. See also Brunson and Knighten (2005), Practice #5, [McGee and Eccles] (2003, Strategy C2), and presentations from the 10th Annual International Alcohol Interlock Symposium ([Robertson, Holmes, and Vanlaar, 2010]). NHTSA offers an ignition interlock toolkit to assist policymakers, highway safety professions, and advocates (Sprattler, 2009). In addition, TIRF [the Traffic Injury Research Foundation] offers an alcohol interlock curriculum for practitioners ([Robertson, Holmes, and Vanlaar, 2010]). (UNC Highway Safety Research Center, 2011, p. 1-32)


The first interlock was developed in 1969. Interlocks with alcohol-sensing devices (instead of performance-based devices) became the standard in the 1980s (Marques and Voas, 2010). Some states implemented pilot programs in the mid-1980s; California was the first to enact legislation allowing interlock use (DeYoung, Tashima, and Masten, 2005). NHTSA issued the first standards in 1992. Interlock programs became more popular as the effectiveness of license sanctions declined, as more-effective devices became available, and as states sought more-targeted solutions than vehicle impoundment (Marques, Voas, et al., 2010).


As of February 2012, all states have laws either mandating or allowing the use of alcohol interlocks (NCSL, 2014b). Laws vary between states; interlocks may be required for all first offenders, only high-BAC offenders (meaning that the driver’s BAC was substantially above the legal limit of 0.08), or repeat offenders. Where they are not mandated by the state, courts or DMVs may impose them. States may also allow interlocks in conjunction with other sanctions, such as allowing an offender to drive during a license suspension but only if he or she installs interlocks (IIHS, 2011b).

“Thirty-six States considered legislation pertaining to interlocks in 2010, with new laws passing in 18 States ([NCSL, 2014b])” (UNC Highway Safety Research Center, 2011, p. 1-32).

Despite widespread laws, only a small percent of eligible offenders have an interlock installed. However, interlock use has more than doubled in the past 5 years, from 101,000 in 2006 to 212,000 in 2010 (Roth, 2010). Use of interlocks is substantially higher when they are required as a prerequisite to license reinstatement. For example, among DWI offenders in Florida who were subject to the State’s interlock requirement, 93 percent installed interlocks once they qualified for reinstatement ([Voas, Tippetts, et al., 2010]). Use of interlocks is also higher when interlocks are offered as an alternative to home confinement via electronic monitoring ([Roth, Marques, and Voas, 2009]). (UNC Highway Safety Research Center, 2011, p. 1-32)


Beirness and Marques (2004) summarized 10 evaluations of interlock programs in the United States and Canada. Interlocks cut DWI recidivism at least in half, and sometimes more, compared to similar offenders without interlocks. After the interlock was removed, the effects largely disappeared, with interlock and comparison drivers having similar recidivism rates. A Cochrane review of 11 completed and 3 ongoing studies reached similar conclusions (Willis, Lybrand, and Bellamy, 2004). One limitation of interlock research is that study participants often are not randomly assigned to interlock or no-interlock groups, so there may be important pre-existing differences between groups. However, the preponderance of evidence suggests that interlocks are an effective method for preventing alcohol-impaired driving while they are installed. (UNC Highway Safety Research Center, 2011, pp. 1-32–1-33)


Interlock programs are managed by private interlock equipment providers. Costs in 2006 averaged about $175 to install an interlock and $2.25 per day while the interlock is installed. The offenders usually pay these costs (Marques, 2006). Illinois passed legislation in 2008 that creates a special fund to reimburse interlock providers when they install a device in the vehicle of an indigent offender ([Savage, Teigen, and Farber, 2009]). (UNC Highway Safety Research Center, 2011, p. 1-33)

Time to Implement

“Interlock programs may require enabling legislation. Once authorized, interlock programs require 4 to 6 months to implement a network of interlock providers” (UNC Highway Safety Research Center, 2011, p. 1-33).

Other Issues

Barriers to Use

Interlocks have demonstrated their effectiveness in controlling impaired driving while they are installed. In light of this success, their limited use may be due to several factors, such as long license suspension periods during which offenders are not eligible for any driving, judges who lack confidence in the interlock technology or who fail to enforce “mandatory” interlock requirements, and interlock costs. See Beirness and Marques (2004), Beirness, Clayton, and Vanlaar (2008), Beirness and Robertson (2005), and [McGee and Eccles] (2003, Strategy C2) for discussion. (UNC Highway Safety Research Center, 2011, p. 1-33)

Public Support

There is strong support for ignition interlocks among the general public. In a national survey, 84 percent of respondents approved of requiring interlocks in the vehicles of convicted DWI offenders ([McCartt, Wells, and Teoh, 2010]). Moreover, almost two-thirds (64 percent) of respondents favored having alcohol detection technology in all vehicles. (UNC Highway Safety Research Center, 2011, pp. 1-32–1-33)

Measuring Effectiveness

The effectiveness of alcohol interlocks is generally measured in terms of recidivism, expressed as the percentage of offenders who have alcohol-related traffic violations while the interlocks are installed. As noted above, the existence of state regulations does not necessarily lead to widespread use; this could be an additional metric. Very few studies look at whether the existence of specific provisions of the regulation leads to fewer alcohol-related crashes in the state. A New Mexico study, for example, found that, although crashes decreased during the study period, because other safety programs were implemented at the same time, it was not possible to determine whether the decrease was a result of the interlock law or other factors (Marques, Voas, et al., 2010, p. 4).

Recent Research on Effectiveness

According to a 2010 survey, about 15 studies of interlock effectiveness had been completed. These studies found rates of between 35 and 75 percent effectiveness when they were installed in a vehicle. However, although these rates were statistically significant, offenders were not randomly assigned (Marques and Voas, 2010).

Two large-scale randomly assigned studies in Maryland found different results regarding the length of the effect. The first, with almost 1,400 subjects, found that offenders with interlocks, as opposed to a license-restriction program, were 65 percent less likely than members of the control group to commit a violation while the interlock was installed. But when the year ended, recidivism rates between the two groups were about the same (K. Beck et al., 1999). The second, analyzing about 1,900 drivers, found that offenders randomly assigned to an interlock program for two years were 26 percent less likely to have another DWI offense during the two-year period following the removal of the interlock. However, the difference between the two groups was only 36 percent for the two-year period of the study. The authors attributed this to the reduced monitoring by the state during the study period. They also suggested that the reduced recidivism post–device removal may have been due to the longer period, during which time offenders may have received substantial reinforcement against drinking and driving (Rauch et al., 2011).

Marques, Voas, et al., 2010, conducted eight separate studies to assess the effects of New Mexico’s mandatory interlock laws (six laws have been passed since 1999, with progressively more-stringent requirements). They found that recidivism was reduced by 65 percent when offenders whose licenses were revoked installed interlocks; first-time-offender recidivism was 61 percent lower with interlocks than without; offenders with long-term license revocations were not very interested in entering interlock programs; greater interlock usage rates were attained with mandatory programs than voluntary ones; and offenders 20 and younger had higher recidivism rates than older offenders, but rates for young offenders were lower with interlocks than without interlocks.

Other recent research has found that closer monitoring of offenders was found to produce fewer noncompliance issues than standard monitoring (Zador et al., 2011). A study in Florida evaluated a new law that requires an offender to install an interlock before his or her license can be reinstated (most laws require a fixed period of time, so this is a stronger sanction). Only about one-quarter of offenders in the study qualified for reinstatement, but, of those, 93 percent had interlocks installed (Voas, Tippetts, Fisher, et al., 2010).

Table B.2. Alcohol Interlock Laws, as of October 2011
State Administrative License Suspension for 1st Offense Restore Driving Privileges During Suspension Ignition Interlocks Mandatory Under State Law for First-Time Offenders Ignition Interlocks Mandatory Under State Law for Repeat Offenders
Ala. 90 days No High-BAC offenders only Yes
Alaska 90 days After 30 daysa All offenders Yes
Ariz. 90 days After 30 daysa All offenders Yes
Ark. 6 months Yesa All offenders Yes
Calif. 4 months After 30 daysa All offenders (in four counties)b No
Colo. 3 months Yesa All offenders Yes
Conn. 90 days Yesa All offendersc Yes
Del. 3 months No High-BAC offenders only Yes
D.C. 2–90 days Yesa No No
Fla. 6 months After 30 daysa High-BAC offenders only Yes
Ga. 1 year Yesa No Yesd, e
Hawaii 3 months After 30 daysa All offenders Yes
Idaho 90 days After 30 daysa No No
Ill. 6 months After 30 daysa All offenders Yes
Ind. 180 days After 30 daysa No No
Iowa 180 days After 90 daysa No No
Kan. 30 days No All offenders Yes
Ky. No Not applicable No No
La. 90 days After 30 daysa All offenders Yes
Maine 90 days Yesa No No
Md. 45 days Yesa High-BAC offenders only Yes
Mass. 90 days No No Yes
Mich. No Not applicable High-BAC offenders only Yes
Minn. 90 days After 15 daysa High-BAC offenders only Yes
Miss. 90 days No No No
Mo. 30 days No No Yes
Mont. No Not applicable No Yes
Neb. 180 days After 30 daysa All offenders Yes
Nev. 90 days After 45 daysa No No
N.H. 6 months No High-BAC offenders only Yes
N.J. No Not applicable High-BAC offenders only Yes
N.M. 90 days After 30 daysa All offenders Yes
N.Y. Variablef Yesa All offenders Yes
N.C. 30 days After 10 daysa High-BAC offenders only Yes
N.D. 91 days After 30 daysa No No
Ohio 90 days After 15 daysa No No
Okla. 180 days Yesa High-BAC offenders onlyg Yes
Ore. 90 days After 30 daysa All offenders Yes
Pa. No Not applicable No Yes
R.I. No Not applicable No No
S.C. No Not applicable No Yes
S.D. No Not applicable No No
Tenn. No Not applicable High-BAC offenders only Yes
Texas 90 days Yesa No Yes
Utah 120 days No All offenders Yes
Vt. 90 days No No No
Va. 7 days No High-BAC offenders only Yes
Wash. 90 days Yesa All offenders Yes
W.Va. 6 months After 30 daysa High-BAC offenders only Yes
Wis. 6 months Yesa High-BAC offenders only Yes
Wyo. 90 days Yesa High-BAC offenders only Yes

SOURCE: IIHS, 2011b.

a Drivers usually must demonstrate special hardship to justify restoring privileges during suspension, and then privileges often are restricted.
b First-time-offender pilot program in four counties—Alameda, Los Angeles, Sacramento, and Tulare.
c Effective December 1, 2012.
Interlock is mandatory unless waived because of financial hardship.
e Effective January 1, 2012.
f In New York, administrative license suspension lasts until prosecution is complete.
Effective November 1, 2011.