Alcohol ignition interlocks: how they work, what research shows and how to live with one
How retests work, how to keep a clean record and how to make the change last
Safety Behind the Wheel Foundation built this guide from National Highway Traffic Safety Administration (NHTSA) guidance and device specifications, Insurance Institute for Highway Safety (IIHS) law summaries, Washington and Virginia statutes and published research on interlock programs. Every source is listed at the end. It focuses on the device itself: how it works, daily life with one and how to make the change last. Our DUI and DWI laws by state guide covers interlock laws state by state, along with blood alcohol limits and license suspensions.
What an ignition interlock does
An ignition interlock is a small breath testing device connected to a vehicle’s ignition. To start the engine, the driver must blow a breath sample with an alcohol level below a preset point, usually .02 [1]. That is one quarter of the .08 level.
Its main job is collision prevention. It keeps a driver who has been drinking from starting that one vehicle. Most interlocks are installed after a drunk driving arrest or conviction, as a condition of driving again. NHTSA notes that many states make offenders pay for the device themselves [8]. Some employers, fleets and families also choose them on their own.
How the device works
When you sit down to drive, the handset asks for a breath sample. If the reading is under the set point, the engine can start. If not, the start is blocked and the result is saved.
Once the engine is running, the device asks for more samples at random times. These rolling retests keep a sober friend from starting the car for you. They also catch drinking after the trip begins [1].
Comparison compiled by Safety Behind the Wheel Foundation
Main interlock features
| Feature | What it does | Why it matters |
|---|---|---|
| Start test | Blocks the start if breath alcohol is at or above the set point | Prevents a drunk start [1] |
| Rolling retests | Asks for new samples at random times while the engine runs | Stops someone else from blowing for you [1] |
| Data logger | Saves every start attempt and its result | Lets your program track progress [1, 6] |
| Camera (some states) | Photographs the person giving the sample | Confirms who blew. Cameras were required in 22 states as of May 2019 [1] |
| Tamper seals | Show if the unit was disconnected | Makes tampering easy to spot [6] |
NHTSA has published model specifications for these devices. These performance guidelines are used, in some form, by many states when they approve devices [6]. Under them, a device should lock out a start at .02. It should tell alcohol apart from acetone and cigarette smoke. It should also pass tests at extreme temperatures, from minus 40 to 185 degrees Fahrenheit [6]. NHTSA calls acetone the most common substance that can confuse these devices [6].
Many devices also ask for a set breathing pattern, such as blowing while humming, which makes them harder to fool [6]. The guidelines leave some choices to each state, such as how retests work and whether an emergency override is allowed [6]. Ask your installer to explain your state’s settings before you drive away.
Who uses an interlock, and where it comes from
Drivers under a court or licensing order
Most interlocks are part of a state program. Ask the court, licensing agency or monitoring program for the list of approved providers. The provider installs the device, trains you, services it and reports to your program. Use only an approved provider, or your time may not count.
Drivers who choose one
Sweden has run a voluntary program since 2012. Offenders can keep their license if they use an interlock for one or two years [9]. In a survey, about 70% of working participants said they joined because they needed to drive at work [9].
Employers and fleets
Some employers put interlocks on company vehicles as part of a written alcohol policy. Fleet suppliers, monitoring software providers and interlock providers offer devices for this use. Before you buy, ask who sees test results, what happens when a driver fails, who calibrates the devices and how driver data is protected. Our guide to workplace driving impairment policies can help you set the rules around the device.
Parents and families
For most families with a new driver, clear rules and planning come first. Our companion guides on personal breath testers and teen driver settings cover other tools. If someone in your home has a court ordered interlock, everyone who drives that car must pass its tests too.
A day with an interlock
Testing becomes part of every trip. In one Canadian program, drivers logged about 10 breath tests on each day they drove: about four start tests and about six retests on the road [5]. A simple routine helps:
- Plan before you eat, drink or rinse. Alcohol left in your mouth can cause a high reading even when almost none is in your blood. In a lab study, alcohol based mouthwash caused a false positive if the test came within 15 minutes [7].
- Wait, then rinse. After mouthwash or anything else with alcohol, wait at least 15 minutes. Then rinse your mouth with water.
- Blow the way you were trained. Use the pattern your installer showed you.
- If a start test fails, pause. Do not blow again and again. Wait, rinse and retest as your program allows. In Washington, a failed start test is not held against release if a retest within 10 minutes reads below 0.04 [10].
- Handle retests safely. Ask how long you have to give a retest. In busy traffic, use that time to find a safe place to pull over.
- Write down anything odd. Note the time, the reading and what you ate or used. Call your provider the same day.
Service, calibration and a clean record
Calibration means adjusting the sensor so its readings stay accurate. Under NHTSA’s guidelines, a device should stay accurate for at least 30 days between calibrations, plus a seven day countdown before lockout. After 30 days, it should clearly tell you to bring the vehicle in within seven days for service and a data download [6]. Each state sets its own service schedule [6].
Missed visits matter. In Washington, failing to show up for required maintenance or calibration counts against release. So does removal of the device by anyone other than an interlock technician [10].
Your log is more than a scorecard. In a study of over 2,000 Alberta drivers, the share of failed tests was the best predictor of who would reoffend after the device came off [5].
Costs and help paying
In most programs, you pay. Drivers in a program usually pay for every stage: installation, calibration, monitoring and removal [1]. Prices vary, so ask for a written quote. It should list the installation fee, the monthly fee, service and lockout reset fees, any camera charges, the removal fee and costs for a second vehicle.
Help exists, but few people use it. About two thirds of states have funds to help participants who cannot afford the device. Yet fewer than 10% of offenders typically draw on that help [1]. Cost is a real barrier. In Sweden’s voluntary program, about 64% of surveyed drivers who did not join named cost as a reason [9]. Ask the court, DMV or provider about help before you decide you cannot afford the device.
What the research shows
Researchers have studied interlocks for decades. The results are strong on one point and sobering on another.
Comparison compiled by Safety Behind the Wheel Foundation
What interlock research found
| Question | What studies found |
|---|---|
| Fewer repeat offenses while installed? | Yes, about 50% to 90% fewer [3] |
| Does the benefit last after removal? | Not by itself; rates return to comparison levels [2] |
| All offender laws vs. no law | 26% fewer drivers at .08+ in fatal crashes [4] |
| Repeat offender only laws vs. no law | 9% fewer [4] |
| Convicted drivers with a device (2019 survey) | 42% [1] |
While installed, it works. A systematic review for the Guide to Community Preventive Services found strong evidence that interlocks cut drunk driving rearrests while in use. The few crash studies also pointed to fewer alcohol related collisions during that time [2]. A 2021 review drew on 71 articles about interlocks. It warned that most studies did not randomly assign drivers to groups, so the exact size of the benefit is uncertain [3].
After removal, the benefit fades. Rearrest rates returned to levels like those of comparison drivers once the devices came off [2]. NHTSA’s guide to proven countermeasures agrees [1].
All offender laws appear to save lives. One study looked at U.S. fatal crashes from 2001 to 2019 [4]. Laws for repeat plus high blood alcohol offenders were linked to a 20% drop in drunk drivers in fatal crashes, compared with no law [4].
Many people who should have one do not. In a 2019 provider survey, only about 15% of people arrested for drunk driving had a device installed [1]. Low participation limits how many collisions interlocks can prevent [2].
Making the change last
Because the benefit fades after removal, time on the device is best used to change habits. Four approaches can help:
- Compliance based removal. The device comes off only after a clean stretch, not just a date on the calendar. Washington requires 180 clean days, and Virginia at least 12 straight months without alcohol related violations for certain offenders [10, 11].
- Treatment when tests show trouble. A 2008 Florida law required treatment for participants with four or more interlock violations. After removal, those who got treatment plus the device had a one third lower rate of repeat offenses than those with the device alone [1]. Safety Behind the Wheel Foundation encourages anyone whose tests show trouble to ask about treatment.
- Close monitoring. Drivers whose data were reviewed weekly, with progress letters, had fewer failed start tests [1].
- Family support. In a randomized trial, a smartphone support app for offenders and their family members led to fewer lockouts. Offenders were also less likely to be drinking at nine months. The authors note that fewer lockouts predict fewer repeat offenses [13].
If your log shows failed tests, treat that as a signal, not a verdict. A licensed health care provider or your program’s treatment staff can help you find support that fits.
Interlock rules by state, in brief
State laws decide who must install a device, for how long and how it comes off. Our DUI and DWI laws by state guide compares them in detail. Four state examples show the range:
- Washington: The restriction lasts one year the first time, five years the second time and 10 years after that. It ends only after the provider confirms 180 clean days in a row [10].
- Virginia: For certain offenders, such as a repeat offender seeking a restricted license, the court must require a device on each vehicle the person owns or registers. It stays for at least 12 straight months without alcohol related violations [11].
- California: An interlock is required to reinstate a license after a repeat conviction. The state drops the mandatory (“hard”) suspension period for offenders who choose to install one [12].
- Texas: A first offender must use an interlock to drive during a post conviction suspension, if one is imposed. A repeat offender must use one during suspension, and to reinstate if the prior conviction was within five years [12].
Washington and Virginia both let a restricted driver use an employer’s vehicle for work without a device, under conditions [10, 11]. Washington’s exception does not cover an employer vehicle assigned only to you and used only for commuting [10].
Limits and common mistakes
An interlock is a strong tool with clear edges:
- It protects only the vehicle it is in. It cannot stop someone from driving a different car.
- It detects alcohol, not other drugs. In a New York study, drivers who cut back on alcohol while on the device used more cannabis, and their use rose further after removal [14]. Our guide to drug and medication impaired driving explains those risks.
- A pass does not mean you are fit to drive. Fatigue, illness and some medicines can still impair you.
Common mistakes include using mouthwash right before a test, letting a friend blow, missing a service date and driving a vehicle without the device while restricted. Built in alcohol detection for all new cars is a separate effort. Our guide to alcohol impaired driving covers where it stands.
Frequently asked questions
Can other people drive my car if it has an interlock?
Yes, as long as they pass every start test and retest. The device cannot tell who owns the car, so each driver must blow. Where cameras are required, photos help confirm who gave a sample [1]. A failed test by someone else may still show up in your record. Ask your program how it reviews those events.
Will mouthwash set off the device?
Alcohol based mouthwash can. In one lab study, breath alcohol after a mouthwash rinse took about 11 minutes on average, and up to 15, to drop below Sweden’s legal breath limit. Almost no alcohol reached the blood [7]. To be safe, wait at least 15 minutes after mouthwash, then rinse with water before you test.
What happens if I fail a retest while driving?
That depends on your state’s program, since NHTSA’s guidelines leave retest rules to the states [6]. In Washington, failing a retest at 0.020 or higher counts against release unless a retest within 10 minutes reads lower [10]. Learn your program’s rules at installation. When a retest comes up, find a safe place to stop if you need to.
How long will I need an interlock?
It depends on your state, your record and how you do on the device. Washington’s restriction runs one year the first time and longer after that [10]. Virginia requires at least 12 straight months without alcohol related violations for certain offenders [11]. When removal is tied to a clean record, violations can push your end date back.
Can I install an interlock voluntarily?
Voluntary use is possible. Sweden, for example, runs a voluntary program for drink driving offenders [9]. In the U.S., ask approved interlock providers and fleet suppliers whether they serve voluntary customers. Ask what the device reports, who sees the data and what service schedule it needs.
The bottom line
Recommendation from Safety Behind the Wheel Foundation
Safety Behind the Wheel Foundation urges drivers to treat time on an interlock as a chance to build habits that last after the device comes off. An ignition interlock does one job very well. It keeps a driver who has been drinking from starting that vehicle. While it is installed, repeat drunk driving drops sharply, and laws covering all offenders are linked to fewer drunk drivers in fatal crashes [2, 4].
The device is not a cure on its own, because the benefit fades once it comes off. Keep every service date, test with care, and use the time to build habits that keep you sober behind the wheel for good. Support from family and treatment can help those habits stick.
Related articles
Sources and further reading
- 1NHTSA. Alcohol Ignition Interlocks. In Countermeasures That Work: A Highway Safety Countermeasure Guide for State Highway Safety Offices, 11th Edition, 2023 (DOT HS 813 490). Published November 2023. Data year: studies through 2022 (state law counts from a 2022 study; camera count as of May 2019; provider survey 2019). nhtsa.gov/…/other-strategies-behavior-2
- 2Elder RW, Voas R, Beirness D, Shults RA, Sleet DA, Nichols JL, Compton R, et al. Effectiveness of ignition interlocks for preventing alcohol-impaired driving and alcohol-related crashes: a Community Guide systematic review. American Journal of Preventive Medicine 40(3):362-376. Published March 2011. Data year: studies published through December 2007. [https://pubmed.ncbi.nlm.nih.gov/21335270/](https://pubmed.ncbi.nlm.nih.gov/21335270/) (doi:10.1016/j.amepre.2010.11.012) pubmed.ncbi.nlm.nih.gov/21335270
- 3Fell JC, Scolese J. The effectiveness of alcohol monitoring as a treatment for driving-while-intoxicated (DWI) offenders: A literature review and synthesis. Traffic Injury Prevention 22(sup1):S1-S7. Published October 2021. [https://pubmed.ncbi.nlm.nih.gov/34672885/](https://pubmed.ncbi.nlm.nih.gov/34672885/) (doi:10.1080/15389588.2021.1980783) pubmed.ncbi.nlm.nih.gov/34672885
- 4Teoh ER, Fell JC, Scherer M, Wolfe DER. State alcohol ignition interlock laws and fatal crashes. Traffic Injury Prevention 22(8):589-592. Published October 2021. Data year: 2001 to 2019. [https://pubmed.ncbi.nlm.nih.gov/34686075/](https://pubmed.ncbi.nlm.nih.gov/34686075/) (doi:10.1080/15389588.2021.1984439) pubmed.ncbi.nlm.nih.gov/34686075
- 5Marques PR, Tippetts AS, Voas RB. Comparative and joint prediction of DUI recidivism from alcohol ignition interlock and driver records. Journal of Studies on Alcohol 64(1):83-92. Published January 2003. Data year: Alberta, Canada interlock and driver records (years not stated in abstract). [https://pubmed.ncbi.nlm.nih.gov/12608487/](https://pubmed.ncbi.nlm.nih.gov/12608487/) (doi:10.15288/jsa.2003.64.83) pubmed.ncbi.nlm.nih.gov/12608487
- 6NHTSA, Federal Register. Model Specifications for Breath Alcohol Ignition Interlock Devices (BAIIDs). Notice, document 2013-10940. Published May 8, 2013. federalregister.gov/…/model-specifications-for-breath-alcoho…
- 7ErnstgĂĄrd L, Pexaras A, Johanson G. Washout kinetics of ethanol from the airways following inhalation of ethanol vapors and use of mouthwash. Clinical Toxicology 58(3):171-177. Published online June 2019. [https://pubmed.ncbi.nlm.nih.gov/31215247/](https://pubmed.ncbi.nlm.nih.gov/31215247/) (doi:10.1080/15563650.2019.1626868) pubmed.ncbi.nlm.nih.gov/31215247
- 8National Highway Traffic Safety Administration (NHTSA). Drunk Driving. Web page, no update date shown; accessed October 2026. Data year: 2024. nhtsa.gov/risky-driving/drunk-driving
- 9Forsman Å, Wallhagen S. Drink drivers’ views of a voluntary alcohol interlock programme for drink driving offenders in Sweden. Accident Analysis and Prevention 124:210-218. Published 2019. Data year: survey of program participants and non-participants (program in place since 2012). [https://pubmed.ncbi.nlm.nih.gov/30677694/](https://pubmed.ncbi.nlm.nih.gov/30677694/) (doi:10.1016/j.aap.2019.01.010) pubmed.ncbi.nlm.nih.gov/30677694
- 10Washington State Legislature. RCW 46.20.720: Ignition interlock restriction (duration, release, employer vehicles). Last amended 2024 (2024 c 306 s 28); accessed October 2026. app.leg.wa.gov/RCW/default.aspx?cite=46.20.720
- 11Code of Virginia. § 18.2-270.1. Ignition interlock systems; penalty. Virginia Law Library; accessed October 2026. law.lis.virginia.gov/…/section18.2-270.1
- 12Insurance Institute for Highway Safety (IIHS). Alcohol interlock laws by state. Updated October 2026. iihs.org/…/alcohol-interlock-laws-by-state
- 13Woodall WG, McCrady BS, Westerberg V, Berteletti J, Martinez L, Brooks M, Starke T, Chirico N. Randomized Trial Shows Smartphone Support App for DWI Offenders and Their Families Reduced Alcohol Use and Ignition Interlock Device Lockouts. Journal of Studies on Alcohol and Drugs 87(4):812-820. Published online December 2025. Data year: 9-month randomized trial (enrollment years not stated in abstract). [https://pubmed.ncbi.nlm.nih.gov/41379082/](https://pubmed.ncbi.nlm.nih.gov/41379082/) (doi:10.15288/jsad.25-00171) pubmed.ncbi.nlm.nih.gov/41379082
- 14Scherer M, Romano E, King S, Marques P, Romosz A, Taylor E, Nochajski TH, Voas R, Manning A, Tippetts S. Cannabis Adaptation During and After Alcohol Ignition Interlock Device Installation: A Longitudinal Study. Journal of Studies on Alcohol and Drugs 83(4):486-493. Published July 2022. Data year: 2015 to 2020. [https://pubmed.ncbi.nlm.nih.gov/35838425/](https://pubmed.ncbi.nlm.nih.gov/35838425/) (doi:10.15288/jsad.2022.83.486) pubmed.ncbi.nlm.nih.gov/35838425
This article reflects the safety priorities of preventing repeat drunk driving and building lasting sober driving habits. It is general education, not legal, medical, insurance or financial advice. Interlock rules change often, so verify details with official sources. For your own situation, contact the court or licensing agency that issued your order, your approved interlock provider, your monitoring program, or a licensed attorney or health care provider. Reviewed October 2026.
Cite this article: Safety Behind the Wheel Foundation. “Alcohol ignition interlocks: how they work, what research shows and how to live with one.” drivewithcare.org, https://drivewithcare.org/alcohol-ignition-interlock-device/. Reviewed October 2026.
Special thanks to our contributors
We appreciate the professionals, experts, and community members who share their knowledge and perspectives to help make this resource better. Your insights are always welcome and appreciated.
Have professional expertise, firsthand experience, or a perspective that could make this resource better?
We welcome your input. Your knowledge and experience can help us make this resource more useful, accurate, and meaningful to the people who rely on it.
Thank you. Your contribution is on its way.
We’re grateful you took the time to share your knowledge with us. Our team reviews every contribution with care. If it helps improve this resource, we’ll update the page and recognize you the way you chose.
Keep learning
Safe Driving Resource Center
Guides, self-checks and articles for drivers, parents of new drivers, and families recovering after a crash.
