U.S. Department of Transportation 1200 New Jersey Avenue SE., Washington, DC 20590

National Highway Traffic Safety Administration

TRAFFIC TECH

Technology Transfer Series

Number 401 November 2010

Evaluation of the New Mexico Ignition 

Interlock Program

Impaired driving is a major factor in vehicle crashes and traffic fatalities. The use of ignition interlocks is growing as a

countermeasure to combat the high rate of offender recidivism for driving while intoxicated (DWI).

New Mexico currently has the highest rate of interlock installations per capita in the nation. The changes in usage, from its

first interlock law in 1999 to its comprehensive interlock law

passed in 2005, make it an ideal location to examine the use of

interlocks. NHTSAtherefore funded a series of studies to evaluate the New Mexico interlock laws, the New Mexico Ignition

Interlock Program, and the impact on impaired driving.

Study #1—Recidivism of Multiple Offenders With and Without 

interlocks: The first study compared license-revoked multiple

offenders who were ordered by the courts to install interlocks

(but prohibited from using those interlocked vehicles) to multiple offenders who were similarly prohibited from driving

but not required to install interlocks under the early interlock

law. Statistically controlling for age, gender, arrest blood alcohol concentration (BAC), and prior DWIs, there were significant differences in DWI recidivism between the two groups.

Multiple offender rearrest rates were 66% lower than the

rearrest rates of those without interlock devices during the

portion of the study period that the interlock was actually on

their vehicles, (see Fig. 1); however, after the interlocks were

removed, there was no appreciable difference between the

group who had used the interlocks and those who did not use

them (see Fig. 2). During the full study period (1999-2002),

including both the time on interlock and after interlock, the

rearrestrate forthose who installed the interlock was 22% less

than the rearrest rate for those without the interlock.

Study #2—Recidivism of High-BAC First Offenders With and Without Interlocks: A second study examined first-time offenders

arrested for aggravated DWI (defined as BAC of .16 g/dL or

greater, refusing breath test, or causing bodily injury while

driving intoxicated) between January 2003 and December

2005. High-BAC first offenders who installed an interlock

under court mandate, were compared with similar high-BAC

first offenders without interlocks. Statistically controlling for

age, gender, arrest BAC, and prior DWIs, results revealed a

strong interlock effect. The first offenders who had interlocks

installed had an overall 39% lower recidivism rate during the

full study period (both during and after interlocks) than the

first offenders who did notinstall interlocks.When comparing

only the period interlocks were on their vehicles, first offenders rearrest rates were 61% lower than those of first offenders

without interlocks. Once the interlocks were removed, there

was still an 18% lower recidivism rate for the interlock group,

but this difference was not statistically significant.

Figure 1: Recidivism of multiple offenders with or without 

interlocks during the period of interlock installation, New 

Mexico, 1999-2002

.10

.08

.06 Noninterlock

Interlock

Fraction Rearrested for DWI

Time During Installation of Interlock (years)

.04

.02

0.00 .25 .50 .75 1.00

Figure 2: Recidivism of multiple offenders with or without 

interlocks during the 3-year period after interlock removal, 

New Mexico, 1999-2002

.25

.20

.15

Noninterlock

Interlock

Fraction Rearrested for DWI

Post-Interlock Time (years)

.10

.05

0.00 .5 1.0 1.5 2.0 2.5 3.0
TRAFFIC TECH is a publication to disseminate information about

traffic safety programs, including evaluations, innovative programs, and new publications. Feel free to copy it as you wish. If you

would like to receive a copy, contact Angela H. Eichelberger, Ph.D.,

Editor, fax 202-366-7394, e-mail: angela.eichelberger@dot.gov.

1200 New Jersey Avenue SE., NTI-132

Washington, DC 20590

Study #3—Voluntary Interlock Installations After Third DWI 

Offense: In the past, New Mexico required a 10-year revocation of licenses for people convicted of a third DWI offense.

When given the option to voluntarily install an interlock

device and continue driving, only 9.8% of these DWI offenders chose to do so, a rate similar to other studies of voluntary interlock installations. The evidence suggested that

voluntary interlock programs do not attract many offenders.

Nonetheless, among those who did install interlocks, the

recidivism rate was 32% lower compared to those who did

not do so. However, because this program is voluntary, the

effect may be self-selection bias.

Study #4—Alternative Sentence of House Arrest: In 2003-2005,

one New Mexico jurisdiction expanded the use of interlocks

through a mandatory alternative sentence of house arrest.

When faced with the choice of either house arrest orinstalling

an interlock, 71% of people convicted of DWI chose to install

interlocks. This is the highest rate of interlock installation

on record in the United States, significantly higher than the

13% rate statewide in New Mexico during that time period

(the statewide rate is much higher now). Offenders without

interlocks were 2.5 times more likely to be rearrested than

those offenders with interlocks, as long as the interlocks were

still installed. This approach succeeded in bringing higherrisk offenders into the interlock program, which resulted in

this jurisdiction having fewer rearrests (15%) than the State

as a whole (29%) during the full study period. Recidivism

reduction while the interlocks were installed was virtually

identical for this jurisdiction and the State as a whole (61%

and 62%, respectively). A district court judge later rejected

the use of the mandatory house arrest to increase interlock

installation. Today, installation rates in this jurisdiction have

declined and are now near the Statewide average. Offenders can claim having no vehicle or no intention to drive and

avoid having to install interlocks on their vehicles.

Study #5—Pattern of Interlock Failures by Day and Time: Offenders with installed interlock devices must pass an alcohol

breath test on the interlock to start their vehicles, as well as

perform retests after the vehicle has been started while the

vehicle’s engine is still running. In the New Mexico study,

10.1 million BAC tests were logged. These tests showed a pattern similar to other interlock studies. Most BAC tests (99%)

were passed. Tuesdays had the fewest lockouts and Saturdays had the most lockouts. Weekday mornings between 6

a.m. and 9 a.m. when most people first try to start their cars

show a big spike in positive BACs. Those hours of elevated

BAC tests shifted to 9 a.m. to 12 p.m. on weekends. In both

cases, the morning BAC test lockouts are indicative of alcohol remaining from heavy drinking the night before.

Study #6—Predictors of Recidivism: Predictors of recidivism

in the New Mexico Ignition Interlock Program are similar

to those found in other studies. Dependable predictors of

future recidivism are younger age, prior DWI, rate of failed

interlock BAC tests, and failed morning BAC tests. No relationship was found between recidivism and procedural violations, such as trying to circumvent the interlock, failure to

take retests, or failed retests; however, that may be a consequence of different procedural codes used by different interlock manufacturers.

Study #7—Discussions With Representatives of Interlock System: Key informant discussions were held with judges, prosecutors, and probation officers involved with the interlock

system. These public servants managing the DWI problem

in New Mexico were generally supportive of the interlock

program, seeing it as an important tool to prevent impaired

driving. Concerns were voiced regarding the financial burden of interlocks on low-income offenders, loopholes in the

laws, and the added burden of administering the interlock

program. Some judges wanted to use interlocks to enforce

abstinence, while others saw the role of interlocks as one of

preventing impaired driving.

Study #8—Discussions With Offenders: Discussions were also

held with DWI offenders. Offenders expressed embarrassment

at having to use the interlocks. Many DWI offenders noted that

interlocks changed the way they drink, appreciated being able

to drive legally, and reported family support for interlocks.

Offenders voiced concerns over costs and device accuracy.

Note: Along with interlock programs, other programs were

implemented in New Mexico during the time of these studies, so changes in traffic safety in New Mexico cannot be

attributed solely to interlocks.

How to Order

To order Evaluation of the New Mexico Ignition Interlock Program 

(131 pages), prepared by Pacific Institute for Research and

Evaluation, write to the Office of Behavioral Safety Research,

1200 New Jersey Avenue SE., Washington, DC 20590, fax

202-366-2766, or download from www.nhtsa.gov. Randolph

Atkins, Ph.D. was the Contracting Officer’s Technical Representative for this project.