Requiring all drivers convicted of driving while under the influence of alcohol to install ignition interlocks on their vehicles has the potential to significantly reduce the number of fatal crashes caused by impaired drivers, according to an analysis by the Insurance Institute for Highway Safety.

One IIHS study determined that fatal crashes caused by drunk driving could be reduced by 16 percent if every state had a law requiring all offenders to install an interlock. Another study suggested that existing laws could be strengthened to be made even more effective.

An ignition interlock that measures the blood-alcohol concentration of a driver requires them to blow into the device before the vehicle can be started. The reading must register below a certain level, usually between 0.02 percent and 0.04 percent. It is illegal to drive with a blood-alcohol concentration of 0.08 percent or higher.

Currently there are 28 states, along with the District of Columbia and four counties in California, where an interlock requirement applies to all drivers convicted of driving under the influence, not just repeat offenders or those whose blood-alcohol content was particularly high. Connecticut began requiring repeat offenders to install an interlock in 2012 as a condition for reinstating their driver's license, then started requiring all offenders to install the device in 2015.

In the first study, IIHS looked at alcohol-related fatal crashes in 2016, when 25 states required first-time offenders to install an ignition interlock. In that year, 10,497 people died in crashes where a driver had a blood-alcohol level above the legal limit; 8,853 deaths were crashes involving an impaired driver in a passenger vehicle.

To analyze the effect of more widespread interlock requirements, IIHS researchers distinguished between laws that required all offenders to install an interlock to reinstate their license and those that only required it in order for an offender to drive during a post-conviction suspension.

IIHS determined that an estimated 543 deaths could have been prevented if all states had interlock requirements for all drivers convicted of drunk driving. Researchers calculated that laws requiring interlocks for repeat offenders and those with high blood-alcohol content would result in an 8 percent reduction in fatal crashes related to impaired driving, while an interlock requirement for repeat offenders alone would reduce fatalities by a statistically insignificant 3 percent.

The study acknowledged that since interlocks can only prevent impaired driving after a person has already been convicted of an offense, their main role is to discourage recidivism. However, a requirement for all offenders to install an interlock could also act as a deterrent to warn people against drunk driving.

In another study, IIHS looked specifically at the ignition interlock laws in Washington State. The state previously only required repeat offenders, those who had been stopped with high blood-alcohol levels, and those who had refused to take a blood-alcohol test to install the device. Researchers determined that recidivism declined after the state expanded its interlock laws.

Starting in 2009, Washington allowed drivers to install an interlock instead of accepting an administrative driver's license suspension before their conviction. Two years later, drivers convicted of drunk driving were required to prove that they had driven with an interlock for the past four months without any violations in order to restore their full driving privileges.

IIHS previously looked at the effects of Washington's interlock laws in 2012, and determined that they helped reduce recidivism. The more recent study sought to determine if the expanded law was having a more widespread effect.

Researchers determined that the new requirements further reduced the recidivism rate. Among first-time offenders, the rate fell from the 7.7 percent expected without the updated law to 5.6 percent.

However, IIHS noted that only 38 percent of drunk driving offenders in the state installed an interlock. Researchers said there are varying reasons for this, including people opting not to drive or simply getting behind the wheel without a valid license. But one major reason was a reduction in sentence, especially among first-time offenders, to charges that did not require an interlock installation.

"Washington's experience shows that more robust interlock laws can cut down on repeat offenses," says Charles Farmer, co-author of the study and vice president for research and statistical services at IIHS. "It also suggests that such changes could have an even greater effect if loopholes that allow people to avoid interlocks by pleading to lesser offenses were closed. It's a perfect example of why legislation, enforcement, and adjudication need to work together for highway safety policies to achieve the desired result."