Study: Mandatory Ignition Interlocks Do Reduce Fatal Crash Rate


Researchers at Johns Hopkins Bloomberg School of Public Health and the Colorado School of Public Health have found that the rate of fatal motor vehicle crashes has dropped in states with mandatory ignition interlock laws. Twenty-six U.S. states have these laws, which require every driver convicted of DUI to install and maintain a device on their vehicle which won’t allow it to start unless the driver is sober. In the other 24 states, only some DUI convictions require the interlock.

“Until recently, there hasn’t been any evidence on whether these laws prevent alcohol-involved fatal crashes, and specifically whether mandatory/all laws are more effective than permissive and partial laws,” said one of the study’s lead authors. “Our study suggests that they are effective.”

The study found a pretty strong association between having a mandatory interlock law and a drop of about 7 percent in the rate of fatal crashes involving at least one drunk driver. The researchers used trend data from the National Highway Traffic Safety Administration’s Fatality Analysis Reporting System, or FARS, and laid it out against the three main types of ignition interlock laws:

  • Mandatory — all DUI convictions result in ignition interlocks being required
  • Partial — only specific DUI convictions will require ignition interlocks
  • Permissive — ignition interlocks are required at judges’ discretion

The team compared changes between pre-and post-passage of the interlock laws, controlling for other traffic safety laws, and also looked at differences between crashes involving drivers with a blood-alcohol level of 0.08 and those where the driver’s BAC was above 0.15. The trend data covers the 32-year period between 1982 and 2013.

Since those laws started getting passed, states with mandatory interlock laws saw an estimated 1,250 fewer people die in drunk driving crashes than states with partial or permissive laws.

“It’s encouraging to see more and more states moving towards this evidence-based policy change,” said the researcher. “Since 2005, we’ve seen over 20 states adopt interlock laws for all drunk-driving offenses. We’d like to see the remaining states follow suit.”

Florida’s ignition interlock law could be described as either partial or permissive. First DUI convictions only result in an interlock at the judge’s discretion, unless the driver’s blood-alcohol level was 0.15 or higher or there was a minor in the car. In those cases and upon subsequent convictions, the interlock is mandatory.

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