A new study finds ignition interlocks, devices that prevent a person from starting a car if their blood alcohol level is too high, prevents deadly accidents. Ignition interlocks are associated with a 7 percent decrease in the rate of fatal crashes involving at least one drunk driver, researchers report in the American Journal of Preventive Medicine . Cars with the device will not start if the driver’s blood alcohol level exceeds a preset limit, HealthDay reports. While all states have some type of ignition interlock law, only half require everyone convicted of a DUI to use the device, the researchers noted. Some states require them only for repeat offenders, or for those caught driving with very high blood alcohol levels.