Every year, states add new legislation dealing with driving under the influence of drugs/alcohol laws in order to deter motorists from making decisions that could affect the rest of their lives. Sometimes these come in the form of sentence changes, such as new or additional penalties, and on occasion they come in the form of relaxed or more lenient changes, depending on the individual in question, the circumstances surrounding the case, or the law itself.
Ignition interlock devices, or IIDs, are some of the newest additions to the sentencing/penalty phase, and they are machines installed directly onto a vehicle that test blood alcohol concentration. You are responsible to pay for the installation and upkeep, including monthly maintenance fees, but it grants those qualified restricted driving privileges.
Several states have added ignition interlock devices, commonly called IIDs, as an alternative to complete loss of driving privileges for certain individuals. Normally, this is an option for those with relatively clear criminal records and driving histories aside from the DUI conviction, but only in states that currently offer an IID option. Some states have not yet adopted or implemented this alternative, and in those states individuals convicted of DUI must seek other options, such as a restricted work license.
An IID works by forcing the individual to pass a Breathalyzer-style test before starting the vehicle and at random intervals while driving. It will not shut off the vehicle immediately if you fail during driving, but will begin to sound very loud alarms to warn you that a shutdown is imminent, forcing you to pull over and park. The vehicle cannot be restarted until the test is passed successfully.
Currently, only 2 states in the United States have no form of ignition interlock law: Alabama and South Dakota. The other 48 states have each implemented these laws wholly or to some lesser extent, such as those that only require/allow and IID for certain offenses.