The new Missouri law is targeted primarily at repeat drunken drivers who are seeking to regain their licenses, but it also applies to anyone convicted of involuntary manslaughter while driving drunk.

Ignition interlocks prevent engines from starting until drivers breathe into an alcohol detector to prove they are sober.

Nearly all states give judges the option of ordering the devices for drunken drivers. In the past few years, about 20 states have passed laws mandating their use for first-time or repeat offenders, according to Mothers Against Drunk Driving.

Missouri's law could have a particularly broad affect because it applies to people convicted of drunken driving as many as 10 years ago who are just now eligible to regain their licenses.

The Missouri Department of Revenue, which issues driver's licenses, plans to send more than 70,000 letters to offenders in early July notifying them of the need to get ignition interlocks to legally drive again. The law requires them to be used for six months.

Offenders must cover the costs for the devices, which can range from $30 to $200 to install and an additional $70 to $100 a month to rent.

Those who drive without the devices after July 1 could face an additional two-year license suspension.

Mothers Against Drunk Driving, which backs ignition interlock requirements for anyone convicted of drunken driving, says the devices have led to significant declines in the number of repeat offenders. It cites a 2008 study by the Pacific Institute for Research and Evaluation that found interlock devices in New Mexico helped decrease repeat offenses by about two-thirds.

Matthew Guilfoil, a defense attorney in Kansas City, specializes in driving-while-intoxicated cases. He says the devices can produce false results from spicy food or cigarettes and can cause accidents by requiring drivers to re-verify they're sober while the vehicle is running. He described Missouri's law as "draconian."

"It's going back and punishing people again for an old offense" by adding a new requirement to reinstate their driver's licenses, Guilfoil said.

A Missouri law passed in 2001 let judges order ignition interlocks for people convicted of at least two drunken-driving offenses. A few years later, Gov. Matt Blunt sought to mandate the devices through the state's license administration because he said judges weren't imposing the requirement enough.

The tougher Missouri law passed in 2008 but didn't go into effect until this July.

Seeking to capitalize on the law, private-sector probation officer Chuck Lee last year began installing and servicing ignition interlocks in the eastern Missouri town of Bonne Terre, which is home to a state prison. The expected business boom has yet to occur, Lee said, though that could change in the coming weeks.

"Of course, most people are probably waiting until the last minute before they break down and shell out the money to have the devices installed," said Lee, president of Securicor Ignition Interlock.

In the Springfield area, Evergreen Interlock, a company from Kansas City, sells and installs the devices at some stores.