(CNN) Since 1993, laws in all 50 states put ignition interlock systems in the cars of some drunken driving offenders. If the device detects alcohol on a driver's breath in excess of the legal limit, the vehicle will not start.

Mandatory laws that require every person convicted of a DUI to use an interlock were associated with a 7% decrease in the number of fatal drunken driving crashes, according to a study published Thursday in the American Journal of Preventive Medicine

"Prior to this study, we didn't have any evidence on whether ignition interlock laws ... reduce alcohol-involved fatal crashes -- which is of course the goal of the law," said Emma McGinty, assistant professor in the Center for Injury Research and Policy at the Johns Hopkins Bloomberg School of Public Health and Colorado School of Public Health. "So that was the big question: Do these laws work?"

Currently, 28 states and the District of Columbia have mandatory (sometimes called "all-offender") ignition interlock laws, according to the nonprofit Mothers Against Drunk Driving . Mandatory laws require every person convicted of a DUI to use an interlock. Partial laws apply only to a subset of DUI offenders, usually repeat offenders, and permissive laws are applied to specific DUI offenders at the discretion of a judge.

There was a question too as to whether mandatory laws or partial laws would work better, McGinty said.

Different laws, different trends

To estimate the impact of these laws, McGinty and her colleagues looked at alcohol-involved fatal crashes over a 32-year period, from 1982 to 2013.

Specifically, the researchers used data from the National Highway Traffic Safety Administration's Fatality Analysis Reporting System and looked for changes in the rates of alcohol-involved fatal crashes from before and after each state's interlock law was passed.

Ignition interlock devices prevent vehicles from starting if they sense alcohol on a driver's breath.

The researchers also measured differences between mandatory, partial and permissive laws, while taking into account other vehicle safety laws passed and trends in crashes during the study period.

"A lot of things affect these trends," McGinty said, including other laws such as states' seat belt laws and the increasing manufacture of airbags over the decades.

The researchers also looked at alcohol-involved fatal crashes in which a driver had a blood alcohol level at or above the legal limit of 0.08.

"We found that the mandatory laws were associated with a 7% decrease in alcohol-involved fatal crashes" among drivers with a blood alcohol level of greater than or equal to the legal limit of 0.08, said McGinty. This translates into about 1,250 prevented fatal crashes in states with mandatory interlock laws between 1982 and 2013.

She and her co-authors also estimated an 8% reduction in fatal crashes occurring among drivers with a blood alcohol level greater than 0.15.

The partial laws were associated with about 2% reduction in alcohol-involved fatal crashes, said McGinty. "So regardless, it suggests the mandatory laws are more effective than the partial laws."

Why ignition interlock can fail

According to Guohua Li , interim chairman and professor of epidemiology at Columbia University's Mailman School of Public Health, the effects reported in this study "are quite modest."

Still, Li -- who was not involved in the research -- finds that the results "are pretty much consistent with previous studies." Even if "this is the most comprehensive study of interlock laws in the United States," Li said, "this one covers a much longer time period and all 50 states, and also it's much more updated than previous studies."

Drunken driving is a complicated issue, and if the effect of ignition interlock laws is small, the reason is enforcement, said Li.

"Its effect is very limited because of economic and social barriers to implement it at full scale," he said, noting that ignition interlocks cost money and raise legal issues.

The cost of installing and maintaining an ignition interlock is typically borne by the offender. According to the Centers for Disease Control and Prevention , costs averaged about $175 to install and $2.25 per day to maintain in 2006, the most recent year studied.

Legal issues include user claims of false positives and loss of privacy: You are visible as you blow into the alcohol-sensing attachment before starting your car.

Also, a lock doesn't really resolve the fundamental issue of alcohol use and alcohol dependence, Li said.

"Even when the ignition interlock is installed, previous studies have found the fact of the device on drunk driving behavior is usually limited to the first six months," he said. "After the first six months, the drivers maybe have figured out a way to bypass the device."

Li is not alone in his opinion.

"Ignition interlock devices have been proven unreliable per a well-documented 2004 study from the California DMV," said Gary Biller, president of the National Motorists Association. The study, which McGinty also cited in her report, found less effectiveness among first-time DUI offenders with interlocks on their cars compared with second offenders.

Biller added that the devices often provide false readings, and the cost of installation and maintenance is high, "compounding issues for problem drinkers."

"DUI attorneys have often said that if a person wants to drink and drive, they will find a way," he said. "The best solution is education and rehabilitation programs, particularly for repeat offenders."

Li also believes that "treatment, rehabilitation program and other interventions" may be necessary. Still, he said, "if it is implemented in more innovative ways," ignition interlocks "may be a very useful tool."

'Welcome news' to advocates

Despite mixed attitudes, use of ignition interlocks has grown since the early 1980s.

"The first 20 years, the numbers were pretty small: less than 30,000 in use in the entire United States. In the last 10 years, it's grown dramatically. There will probably be 400,000 in use at the end of this year. So massive increase in the usage," said Brad Fralick, director of government relations for Intoxalock, a maker of interlock systems.

The National Highway Traffic Safety Administration "strongly supports the expansion of ignition interlocks as a proven technology that keeps drunk drivers from getting behind the wheel," said Bryan Thomas, a spokesman for the administration within the Department of Transportation.

Advocates also appreciate ignition interlock laws and the results of the new study.

"This is really welcome news for MADD. This is what MADD has been saying for years now, and this is another study that exactly backs up what we've been trying to do, which is to pass (mandatory) laws in all 50 states," said J.T. Griffin, chief government affairs officer at MADD.

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"Unfortunately, drunk driving is still ... a third of all traffic deaths ," Griffin said. "And we've done a good job to bring deaths down, but there's still a lot of work to do. There's still 10,000 people a year dying because of drunk driving."

Though alcohol-intoxicated driving is still a major issue, Columbia University's Li suggests policymakers refocus their attention. After all, ignition interlock systems don't prevent other types of impaired driving.

"Driving under the influence of non-alcoholic drugs has been increasing progressively in the US over the past 15 years," Li said. "We need to pay more attention to other drugs."