The driver impairment study volunteer at the University of Iowa National Advanced Driving Simulator drives in the NADS-1 driving simulator after being given a mix of alcohol and juice of choice on August 20, 2013 in Coralville, Iowa.

— Timothy Brown has put hundreds of drunken drivers behind the wheel.

In the research center where he works, the drivers ingest vodka or 151-proof alcohol and get behind the wheel of a Chevrolet Malibu mounted in a metal pod about the size of a two-car garage. Then they take a spin in what's considered the world's most sophisticated driving simulator, while Brown and his colleagues gather data.

Brown is using that data to better understand the difference in driving abilities of someone who is sober, someone who has had a few drinks and someone who has had a few more drinks. That work has been made especially timely by a controversial National Transportation Safety Board recommendation to lower the legal limit of intoxication to a blood alcohol content of 0.05 from 0.08.

Brown, senior research associate at the $100 million National Advanced Driving Simulator in Iowa, is looking to isolate the precise differences in driving performance with no alcohol in the blood and at a level of 0.05. His work is expected to shed light on the national debate.

"My heart doesn't tell me anything," Brown said when asked for his best guess on whether 0.05 was serious impairment. He acknowledges that diminished performance happens at 0.05 but would not elaborate "because I'm a researcher and the data drives me."

Over Labor Day weekend, one of the busiest holiday driving weekends when an estimated 30 million drivers will take to the roads, authorities will be on the lookout for those who attempt the potentially dangerous mix of drinking and driving. But what constitutes dangerous driving is once again up for debate.

Calling impaired driving "a national epidemic," NTSB Chairwoman Deborah Hersman made the 0.05 recommendation in May. It was one of several proposals that include high-visibility DUI enforcement, expanded use of alcohol-sensing technology and ignition interlock devices, and more DUI treatment courts.

Research suggests that lowering the legal limit of intoxication to 0.05 could save 500 to 1,000 lives a year.

But many safe-driving advocates are conspicuously silent on the issue of whether 0.05 is high enough impairment to merit criminal charges. Mothers Against Drunk Driving is standing down, as is Illinois Secretary of State Jesse White. The venerable Insurance Institute for Highway Safety, which notes that it never takes formal positions on policy, said police will have trouble enforcing 0.05.

At the core of concerns about 0.05 is the tricky issue of when alcohol impairment becomes criminally negligent. How does slight alcohol impairment differ from impairment caused by drowsiness, cellphone use, medication, aging or other conditions? Is it reckless to get behind the wheel after two glasses of wine at a dinner party? A large beer at a Blackhawks game? A couple of cocktails at a reception?

Research on the topic is voluminous — and resembles a weaving car.

The National Sleep Foundation states that drowsiness is very similar to alcohol impairment and "can impair driving performance as much or more so than alcohol," according to a report on the topic. Being sleepy can slow reaction times, limit vision and create lapses in judgment and delays in processing information, the foundation states.

"In other words," the foundation reports, "driving sleepy is like driving drunk."

A 2003 study by University of Utah showed that motorists who talk on cellphones — hands-free or not — are as impaired as drivers at a 0.08 BAC. Study participants drove slower and hit the brakes and accelerated later than those driving undistracted. Drunken drivers drove slower than cellphone users and undistracted drivers but more aggressively. They also followed more closely and hit their brakes more erratically, the research showed.

As to whether such a thing as responsible drinking and driving exists, some research shows that lane deviations and attention lapses occur at a BAC as low as 0.001. MADD and the National Institute on Alcohol Abuse and Alcoholism recommend no one drive after drinking.

But the American Beverage Institute, which represents restaurant and bar owners, calls the 0.05 recommendation an effort to "criminalize perfectly responsible behavior," saying that less than 1 percent of traffic fatalities in the U.S. are caused by drivers with a BAC from 0.05 to 0.08. The organization points to National Highway Traffic Safety Administration data showing that 70 percent of drunken driving deaths involve a driver with a BAC of 0.15 or higher.

In making its recommendation in May, the NTSB noted that more than 100 countries, including many in Europe, have set 0.05 as the legal limit of intoxication and experienced significant drops in traffic fatalities after doing so. Drunken driving accounts for nearly 10,000 traffic fatalities a year in the U.S.

"The research clearly shows that drivers with a BAC above 0.05 are impaired and at a significantly greater risk of being involved in a crash where someone is killed or injured," the NTSB's Hersman said in recommending the lower level.

Sarah Longwell, managing director of the American Beverage Institute, contends the NTSB research lacks context. The significantly greater risk that the NTSB points out is no different than the risk that accompanies listening to a loud car radio or having a passenger talking to the driver, she said.

Emphasizing other countries that set their legal definition of intoxication at 0.05 is "an apples to oranges comparison" to the U.S., Longwell said. Many of those countries have "vastly different" driving, mass transit and drinking cultures, she said. In addition, the countries imposed "other draconian measures," including random breath testing, that contributed to the decline in traffic fatalities and would be unacceptable in the U.S., Longwell added.

"We're going to stand by 0.08 as the law," she said.

'Pretty harsh'

Clarifying alcohol's effect on the body can be tricky.

Generally speaking, the liver, brain, pancreas and stomach break down and eliminate alcohol through enzymes that convert the substance into water and carbon dioxide, the U.S. Department of Health and Human Services reports.

During that conversion, one of the enzymes metabolizes into "a highly toxic substance and known carcinogen," acetaldehyde. That substance and an alcohol-metabolizing enzyme known as cytochrome contribute to the development of cancers in the respiratory tract, liver, colon or rectum and breast, Health and Human Services research shows.

Alcohol's effect on the brain is considered harmful but somewhat uncertain. In its 24-page "Beyond Hangovers: Understanding alcohol's impact on your health," even the National Institute on Alcohol Abuse and Alcoholism states, "There still is much we do not understand about how the brain works and how alcohol affects it."

What research does show is that alcohol can slow communication between chemical neurotransmitters that carry messages between the brain's estimated 100 billion neurons. Some research indicates that acetaldehyde may contribute to that impairment. Lab animals that received acetaldehyde exhibited impaired coordination and memory and sleepiness, according to research published in 2006 in the journal Alcohol Research & Health.

Brain regions most vulnerable to alcohol include the cerebellum, which controls motor skills; the limbic system, where memory and emotion are centered; and the cerebral cortex, which connects to the nervous system and deals with the ability to think, plan, remember, solve problems and interact socially.

The metabolism of alcohol varies based on genetics, including variations in those enzymes, and environmental factors such as the amount a person drinks and his or her diet, the Department of Health and Human Services states.

"Regardless of how much a person consumes," a department report notes, "the body can only metabolize a certain amount of alcohol every hour," an amount "that varies widely among individuals and depends on a range of factors, including liver size and body mass."

That wide variance may contribute to lawmakers' reluctance to embrace 0.05 as the new drunk.

No Illinois legislators are pitching bills to lower the legal level of intoxication, said state Rep. John D'Amico, D-Chicago, chairman of the Vehicles and Safety Committee.

"You don't want anybody driving drunk, obviously," D'Amico said. "But this would be limiting everybody to one beer. That'd be pretty harsh."

He also said the lower level "would have a domino effect." Restaurants, bars and sports stadiums would suffer.

Call for ignition interlocks

Illinois Secretary of State White, considered a leader in fighting drunken driving, is taking a pass on tinkering with the 0.08 level (adopted in 1997), a spokesman said. White is planning instead to push for legislation that would expand for repeat offenders the use of ignition interlocks — in-car breath testers that prevent the engine from running if alcohol is detected on the driver's breath, the spokesman said.

Wider use of ignition interlocks is central to MADD's efforts, too. J.T. Griffin, the organization's senior vice president of public policy, said MADD supports 0.08 as the limit of legal intoxication in large part because research over 50 years shows definitively that everyone is seriously impaired at that level. Impairment below 0.08 becomes a little more uncertain.

Griffin said pushing to make 0.08 the law "was such a tough battle to fight. We sort of established it as the across-the-board level."

MADD is taking a more practical approach — including recommending that ignition interlocks be mandatory for all DUI offenders — in continuing its fight against drunken driving. Twenty-one states and four counties in California require interlocks for all drunken driving offenders. In Illinois, first-time DUI offenders must obtain one if they want to drive.

"We're doing a lot of really positive things," Griffin said, "and we feel like we've got a lot of momentum. To shift to 0.05 really goes against what we're doing."

Like many, Griffin said impairment happens below 0.08, but "what that level is, I don't really know."

Which is why Brown's work at the simulator may be illuminating. He said he's downloaded the data file from a key 2008 project that focused on blood alcohol levels and impaired driving and will begin re-crunching the analysis in a couple of weeks. He may have preliminary results in October, he said.

But getting there won't be as exciting as watching the pod slide, tilt and spin in the research center.

"This," Brown said, "mainly involves sitting in front of a computer screen."

tgregory@tribune.com