Your heart begins to race as you approach the officers standing in the road and the simple, boldface signs: “Please have your license, registration and proof of insurance ready.” You smile politely—eyes, down, lips closed (God, if only you had a breath mint)—as you roll down the window.

“Good evening,” says the officer. “Been drinking tonight?”

“Just a couple, sir,” you admit. That number doesn’t sound too incriminating. You don’t feel drunk, even though you know you had more.

He knows, too. He can smell them.

“OK, I’m going to ask you to take your car right over there and park it in one of those spaces,” he says. As you obey, the call goes out over the radio, an unmistakable description of you and your car. Then, your situation: “Driver coming in now. Possible impairment.”

Whether you realize it or not, the next few minutes could be some of the most significant of your life. A DUI arrest and conviction can mean humiliation and inconvenience (your name in the paper, court appearances, suspended driver’s license), financial costs (heavy fines, astronomical insurance rates) or even the loss of your freedom (time in prison). And if you’re in an accident involving injury or death, a DUI means an automatic felony.

“It affects everything, for forever,” says Manatee County assistant public defender Lawrence Eger. “It is the pebble in the water.”

From 2001 to 2005, Sarasota County averaged 500 alcohol-related crashes—leading to 350 injuries—per year. Each year, Sarasota hosts about 15 sobriety checkpoints, each one yielding five or six DUI arrests out of approximately 500 cars. On Aug. 17, 2007, Sarasota law enforcement netted nine drivers for DUI in just four hours at a single sobriety checkpoint near Phillippi Estate Park.

The social stigma—and legal penalties—for driving under the influence of alcohol have risen sharply in the last few decades. You know that you shouldn’t drive drunk. But you may not realize exactly how much—or how little—alcohol you need to get to that state. In Florida, you will be judged DUI if you have a blood alcohol level of .08. But the police can also arrest you for DUI if you show enough signs of intoxication to be considered impaired—even if your blood alcohol is below the legal limit.

To learn more about what it takes to be arrested for drunken driving, we observed a sobriety checkpoint, talked to law enforcement, attorneys, doctors and other experts, and ran an experiment of our own. We invited five men and women of varying ages and weight to spend a few hours in our offices drinking under the supervision of two sheriff’s officers. The officers periodically tested them to see what signs of impairment they were showing. We thought we knew quite a bit about drinking and its physical and legal consequences, but what we learned surprised us.

Once you’ve been singled out at a checkpoint, you’re told to park away from the road for an examination. As you get out of the car, you see the drivers going through the checkpoint—the non-suspect ones—looking curiously at you.

A deputy approaches with a thin, foot-long, red-tipped rod. “Keep your head still and follow the tip with your eyes,” he tells you. What he’s looking for are uneven, jerky movements as your eyes move from side to side.

That jerky motion indicates an impairment of fine motor function, one of the first signs of inebriation and a condition that hinders your ability to drive a car. Research shows that, on average, the eyes begin to have that “lack of smooth pursuit” when your blood alcohol level hits .08. The traffic officer looks for six specific clues in your eye movement; if he spots at least four, he’ll continue the examination. And you’re one step closer to a DUI arrest.

“You can practice the walk-and-turn and you can practice the one-leg stand,” says Deputy Chris Butler of the Sarasota County Sheriff’s Office. “But you can’t practice the eyes.”

The walk-and-turn and one-leg stand are the other two field sobriety tests favored by Sarasota County law enforcement. During these, you’re asked to perform almost comically simple tasks that combine counting, balance and, just as important, following specific directions. You might stumble or lose count, but the officer is also aware of more subtle mistakes, like turning the wrong way or too quickly. When you’re intoxicated, it’s more difficult to concentrate on multiple things at once.

And don’t plead ignorance if you make a mental error. “We’re going to give you the instructions once,” says sheriff’s office Sergeant Darrell Seckendorf, who oversees most Sarasota County sobriety checkpoints. “Then we’re going to demonstrate the test, while saying the instructions again. And then we’re going to ask you if you understand.”

You know you’re really perfectly fine—hardly tipsy, much less drunk. But you wavered and stumbled through both those tests. The officer now believes you to be impaired, and he has other ways to verify that. You’re taken to “the BAT mobile,” the sheriff’s office’s mobile breath alcohol testing van, which is present at every checkpoint in Sarasota. Technically speaking, you can refuse the breathalyzer. But take a look at the clause printed on your Florida driver’s license: “Operation of a motor vehicle implies consent to any sobriety test required by law.” Refuse once and you’ll lose your license for a year—which is actually a civil, not criminal, penalty, because a driver’s license is not considered a basic civil right.

Refuse on two separate occasions, and you’ve committed a first-degree misdemeanor.

A blood alcohol level of .08 or higher is the ball game: You’re legally intoxicated. If your blood alcohol level is lower or you refuse the breathalyzer, the officers must decide then and there if they have enough evidence from their own observations to arrest you.

The traffic unit of the Sarasota County Sheriff’s Office frequently receives statewide recognition for its high number of annual DUI arrests (many officers perform more than 100 arrests per year). In 2006, 1,391 people were arrested in Sarasota for drunken driving. And before you accuse Seckendorf and his crew of having a quota, know this: More than 95 percent of those DUI arrests result in conviction.

But now is not the time to mount a legal defense, says Eger: “If you want to defend yourself, you do it in court, not on the street to law enforcement. If they’re not going to arrest you, they’re not going to arrest you. If they are going to arrest you, nothing good can come of you opening your mouth.”

At 11 p.m., your evening is just getting started. You’re booked on-site and sit in a “jail van” until the checkpoint is closed—usually around 3 a.m. Then you and everyone else who was arrested are taken downtown to the North County jail. You’ll spend a minimum of eight hours in a cell before you’re deemed fit for release.

So what does it take to get to that point of intoxication?

Many variables are involved. Age, weight, sex, drinking habits and the amount of food in your stomach all affect both your blood alcohol level and level of impairment. Drugs—whether prescription or recreational—can interact with alcohol in serious and sometimes unpredictable ways. Warm weather changes your hydration level and enhances alcohol’s effects.

Opportunity factors in, too. “Retirees may have time to sit around and drink all day,” says Dr. Debra Federer, an emergency room physician for Doctors Hospital of Sarasota. “And as you get older, you don’t have the ability to deal with alcohol as well. You start feeling better to the point that you drink more. People really don’t have any idea how much control they’ve lost.”

To explore these variables, we gathered our volunteers—two women, ages 24 and 59, and three men, ages 29, 34 and 51—in our editorial office while Seckendorf and Butler, a breathalyzer expert, set up their equipment. Then we started passing out drinks. Each volunteer had eaten lunch about two hours before, and each drank only one type of alcohol: 1.5 ounces of rum mixed with Coca-Cola, four-ounce glasses of wine or 12-ounce cans of Budweiser. Every half hour, the officers administered field tests and had them blow into a portable breathalyzer. Before the volunteers left with their designated drivers two hours later, we’d all learned quite a bit.

Some people show signs of impairment after just one drink—especially those who rarely drink. Our oldest volunteer (also the lightest in the group, weighing 135 pounds) usually drinks only once or twice a month. After half a glass of wine, she was the first to draw the officers’ attention with increased chatter at an increased volume. She hit .08 after only three glasses of wine. “And keep in mind,” Butler emphasizes, “those were only four ounces each. That’s a lot smaller than a restaurant glass of wine.”

People become intoxicated at different rates. An hour into our study, the 51-year-old man, who weighs 210 pounds, was asked to touch the red tip of the indicator used to check eye motion with his index finger. He missed. And giggled. Though everyone had consumed between two and three drinks, only he and the youngest woman, weighing 150 pounds, would have been considered for a possible arrest based on behavior and field sobriety tests. Both had consumed three rum-and-Cokes.

Each volunteer then blew into the portable breath tester. At that point, none of the panel—not even the two singled out as possible arrests—was above a .06 blood alcohol level.

You can be seriously impaired even if your blood alcohol level is below the legal limit. After two hours of steady drinking, none of our volunteers was comfortable continuing. In fact, most had claimed an hour earlier that they already felt past the point of driving. “I can’t believe anyone would get in a car and drive feeling the way I do right now,” one panelist said after registering a .067. “I’m glowing. I’ve got gills.”

Still, by the end of our experiment, only two had reached a .08 blood alcohol level. The officers told us that, based on the other tests, they would have considered four of the five panelists for possible DUI arrest.

Some people can drink a lot without showing many signs. The lone non-suspect at the end of the experiment was our 34-year-old beer drinker, who normally drinks about 15 beers a week and, at 280 pounds, was the heaviest of the group. Twenty minutes after finishing his sixth beer, his blood alcohol level was .05 and he was too full to continue drinking. (Beer is absorbed into the body more slowly than other forms of alcohol.) The officers saw no signs of impairment.

On the other hand, the lightest and youngest man, weighing 150 pounds, blew a .116 after five rum-and-cokes. But because he showed four of six clues when he was given the test for uneven eye motion, he was a borderline suspect without the breathalyzer results.

Even if you can’t smell alcohol on your breath, someone else can. About an hour into our experiment, other magazine employees began saying that it smelled like a bar when they walked by our office.

And our conversation with the officers that afternoon yielded a few other insights.

You don’t have to be wavering all over the road to get arrested. According to Seckendorf, most DUI arrests begin as routine traffic stops for equipment or moving violations—a broken headlight or failure to use a turn signal. Only when the officer gets to the car window do the smell of alcohol, slurred speech or other indicators arouse suspicion that the driver is impaired.

Don’t bother telling the cops that the reason you blew high was that you just finished a drink and there’s residual alcohol in your mouth. To combat that defense, suspects are observed for a minimum of 15 minutes before officers administer the breath test. “Eleven minutes is the very longest it would take for alcohol to dissipate from the mouth,” says Butler.