Illinois uses three different types of chemical tests to calculate your blood alcohol level: breath, urine and blood. Of the three, the breath test is used most often, as it is the least invasive and the easiest for officers to perform. The first machine that was able to analyze a person's breath to determine their BAC was invented in 1938 in Indiana by Dr. R. N. Harger. It was known as the "Drunkometer." In 1953, Dr. Robert Borkenstein revolutionized the field by inventing a machine called the Breathalyzer, which was easier to transport and use. Despite their long history of use in the United States, breath tests can be inaccurate, however. For example, common solvents and cleaning products that are similar to alcohol can cause a high BAC rating, even if the person tested isn't drunk. If you are arrested for DUI and the breath alcohol test results seem inaccurate to you, make sure to have your own independent chemical test done.

Before a police officer can arrest you and take you in for chemical testing, the officer must establish probable cause. This is usually done through observation of such signs of drunkenness as red eyes, a flushed face, and the smell of alcohol, or on the basis of field sobriety tests. Field sobriety tests usually consist of a series of tasks that an officer has a suspected drunken driver perform. The tasks require reflexes, balance, and the ability to multi-task-all attributes that are in short supply among people who are intoxicated.

There are many types of sobriety tests out there, but there are three tests specifically recommended by the National Highway Traffic Safety Association: the walk-and-turn, the one leg stand, and the horizontal gaze nystagmus test. These three tests are the ones you'll see police officers used most often, because they have been tested extensively by NHSTA and the results are accurate more often than not. The walk-and-turn has an accuracy rate of 68%, the one-leg stand has an accuracy rate of 65%, and the horizontal gaze nystagmus test is the most accurate of all, identifying 77% of drunk drivers.

What happens if you refuse a sobriety test? In Illinois, you are consenting to take a chemical test if a police officer requests it simply by placing your keys in the ignition and starting the car. This is known as "implied consent." If you are driving and refuse to take an alcohol test, you'll still lose your license for 6 months. Refusing to take a sobriety test also goes on your driving record.

What about personal sobriety tests? These devices are small, often key-chain sized, and just right for stuffing in a pocket or a purse and traveling with you to the bar. In theory, they provide a convenient way to measure your BAC before you get behind the wheel, but can they really keep you out of trouble? Unfortunately, these devices are not as accurate as the larger and more expensive devices used by law enforcement. Sometimes they read right, sometimes they don't. They can be a fun party game, but it's not safe to trust your life and your personal freedom to these gadgets. For example, in July 2007 a New York CBS affiliate, Channel 2 WCBS, tested out some of these devices with the Westchester Police Department. They noted wildly inaccurate readings, both too low and too high. Again, it's best not to drive if you've been drinking, period.

Law enforcement tip-offs of DUI

How do the police know which cars to stop, anyway? The reality is that no matter how carefully you try to drive, if you are impaired, your driving will visibly suffer. Police officers look for tell-tale signs of impaired driving, including:

• Swerving for no apparent reason

• Weaving, inability to stay in your lane

• Driving too slowly.

• Inappropriate stops

• Riding in the center line. 

Police officers also hold Illinois DUI checkpoints every so often. At a checkpoint, cars are stopped at random-sometimes every car is stopped, sometimes every other car, etc. When an officer stops a car at a checkpoint, he or she will request the driver's license, registration and insurance card. As the driver of the stopped vehicle interacts with the officer, the officer observes the driver for signs of intoxication from alcohol or other drugs. Signs of intoxication can include the smell of alcohol, the sight of open containers, slurring speech, a flushed face and red eyes.

There is one final law enforcement tip-off of Illinois DUI: blood tests performed by emergency room personnel to determine the presence of alcohol or drugs in the patient's blood. Under Illinois law, doctors and nurses must provide such test results to law enforcement if requested.