If a police officer pulls you over on suspicion of DWI, he or she will begin evaluating you to see if you are intoxicated the moment they step up to the car. Does your breath smell like alcohol? Are your eyes bloodshot? Is your speech slurred? If so, the officer will likely ask you to step out of the car and take a "field sobriety test." This is a series of specific actions that test your balance, reflexes and mental capabilities. For example, you may be asked to stand on one leg, walk in a straight line and turn, count on your fingers, track an object moving horizontally with your eyes, and/or say the ABC's backwards. Three tests in particular are recommended by the National Highway Traffic Safety Association: the one-leg stand, the walk-and-turn, and the horizontal gaze test.

You can refuse to take a field sobriety test, although the officer may not tell you that. Refusing to take the tests will in all likelihood lead to your arrest, however. One problem with field sobriety tests is that both environmental conditions and physical conditions other than intoxication can cause a person to perform poorly. For example, a neurological condition such as Parkinson's disease could affect balance, as could performing the test on uneven ground. Even the three tests recommended by the National Highway Safety Association are far from perfectly reliable: the one-leg stand has a 65% possibility of predicting whether or not you are above the legal limit, the walk-and-turn has a 68% percent probability, and the horizontal gaze test has a 77% probability.

If you are pulled over on suspicion of drunk driving in Texas and you know you've been drinking, many Texas DWI attorneys advise you to refuse to perform the tests because there are so many other factors besides your level of intoxication that could affect the results. Remember, even your BAC is below .08%, you can still be charged with Texas DWI if you not functioning normally. Your performance on the field sobriety tests can be used against you in court, so it's in your best interest to provide the prosecution with as little evidence against you as possible.

Of course, field sobriety tests are not the only method the police can use to determine whether or not you've been drinking. In 1937, an Indiana University professor named Rolla Harger invented the first machine that could estimate the concentration of alcohol in a person's blood. Amusingly, this device was known as the "Drunkometer." In 1954, a more portable device called the "Breathalyzer" was invented. Although "breathalyzer" is really a brand name, today most people refer to any device that can measure BAC through breath as a breathalyzer.

Breath alcohol tests work by measuring the concentration of alcohol in your breath. With each breath you take, tiny blood vessels in your lungs called alveoli take in oxygen and give off carbon dioxide-and, if you've been drinking, alcohol. The breath test machines measure the concentration of alcohol in your breath and apply a standardized equation to estimate how much alcohol there is in your blood.

You can refuse to take a field sobriety test without a penalty. However, refusing to take a breath test for alcohol is against the law based on the principle of implied consent, which states that by operating a vehicle you are consenting to allow the state to test you for alcohol. It's important to realize that breath alcohol test results are not always accurate, either. Several factors can skew the machine's BAC reading either up or down, including exposure to chemical fumes from cleaning products, acid reflux and heartburn, and your individual lung capacity. So, if you are charged with DWI and you don't believe you were intoxicated at the time, it's best to hire a Texas DWI attorney who can help you challenge the results of the breath test.

What about personal sobriety tests? These small, often key-chain-sized devices are marketed as a way to test yourself before you get in a vehicle. Can these help prevent you from getting a Texas DWI? While they might be helpful in showing you or a friend that you've had too much to drink, they are not accurate enough for you to rely on completely. For example, in November 2006, Popular Mechanics tested 3 personal sobriety test devices and found inconsistent results with each. Also, remember that in Texas, if you are impaired you can be charged with DWI no matter what your blood alcohol concentration is.

Law enforcement tip-offs of DWI

How do the police know which drivers to pull over for DWI in Texas? Since alcohol affects both judgment and reflexes, there is a specific set of driving behaviors that police officers are trained to use to spot Texas drunk drivers. Some behaviors that are almost guaranteed to get you pulled over include: weaving, straddling two lanes at once, turning abruptly and/or with a wide radius, drifting from lane to lane, narrowly avoiding collisions, going more than 10 miles under the speed limit, and making sudden starts and stops. Speeding is only used as a measure of how intoxicated a driver might be if it occurs along with other behaviors such as those mentioned above. Driving fast while at the same time maintaining control of your vehicle requires sharp reflexes. Intoxicated people, of course, are not known to have sharp reflexes.