How often do you practice standing on one leg, with your hands down at your side, for thirty seconds, in front of a mirror and if you are unable to do so even though you have had nothing to drink, you have to take yourself to jail and hang out with felons for up to 48 hours? Probably not too often.
However, if you are pulled over and the police officer smells alcohol on your breath then he or she will probably ask you to take the standardized field sobriety tests that are laid out in the National Highway and Safety Administration's Manual. First, the officer will likely ask you to perform the HGH "eye test." Then, the officer will ask you to walk on a straight line for 9 steps each way.
The final road side olympic test will be the one leg stand. Most people stone cold sober cannot pass this test. In fact, if someone is overweight or over the age of 65 the cop's own manual states that the tests are not reliable to show signs of intoxication.
Basically, the tests are set up for you to fail. However, just because you failed them does not mean that you will be convicted of DWI. It simply means that the cop felt he had probable cause to arrest you. That is very easy to accomplish in most occasions. The state must prove you were intoxicated at the time you were driving beyond a reasonable doubt which is the highest burden of proof in the American system of jurisprudence.