Before administering a field sobriety test on the side of a Massachusetts roadway, law enforcement officers have to go through training to learn how to correctly administer them. Two of these are the walk-and-turn test and the one-leg stand test. These measure balance, which can be precarious for someone who has a blood alcohol concentration at or above the legal limit of 0.08 percent.
According to AAA, an officer may suspect that drivers are under the influence of alcohol if they have to put their arms out to catch their balance while taking heel-to-toe steps, or while standing on one foot and holding the other about six inches off the ground. If a driver also steps off the line in the first test, or puts the foot down in the other, the officer may determine that those two errors are enough reasonable proof that a person is impaired and can be required to take a breath or blood test.