When drunk drivers are prosecuted in Georgia and around the country, breath test results are almost always the most important form of evidence. Suspected drunk drivers usually take two breath tests. The portable devices that police officers use to conduct roadside breath tests are not considered reliable enough for court, so DUI suspects are tested a second time at police facilities.
The machines used to conduct breath tests are extremely sophisticated, but they do not always provide accurate results. When breath test results are excluded in a drunk driving case, it is usually because the machine used to conduct the test was faulty or because the police officer who made the arrest violated rights protected by the U.S. Constitution.
Unreliable breath test machines
The large breath-testing machines found in police stations are accurate to the third decimal point when they are regularly recalibrated and used correctly, but that does not always happen. In New Jersey, 18,000 breath tests were invalidated because a state trooper made a calibration error. In Massachusetts, calibration errors became so common that a judge invalidated every breath test conducted in the state for eight years.
The Fourth Amendment
Breath test results may be inadmissible in court even if the equipment used works properly. The Fourth Amendment protects motorists from unreasonable search and seizure, and it prevents police officers from pulling vehicles over or ordering breath tests for no reason. Vehicles may only be pulled over if police officers witness a motor vehicle infraction or have good reason to believe that a crime has been or is being committed, and they can only order DUI suspects to take breath tests if they observe signs of intoxication like slurred speech or bloodshot and watery eyes. When police officers act without probable cause or reasonable suspicion, breath tests may be excluded.
Reasonable doubt
Drunk driving suspects are presumed innocent, and prosecutors must establish guilt beyond all reasonable doubt to convict them. This is usually done by introducing toxicology test results, but this evidence is not always reliable. If breath-testing machines are not properly maintained and regularly recalibrated, the results they produce may not be admissible. Toxicology evidence may also be excluded if police officers violate rights protected by the Fourth Amendment.