The police can only make a legal arrest if they have probable cause to believe the driver is under the influence of alcohol or drugs (or has a blood alcohol concentration of at least 0.08). Probable cause can be based on one fact or on a constellation of facts. For example, if the first thing you say to an officer who stops you is “I am way too drunk to drive,” that admission alone will probably justify a DUI arrest.
In the absence of “smoking gun” evidence of DUI, the police will rely on a variety of clues to support a belief that a driver is probably under the influence of alcohol. Those clues might include:
Police officers are trained to believe that the last two factors provide particularly strong evidence that a driver is under the influence of alcohol. In most instances, the police will administer field sobriety tests (FSTs) that are designed to produce that evidence.
When the police arrest a driver without probable cause, evidence obtained after the arrest cannot legally be used as evidence in a DUI prosecution. A DUI defense lawyer will file a motion to suppress (throw out) that evidence because it resulted from an illegal arrest.
Breath and blood test results are the most important evidence that can be suppressed due to an illegal DUI arrest. When those test results are suppressed, prosecutors often dismiss cases rather than going forward with a trial.
Judges understand that the police are itching to make DUI arrests. In the past, many judges were skeptical of arrests that were supported by thin evidence of DUI. For example, since it is not illegal to drive after drinking half a glass of beer, judges will not allow an arrest to be based solely on the odor of an intoxicant.
To enhance their evidence of probable cause, police began to rely on field sobriety tests. They developed a range of tests, from touching the nose with the index finger to counting backwards or reciting a portion of the alphabet. Depending on the officer’s testimony, judges accepted some arrests based on those test results but concluded that other arrests were illegal. Since none of those tests had been scientifically validated, many cases were dismissed when arrests hinged on FST results.
Concerned that too many DUI arrests did not lead to convictions, The National Highway Traffic Safety Administration (NHTSA) decided to develop standardized FSTs that were scientifically valid. The three standardized FSTs (described elsewhere on this website) are:
Since the tests were “validated” without using double-blind tests administered to randomly selected test subjects, there is nothing scientific about NHTSA’s research methodology. And since the validation involves tests given in controlled environments, not tests on the side of the road at night, with passing headlights, flashing red-and-blue squad car lights, and adverse weather conditions, the tests have little to do with the real world.
Only the three standardized tests mentioned above have been “validated.” No nonstandard test has ever been accepted by the scientific community as being a reliable basis for determining whether a test subject is probably under the influence of alcohol.
We agree with NHTSA that drivers who pass all the standard FSTs are probably sober. On the other hand, critics who have analyzed NHTSA’s data have concluded that drivers who do not pass the tests have about a 50-50 chance of being sober. NHTSA combines the statistics of test subjects who pass and test subjects who fail to conclude that the tests usually produce the right result. In fact, the tests usually produce the right result when drivers pass, but are little better than guesswork when drivers fail.
Field sobriety tests probably do identify drivers who are seriously drunk. Unfortunately, many drivers who are not under the influence of alcohol also fail the standardized FSTs. There are many reasons that drivers might “fail” either a standardized or a nonstandard FST. Head injuries, leg and back injuries, a variety of other medical problems, fatigue, and old age can all have an impact on FST results. One recent study found that standardized FSTs, even when administered to healthy test subjects, yield a significant percentage of false positives — that is, sober people are falsely determined to be under the influence of alcohol.
While most judges accept the doubtful proposition that the results of standardized FSTs can be relied upon to establish probable cause to support an arrest, it is important to understand that NHTSA regards the test results as valid only if the tests are administered properly. A skilled DUI defense attorney who has studied the NHTSA training manual will often undermine an officer’s testimony by asking cross-examination questions that expose the officer’s failure to perform the tests correctly.
It is also worth noting that NHTSA originally recommended that police officers take an 8-hour refresher course every two years. That recommendation was based on the realization that police officers often fail to absorb or retain their initial training and develop sloppy habits over the course of time as they administer FSTs. Police departments resisted that recommendation because they did not want to pay for the refresher training.
An experienced DUI defense lawyer can discredit non-standard and standardized DUI tests during suppression hearings. Even if jaded judges accept FST test results, juries are less likely to give them any weight during a DUI trial after a defense lawyer points out all the reasons to believe that FST performance has nothing to do with the ability to drive safely or with blood alcohol concentrations.