When police suspect a driver is under the influence of drugs or alcohol (DUI), they have a number of different methods to confirm or dispel their suspicions. Field sobriety tests (FSTs) are typically at the top of this list. Field sobriety tests are roadside performance tests that are supposed to give an indication of the driver's level of intoxication.
First, it's important to know that drivers generally aren't required to participate in FSTs. Although during DUI stops police routinely ask drivers to perform one or more FSTs, a driver typically has the right to refuse. But, assuming a driver agrees to participate and performs poorly on an FST, here are some of the possible implications.
To lawfully arrest someone for a DUI, police need probable cause. In other words, police need evidence that the person was under the influence of drugs or alcohol while operating a vehicle.
According to studies sponsored by the National Highway Traffic Safety Administration (NHTSA), three "standardized" FSTs are accurate predictors of a driver's blood alcohol concentration (BAC). The standardized FSTs are:
If a driver fails one of these three tests, it supposedly indicates the driver likely has a BAC of .1% or more (which is illegal in every state).
So, poor FST performance can supply an officer with probable cause to arrest a motorist for driving under the influence. (Of course, other factors like bad driving, the odor of alcohol, and breath test results can also play a part.)
A driver's field sobriety test performance can also be a factor at a DUI trial.
In every state, a driver can be convicted of a DUI for either having a BAC that's above the legal limit or being actually impaired by drugs or alcohol. At a DUI trial, prosecutors have the job of proving the driver's BAC or impairment.
Prosecutors normally prove BAC using blood or breath test results. Failed field sobriety tests—though possibly relevant to prove a driver's BAC level—are most often used in DUI trials when the prosecutor needs to prove actual driver impairment. The prosecutor can have the arresting officer testify in court about how the driver performed on the FSTs. Or, in some cases, a prosecutor might show the jurors a video of the driver actually performing the tests.
Whether the jurors hear it from a testifying officer or see it for themselves, a driver fumbling around and struggling with FSTs doesn't look good. With this kind of evidence, the jurors are all the more likely to convict the driver of the DUI charge.