Police typically use three methods for determining whether a driver is under the influence of drugs or alcohol: general observation, field sobriety tests (FSTs), and breathalyzers.
Observations. Generally, an officer’s decision to conduct a DUI stop or investigation starts with an observation indicating possible driver impairment. An officer might see the motorist swerving or driving unusually slow. Or while conducting a traffic stop, an officer might note the driver has bloodshot eyes, smells of alcohol, or is behaving strangely. An innocuous explanation might exist for an officer’s observations. But, typically, an officer who notices anything that could signal intoxication will at least want to investigate to be sure the driver doesn’t pose a danger.
Field sobriety tests. While conducting a DUI stop, an officer might ask the driver to perform field sobriety tests (FSTs). Officers use lots of different FSTs. But the most common ones are known as the “standardized” FSTs: the horizontal gaze nystagmus, walk-and-turn, and one-leg stand. Police rely on FSTs to gauge the level of a person’s intoxication. Poor FST performance can persuade an officer to make a DUI/DWI arrest. Generally, a driver isn’t required to participate in FSTs and can’t be formally penalized for refusing.
Chemical testing. At the roadside, an officer might ask a driver to take a breathalyzers test. Typically, a pre-arrest breath test is optional—meaning the driver is free to refuse. The breath-test machines officer use in the field is sometimes called “preliminary alcohol screening” (PAS) devices. PAS results give police an idea of what a driver’s blood-alcohol level but are generally less accurate than breath-test machines—called “evidential breath test” (EBT) divides—that police use post-arrest, once the driver has been taken to the police station.