The McPherson Law Firm


The McPherson Law Firm


The McPherson Law Firm


DWI and DUI Terms and Definitions

Judd McPherson • Feb 05, 2021


Reasonable suspicion is a general criminal law concept that also applies to driving under the influence. It requires that officers have an objectively reasonable basis for suspecting criminal activity before detaining someone. It’s a standard that’s lower than probable cause (discussed below). Before pulling someone over, an officer needs only a reasonable suspicion that the motorist has violated the law. An officer can pull you over, for example, after witnessing you swerve across the road, drift in and out of a lane, or commit some other traffic violation.



Probable cause is another standard that applies to all kinds of criminal cases: It is the measure by which judges evaluate arrests. There’s probable cause for an arrest if the facts support an objective belief that the suspect has committed a crime. So, after pulling you over, but before arresting you for DUI, a police officer must have probable cause to believe that you were too impaired to drive.



Police officers use a set of preliminary tests known as field sobriety tests (FSTs) to determine if a DUI suspect is intoxicated. These physical tests—as distinguished from chemical breath and blood testing—assess the driver’s physical dexterity and ability to follow directions. The most common FSTs are:

  • standing on one leg
  • walking and turning
  • counting backwards
  • reciting the alphabet
  • placing finger-to-nose, and
  • Horizontal Gaze Nystagmus (HGN), a test in which the suspect follows the officer’s moving finger.



If field sobriety tests don’t eliminate the likelihood of intoxication, police officers generally administer chemical testing: either a breath test performed with a device known as a breathalyzer or blood sampling. A third, less favored chemical test involves urine analysis (discussed below). All three tests are administered to determine the suspect’s blood alcohol content (BAC).



BAC refers to the proportion of alcohol found in a driver’s blood. BAC—measured as weight per unit of volume—is reported as a percentage of one percent of the driver’s blood. For example, a BAC of.20%, means that 20/100 (or 1/5) of one percent of the driver’s blood is comprised of alcohol. Typically, the level at which a driver is deemed too intoxicated to drive is.08%.

In all states, it’s illegal to drive with a BAC of .08% or more—called DUI per se—regardless of whether you’re ability to drive safely was actually impaired.



The law prohibiting drunk driving is known in most states by the acronym DUI (for driving under the influence). But many states, including Missouri, use other acronyms, including:

  • DUIL (driving under the influence of liquor)
  • DWI (driving while intoxicated)
  • OMVI (operating a motor vehicle while intoxicated)
  • OWI (operating while intoxicated), and
  • OUI (operating under the influence).

Regardless of the acronym, the meaning is generally the same: driving under the influence of alcohol or drugs.



Don’t make the mistake of confusing a DWI with “drunk driving.” If you’re arrested for a DWI, the state prosecutor doesn’t need to prove you were “drunk.” Generally, all the state has to show is that you consumed enough alcohol or took enough drugs so that your ability to control the motor vehicle was impaired.



Every state has implied consent laws that generally require a driver to submit to a chemical test when law enforcement is investigating a potential DUI. If you do not take the requested test, it will be deemed that you refused the test. This is true even if you simply remain silent, meaning that you do not have to actually say the words “no” when asked to take a test.



An ignition interlock device (IID) is similar to a breathalyzer (a device used to measure blood alcohol content). An IID is connected to a vehicle dashboard or another location inside the vehicle; it requires that the driver breathe into the device before starting the vehicle. If the ignition interlock device detects BAC that’s above the programmed limit, then the engine of the vehicle won’t start.



An SR-22 is a document that verifies that someone has automobile insurance. (The “SR” stands for “safety responsibility.”) An insurance company typically prepares the SR-22 and files it with the department of motor vehicles (DMV). The SR-22 is not an insurance policy; it is evidence that the driver has a policy.