DWI Vocabulary

Alveolar Breath

This is the breath that comes from the deepest parts of the lungs, and is the breath that breath machines are meant to measure for BAC.


blood alcohol concentration

Blood Alcohol Concentration

aka, BAC.  The percentage of alcohol in a person’s blood


Breath Alcohol Concentration

Breath Alcohol Concentration

aka, BrAC.  The percentage of alcohol in a person’s Alveolar breath sample.


These are the things that officers are trained to look for during the administration of the Standardized Field Sobriety Tests.


driving indicators that allegedly indicate a person may be intoxicated

Divided Attention Test

A test that requires the subject to concentrate on mental and physical tasks at the same time.  This is the foundation of the Standardized Field Sobriety Tests.


Driving While Intoxicated.


Driving Under the Influence.  There is no DUI in Texas, but since many people use the terms DWI and DUI synonymously, I have included it here.

DWI Detection Process

This is the process NHTSA says officers must use in order to determine whether a person under suspicion of DWI should be arrested.  It includes three phases, and the ultimate arrest/no arrest decision should be based on the totality of the circumstances – meaning, it should be based on the officer’s observations and judgments from all three phases of the DWI detection process.  The three phases are:

Phase One – Vehicle in Motion.  Includes observations of safe and unsafe, legal and illegal driving activity.  The officer should not base an arrest decision on one single act of unsafe or illegal vehicle action, but should consider all factors, including the use of turn signals when pulling over after being signaled by the officer to do so, proper response to emergency lights, coming to a controlled and safe stop in a safe and appropriate area to stop the vehicle.

Phase Two – Personal Contact.  Includes the officer’s observations of proper speech patterns, slurred or clear speech, a suspects response to questions, and ability to follow instructions.  During this phase the officer will typically ask the driver for their driver’s license and other documentation, then begin asking other questions while the person is looking for those documents in order to determine whether the person can effectively divide their attention among multiple tasks.

Phase Three – Pre-Arrest Screening.  Includes administration of the Standardized Field Sobriety Tests and the arrest/no arrest decision.


Any means by which some alleged fact that has been submitted to investigation may either be established or disproved.  Evidence of a DWI violation may be of various types:

– Physical (or real) evidence:  something tangible, bisible, or audible.

– Well established facts (judicial notice).

– Demonstrative evidence: demonstrations performed in the courtroom.

– Written or doccumentation.

– Testimony.

Field Sobriety Test

Any test that seeks to determine whether a DWI suspect can adequately divide their attention between physical and pychological tasks.  Some of these tests are Standardized, while others hold no scientific validation whatsoever.

Horizontal Gaze Nystagmus

An involuntary jerking of the eye.  There are many causes of HGN, including medical conditions, head trauma, flashing or rotating lights, objects passing within the field of vision of the test subject, and others.  One of the greatest problems with the way HGN is used by police officers is that officers often attribute any signs of HGN in a DWI suspects eyes to alcohol without adequately screening whether there are other causes of the HGN.

Illegal Per Se

Legally speaking, the term means “Illegal in and of itself.”  Within the DWI law enforcement community, this term implies that a person is guilty of DWI or DUI if they have a certain BrAC or BAC level.  In truth, Texas does not have an Illegal Per Se law.  The law states that a person may be found guilty of DWI if they test over .08 BrAC or BAC, but does not mandate that they must be found guilty.


is simply an involuntary jerking of the eye.  There are many, many causes, and different types of nystagmus.

One-Leg Stand

a type of Standardized Field Sobriety Test that


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