In a DWI case, the State of Texas is looking to prove that you were intoxicated while driving. Typically, the State is easily able to prove that the individual was operating a motor vehicle in a public place. However, there is a third and most important element of a DWI or intoxication manslaughter case: that is the element of intoxication.
In order to prove intoxication the State must prove that a. the citizen accused had a blood alcohol content of .08 or above at the time of driving or the individual had lost the normal use of their mental or physical faculties due to the introduction of alcohol, drugs, a dangerous drug, a controlled substance or any combination thereof.
In most cases, the individual does not provide a blood or breath specimen. Accordingly, the only way that the State can prove that they had lost the normal use of their mental or physical faculties, is by the testimony of the arresting officer and the video tape from the camera that is mounted behind the rear view mirror in the arresting officer's patrol vehicle.
The arresting officer generally gives three standardized field sobriety tests which are recommended under the National Highway, Traffic, and Safety Administration's standardized field sobriety test manual. The first is the horizontal gaze nystagmus test. Although the jury doesn't get to judge the police officer's assessment of the suspect's performance on the HGN test, the jury can judge whether or not the individual was able to follow the police officer's instructions. One instruction is whether the individual turned their head rather than simply following the pen light with their eyes as the cop passes it in front of their eyes.
The second two tests are the walk and turn test and the one-legged stand test. The officer is looking for a set number of clues to make their determination of whether or not they have probable cause to arrest someone.
However, the ultimate test is whether the judge or jury believes that the State proves that the individual was intoxicated by either theory stated above beyond a reasonable doubt. Probable cause is a much lesser standard than beyond a reasonable doubt, so making the initial arrest does not require the higher burden of proof which is required for the prosecutor to prove their case in court.