DUI cases always turn on the issue of whether the defendant was under the influence of alcohol or drugs when operating or driving the vehicle. This argument tends to be crucial only because it is meant to indicate that the defendant has consumed so much alcohol or other drugs that he/she is incapable of operating the vehicle in a manner like that of an ordinary prudent driver under similar conditions who is operating the vehicle in a reasonable manner.
The extent of impairment or intoxication needed to satisfy the requirement of the DUI statute depends on the state. Within each state, the court rulings vary accordingly. The terms used in the statutes also vary, but regardless of the language used, the prosecution must show that the defendant was not in compliance of the DUI law when driving the vehicle and the influence of alcohol or drugs was so immense as to make him/her a danger to others in the public.
To establish evidence in the case, the prosecution commonly relies on testimony of witnesses, including police officers, indicating a variety of symptoms including mental and physical that the witness observed. Typically symptoms are revealed to the witness or the arresting officer through conversations and sobriety tests performed on the scene or observing the defendant driving in a manner indicating lack of control.
Observance of these symptoms is clearly subjective and thus possibly flawed in their prosecutorial effectiveness with DUI cases. To avoid this controversial issue, the DUI offense is often prosecuted under a per se statute, where the term “under the influence” is reduced to driving with blood alcohol level of about .08 to .1 based on the statute.
These types of statutes remove the problematic requirement of establishing a DUI defendant’s “under the influence” culpability via witnesses’ testimony, and replace it with a more easily proven scientific test. In these types of DUI cases, the evidence is limited to the result of some chemical tests performed on the DUI defendant showing that their blood alcohol level was over the legal limit.
Prosecutors strive to show that the blood alcohol limit was over the legal limit to create a presumption that the defendant drove under the influence of alcohol. Thus, the burden shifts to the defendant to prove he/she was not under the influence of alcohol. A common approach is to prove the device or method used to administer the DUI blood alcohol test was inaccurate or faulty. The defendant is also allowed to introduce evidence, such as his/her own testimony, testimony of other witnesses and chemical tests performed by him/herself that rebut the presumption of being under the influence of alcohol by having an alcohol level above legal limit.