If a person is charged with driving under the influence, the focus is the condition of the driver as being either mentally or physically impaired as the result of consuming alcohol, drugs (DUID), or a combination of alcohol and drugs.  The legal definition for being “under the influence” in California is similar to that in other states.  In order to be considered under the influence, the driver’s “physical or mental abilities are impaired to such a degree that he or she no longer has the ability to drive a vehicle with the caution characteristic of a sober person or ordinary prudence, under the same or similar circumstances.”
In DUI / DWI cases, the prosecutor will often use indirect, or circumstantial evidence, that implies that the motorist is under the influence of alcohol.  The prosecutor will try to introduce evidence of driving patterns, physical observations such as slurred speech, odor of alcohol, unsteady gait, or red eyes, field sobriety test performance, and chemical test results.
Experts in the field agree that typically, mental impairment precedes physical impairment.  One aspect of field sobriety tests are that the tests are divided attention tests seeking to determine whether the motorist is suffering from any mental or physical impairment.   Not only are field sobriety tests inherently unfair – they usually take place on the side of the road with cars whizzing past along with a hundred other distractions – physical impairment alone is not evidence of being under the influence.  If a motorist is demonstrating physical impairment, but no mental impairment, then the impairment must be coming from a source other than alcohol.  Physical impairment can come from any number of sources other than alcohol.
For example, an arresting officer may cite a driver’s red and watery eyes or inability to stand on one foot as a sign that the driver was under the influence.  However, irritated eyes can stem from any number of factors – allergies, fatigue, air pollution, etc.  An inability to balance on one foot can be attributed to an old injury or just poor balance.
The so-called signs of being under the influence can be attributed to any number of factors.  A skilled California drunk driving attorney will take a thorough history from a client to determine if the circumstantial evidence can be attributed to illness, injury, or other conditions that can appear as evidence of impairment, and use that information to help a client win his or her case.