In order to be convicted of a DUI, the prosecutor must prove beyond a reasonable doubt that the defendant was driving under the influence of alcohol. The types of evidence which normally support this charge are: driving pattern, physical appearance, performance on field sobriety tests, and the results of chemical tests. However, the fact is that many people can drive at or above the legal limit (at .08% BAC) and have a perfect driving pattern, none or few physical symptoms of intoxication, and perform the field sobriety tests with flying colors. This can primarily be explained by the theory of tolerance. Some people have a high tolerance, be it by their very biology, or through years of drinking.



Developing a tolerance to alcohol is the body's way of adapting to alcohol use. The intoxicating effect of alcohol decreases as a result of continuous consumption. The system adapts to alcohol very quickly, as the fact that it has accustomed itself to alcohol is evident even during a single occasion of intoxication. The tolerance is not permanent; it decreases with time as one stops using alcohol or cuts down on consumption.

The body has several means of neutralizing the effect of alcohol. The best known of these is metabolic tolerance, where the body aims at neutralizing the effects of alcohol by accelerating the rate with which it metabolizes alcohol. Metabolic tolerance slowly develops as a result of prolonged, high-scale consumption of alcohol.

The most important means is the neural tolerance where the tolerance of the brain and the rest of the nervous system increases. The nervous system tries to function normally in spite of the alcohol. This is why it will before long adapt itself to the alcohol in the body. The intoxicating effect of alcohol decreases. Prolonged, high-scale consumption of alcohol is not needed in order for neural tolerance to develop. Rather, occasional, repeated drinking is enough.

Therefore, the law has adapted to convict people with alcohol tolerance. In the early 1980’s, California passed the first “per se" statutes. Per se is Latin, meaning "by itself," or inherently. Today, Vehicle Code Section 23152(b), is the per se statute on record. Section 23152(b) states: “It is unlawful for any person who has a .08 percent or more, by weight, of alcohol in his or her blood to drive a vehicle." This section states further: “It is a rebuttable presumption that the person had a .08 percent or more, by weight, of alcohol in his or her blood at the time of driving the vehicle if the person had a .08 percent or more ,b y weight, of alcohol in his or her blood at the time of the performance of a chemical test within three hours after driving."

In a jury trial, the jury instructions, per CALJIC read:

“If the evidence establishes beyond a reasonable doubt that a chemical analysis of the defendant's blood, breath, or urine established that there was 0.08 percent or more by weight of alcohol in the defendant's blood at the time the test was performed, the trier of fact may, but need not, infer that the defendant was under the influence of an alcoholic beverage at the time of the alleged offense."

The most common type of tests administered to determine blood alcohol content are breath and blood tests, although urine tests may be given where the other tests are not available or where drugs are suspected.