Much of the evidence in a DUI case is circumstantial in nature. A police officers observations of the suspect including any objective symptoms of impairment and the suspects performance of field sobriety tests is a good example. This is not direct evidence of alcohol impairment but is circumstantial evidence of potential impairment. When a Prosecutor in a DUI trial discusses poor performance of field sobriety tests, what he or she is doing is asking the jury to infer from this factual evidence that the defendant was impaired to drive.
However, circumstantial evidence is open to interpretation and there is often a plausible alternative explanation that can be presented to a jury. For example, a person may have red and watery eyes because he or she has allergies, has been in a smoky environment or is just tired. Poor physical performance on field sobriety tests may be explained by the fact that the defendant was tired, nervous of just suffers from bad balance in normal circumstances.
The law says that if there are two reasonable interpretations to be given to circumstantial evidence, one which points to guilt and one which points to innocence, the jury must adopt the interpretation that supports innocence. One example that I like to give to a jury is the “cat in a box” analogy. I ask the jury to imagine that a cat is placed in a box with a mouse. The box is sealed. Upon opening the box, the mouse is gone.
The only reasonable explanation is that the cat is guilty of eating the mouse. However, if the cat is placed in the same box with the mouse, but this time upon close inspection there is a small hole in the box for the mouse to escape, there is an alternative reasonable explanation for the mouse's disappearance, that is, he escaped. In these circumstances, a jury must acquit the cat of killing the mouse by adopting the reasonable inference that points to the mouse escaping.
Much of the evidence in a DUI jury trial can be dealt with in this way, by educating a jury as to alternative, reasonable explanations for the defendants driving pattern, objective symptoms and performance of field sobriety tests. This is often the key to beating a DUI charge in Los Angeles .