Chemical Test Presumptions in Drunk Driving Cases

If you have ever been arrested for drunk driving you were probably given some sort of chemical test to determine your blood alcohol content. The most common chemical tests include a breath test or blood test. Many jurisdictions have statutes that allow prosecutors to give presumptive effect to chemical test readings of blood-alcohol content. In other words, if a person's blood alcohol content is at a certain level, the court will presume automatically that the person was driving under the influence of alcohol. Depending on the jurisdiction, some statutes will use the word "presumption," while others use the term "prima facie evidence."

In many jurisdictions if there was an alcohol concentration of less than 0.08%, the fact will not give rise to any presumption that the person was or was not under the influence of alcohol. However, the fact may be considered with other evidence to determine whether the person was under the influence of alcohol. If there was an alcohol concentration of 0.08% or more, it will be presumed that the person was under the influence of alcohol.

It is generally held that a statutory presumption concerning chemical tests in drunk driving cases can exist only when the chemical tests were administered to motorists in strict conformity to the applicable statute. Defense counsel will generally argue that the presumption does not arise if the chemical test was performed for some reason other than determining whether the motorist was driving under the influence of alcohol.

In order for a presumption in a criminal action to be constitutional, there must be some sort of rational connection between the fact proved and the ultimate fact presumed. In other words, one must be able to say with substantial assurance that the presumed fact is more likely than not to flow from the proved fact on which it depends. Defense counsel can often attack the validity of chemical test results as not having been satisfactorily proven for the particular defendant on trial because most chemical tests are administered a substantial period of time after the motorist was observed driving, and because the principles on which chemical test machines operate are based on the physiological features of a hypothetical average motorist.

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