If you are arrested in California for DUI, you will probably be charged with two misdemeanor offenses: driving under the influence of alcohol (Vehicle Code Section 23152(a); and driving with a blood alcohol content (BAC) of 0.08% or higher (Vehicle Code Section 23152(b). Regardless of their BAC, a person is under the influence, in violation of Vehicle Code Section 23152(a), if, at the time of driving, they are too impaired by alcohol to drive safely. A person is guilty of violating Vehicle Code Section 23152(b) if, at the time they are driving, their BAC is 0.08% or higher.

Proving a person's BAC at the time they are driving can be complicated.

Blood Alcohol Concentration

When you drink alcohol, it enters your stomach and then makes its way into your bloodstream. The percentage of alcohol in your bloodstream is known as your BAC. Your BAC will continue to rise as more alcohol enters your bloodstream.

Because all people are different, there is no way to know how much or how quickly your BAC will increase by drinking a specific drink. For example, if you drink two shots of vodka, your BAC might increase to 0.06%, while another person's BAC might increase to 0.09% by drinking the same amount.

At some point, your BAC will stop rising and begin to fall as your body metabolizes the alcohol. The speed at which your BAC rises and falls is influenced by many factors, including how much food you have in your stomach; your weight; how quickly you drank; your metabolism; your gender; etc.

Determining BAC

The latter point is important when the government is trying to prove that somebody is driving drunk. Since your BAC drops over time, eventually dropping to 0.00%, the government does not have unlimited time to test your blood.

The “3-Hour Presumption Rule” recognizes this fact. This rule, which is part of the same vehicle code that defines driving under the influence, presumes that, if a blood or breath test conducted within three hours of driving shows a BAC of 0.08% or higher, the driver's BAC was 0.08% or higher at the time of driving.

At a DMV hearing, this is treated as a “rebuttable presumption.”  That means we need to present some evidence to rebut the 3-hour presumption by showing that the blood or breath test does not accurately reflect the BAC at the time of driving.

At a criminal trial for DUI, the 3-hour presumption is treated as a “permissive inference.” That means that jurors may, but are not required to, apply the 3-hour presumption. As a practical matter, that still places a burden on us to convince jurors that the BAC was lower at the time of driving than at the time of the test.

Challenging the 3-Hour Presumption Rule

If you are facing DUI charges in California based on the results of a chemical test, the 3-Hour Presumption Rule will likely be relevant to your case. However, just because your BAC was determined within three hours of your driving, that does not mean you are out of luck. In some circumstances, you can challenge this rule.

For example, if you can prove that your BAC increased between when you were driving and when you were tested, the 3-Hour Presumption Rule may not be applicable. If there was a brief time between when you were drinking and when you took the test, your BAC was probably rising, meaning it was higher at the time of the test and at the time you drove.

The 3-Hour Presumption Rule also only applies when the test is accurate. If the testing device were inaccurately calibrated, this rule should not apply. There are medical conditions which can affect a breath test. And blood tests are not always accurate. Blood samples must be mixed with the proper amounts of preservative and anticoagulant, stored properly, and analyzed using a reliable method. Our firm has successfully defended many drivers whose blood and breath test results were 0.20% and higher.

When Your BAC Is Under 0.08

While the government can take advantage of the 3-Hour Presumption Rule to prove that you were driving under the influence, the reverse is not true. If you take a chemical test and your BAC is under 0.08, that does not definitively prove that your BAC while you were driving was under 0.08.

If the government has other evidence that suggests that you were driving under the influence, it can still charge you with a DUI. Furthermore, the government can present the results of the chemical test at trial as evidence that you have been drinking.

For example, if the test was performed two hours after you were driving and showed you had a BAC of 0.07, the government could use that information to argue that you were likely legally drunk while driving. Combined with other evidence, that could convince a jury beyond a reasonable doubt.

At the same time, you also have the right to introduce evidence of your tested BAC at trial. A low value could help you get acquitted.

The 3-Hour Presumption Rule Lowers the Bar of Proof for the Government

This rule makes it easier for the government to prove that you were driving under the influence. The 3-hour presumption makes it possible for them to convict you even when they do not have clear proof of your BAC while you were driving.

DUI is all we do and we do it well. The Crawford Law Firm, Inc., is located in Santa Cruz County, California, providing representation for DUI Defense, DUI Marijuana, DUI Drugs, and Vehicular Manslaughter. Contact us today to schedule an appointment.