Exceeding the “legal limit” while Driving has become synonymous with the offense of DUI. The “legal limit” refers to a driver’s blood alcohol concentration (BAC). Driving with a BAC in excess of the limit set by law (0.08% for most California drivers) is one way of violating the prohibition against driving under the influence, but it isn’t the only way.

Evidence of DUI

Years ago, prosecutors proved that a driver was under the influence of alcohol by presenting Evidence that the driver’s ability to drive safely was impaired because of alcohol consumption. “Bad driving” was the most common evidence of impairment. Typical examples of “bad driving” included:

• Weaving within a lane
• Crossing a centerline
• Driving recklessly
• Making erratic changes in speed
• Failing to move forward after a red light changes to green

Other evidence used to prove alcohol impairment included a driver’s:

• Slurred speech
• Bloodshot eyes
• Fumbling for a driver’s license or vehicle registration
• Irrational responses to questions
• Belligerent attitude
• Inability to exit the car without holding onto the door
• Poor balance

Much of that evidence was subjective, and skillful cross-examination often suggested to jurors that arresting officers were exaggerating their observations for the sake of justifying an arrest. Alternative explanations for the officer’s observations (such as fatigue or a medical condition) often created reasonable doubt about the driver’s impairment.

Per Se Offenses

Dissatisfied that not every arrested driver was convicted, government agencies and advocacy groups wanted a form of evidence that did not depend on subjective evidence. Scientists were encouraged to give the government what it wanted. Based on less-than-rigorous research, scientists concluded that impairment could be correlated to a specific BAC.

There isn’t much doubt that even the most experienced drinkers are incapable of driving safely with a BAC in excess of 0.15%. In fact, more than half of all drivers who cause a fatal accident after consuming alcohol have a BAC of at least 0.15%. Many states therefore adopted laws making it illegal to drive with a BAC of 0.15% or higher. Proof of impairment was not required if the government could show that the driver had a prohibited BAC within a reasonable period of time after driving. However, if a driver tested lower than 0.15%, the government could still win a conviction by using other evidence (like bad driving) to prove that the driver was “under the influence.”

Advocacy groups and some government agencies were soon arguing that 0.15% was too high and impaired drivers were continuing to avoid conviction. In fact, most impaired drivers who caused actual harm were convicted. Drivers who caused no harm were less likely to be convicted, but advocacy groups insisted that the potential for harm justified the arrest of drivers who are even slightly impaired.

States responded to those arguments in different ways, but a standard of 0.10% eventually became common. Even that did not satisfy the advocacy groups. Based on questionable research, they argued that 0.08% should be the standard of impairment. In fact, most people drive perfectly well at 0.08%, particularly adults who have developed a tolerance for alcohol and understand how to compensate after drinking a couple glasses of wine.

However, based on evidence that a 0.08% BAC can cause slight delays in reaction times (comparable to the delay caused by ordinary fatigue), advocacy groups lobbied for a 0.08% BAC standard. That standard was eventually adopted by all states after Congress threatened to cut off federal highway funding for states that failed to enact a law making it unlawful to drive with a BAC of 0.08% or higher. Those laws are known as “per se” offenses.

DUI Below 0.08%

Per se offenses did not replace DUI laws. Instead, they became a separate and sufficient way of proving that a driver was under the influence.

In some cases, prosecutors still prove DUI the old-fashioned way. That usually happens when testing devices are unavailable or fail to return valid results, or when chemical tests are not administered within the required time frame.

It is possible, however, to charge a DUI even if a valid test result shows that the driver was under the legal limit while driving. That usually happens when an officer sees erratic driving, makes a DUI arrest, and is disappointed to learn that the test result is under 0.08%.

If the test result is close to 0.08%, a prosecutor might argue that the driver was above 0.08% at the time of driving, but that alcohol was eliminated from the driver’s blood before the test was taken, resulting in a lower test result. Prosecutors generally need to present expert testimony to make that case. The testimony is often based on unsupported assumptions and can usually be countered with aggressive cross-examination or with a defense expert.

In other cases, a prosecutor might argue that a driver’s BAC was low because the driver combined alcohol with other drugs. Usually that argument is based on sheer speculation, although it is sometimes supported by testimony from a police officer who claims to be a Drug Recognition Expert (DRE). The testimony of a DRE is about as reliable as voodoo, and a strong cross-examination will typically expose its weaknesses.

As a last resort, prosecutors will argue that the driver was affected by alcohol more than a typical driver and was impaired despite having a low BAC. Prosecutors rely on studies purporting to show that a BAC that is lower than 0.08% can cause impairment in some people, but those studies have never been scientifically validated. Defense experts can explain why those studies are flawed and why, in any event, there is no evidence that the defendant is among the portion of the population that might be impaired with a BAC of less than 0.08%.

In the end, the “legal limit” has been drilled into the heads of jurors so often that jurors tend to see it as a line in the sand. If a driver hasn’t reached the legal limit, most jurors are doubtful that the defendant should be convicted of DUI. That’s why reasonable doubt can often be established when a driver tests less than 0.08%, and why prosecutors are usually willing to plea bargain those cases.

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