Criminal Law

What Is an “Impairment” DUI?

By John McCurley, Attorney
Read about what it means to be charged with an “impairment” DUI.

Oftentimes, when a prosecutor charges a person with driving under the influence, there are two charges: one alleging “per se” DUI and another based on impairment. To prove a per se DUI, a prosecutor must show the driver had a blood or breath alcohol concentration (BAC) of .08% or more. Proving an impairment DUI, on the other hand, requires evidence that the driver was affected by the alcohol or drugs ingested.

Elements of a DUI

The definition of each crime can be broken into elements of its simplest parts. The prosecution must prove each element in order to get a conviction. For a DUI conviction, prosecutors typically must prove the motorist was:

Some states also require prosecutors to prove the defendant was driving on a public roadway rather than private property.

The only difference between a per se and an impairment DUI is how the prosecution proves the motorist was under the influence. For per se DUIs, the prosecutor needs to prove the driver’s BAC. And for impairment DUIs, proving actual impairment is necessary.

Proving Impairment

All states have impairment DUIs, but states vary on the level of impairment prosecutors must show to prove the crime.

Any Effect

In some states, the prosecution can establish the impairment element by showing drugs or alcohol affected the driver’s physical or mental abilities in any way. In other words, the prosecutor doesn’t have to prove the motorist was drunk or incapable of driving safely.

In Florida, for instance, impairment to “some degree” is enough for a DUI conviction. And Arizona prosecutors need to prove only the “slightest degree” of intoxication.

Appreciable Impairment

However, in other jurisdictions, prosecutors must prove a higher level of impairment. For example, in California and Washington D.C., a DUI conviction requires proof that the driver was impaired to “an appreciable degree.” Similarly, in Illinois, proving a driver was the under the influence involves showing the drugs or alcohol rendered the driver “incapable of driving safely.”

Evidence of Impairment

Whereas a per se DUI prosecution typically focuses exclusively on the driver’s BAC, an impairment prosecution usually requires the jury to concentrate more on the driver’s behavior. Signs of impairment might include:

  • erratic driving
  • poor performance on field sobriety tests
  • slurred speech
  • unusual behavior, and
  • red or glazed over eyes.

Though proof of BAC isn’t required for an impairment DUI conviction, prosecutors sometimes use the driver’s BAC as supporting evidence.

Generally, prosecutors prove impairment at trial through the testimony of the arresting officer. Officers typically explain any training they’ve received on looking for signs of intoxication and which of those signs they observed in the defendant’s case.

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