Common Defenses to DUI/DWI Charges

Fighting driving-under-the-influence charges in court.

By , Attorney · University of San Francisco School of Law

DUI laws differ by state, and the facts of every case are different. However, there are a handful of DUI defenses that are commonly used in most jurisdictions. Which DUI defenses might work in a given situation depends—among other things—on the type of DUI the person is charged with and the kind of evidence the prosecution is relying on.

Types of DUI/DWI Charges

Generally, there are two types of DUI charges: "per se" charges and charges based on impairment. In some cases, a defendant might be charged with one or the other. But it's fairly common for a defendant to face both types of charges for a single incident. A defendant can be found guilty of both. However, the court normally would impose penalties for only one of the convictions.

Generally, a motorist can be convicted of a per se DUI for driving with a blood alcohol concentration (BAC) of .08% or more. (Some states also have per se drug DUI laws.) An impairment DUI conviction, on the other hand, requires proof that the driver was actually "under the influence" or "intoxicated" by the substances taken.

Common DUI/DWI Defenses

Prosecutors must prove all the elements of a DUI to get a conviction. In other words, the prosecution must show the motorist was:

  • driving or in actual physical control of a vehicle, and
  • under the influence or in violation of the state's DUI per se laws.

Fighting a DUI charge normally involves efforts to prevent the prosecution from proving one or both of these elements. So, most DUI defenses focus on:

  • disputing the "driving" element
  • providing an innocuous explanation for why the driver appeared intoxicated, or
  • discrediting or getting the court to throw out blood- or breath-test evidence.

The best strategy will always depend on the facts and the type of DUI charge that you're fighting.

To be convicted of a DUI, there needs to be proof that you were driving—right? Not necessarily. It depends on how the state defines a DUI. In a few states, the prosecution must prove you were actually driving before you can be convicted of a DUI. However, in most states, "operating" or "being in actual physical control" of a vehicle is enough for a DUI conviction.

States that Require Proof of Actual Driving for a DUI Conviction

In the few states where driving is a required element of a DUI charge, you might be able to mount a successful defense if the officer didn't actually catch you driving. In these states, the prosecution must prove beyond a reasonable doubt that you were driving. So, when the officer didn't observe you behind the wheel with the car in motion, it can be difficult for the prosecution to come up with the necessary proof. And if you have a way to affirmatively show you weren't driving while under the influence, even better.

States Where You Can be Convicted of a DUI Without Proof of Vehicle Movement

But in most states, proof that you weren't driving won't get you completely off the hook. A jury might decide that you were in actual physical control of your vehicle even if you weren't actually driving down the street. In making the determination, the jury will typically be looking at how close you were to being able to start up your vehicle and drive away. So a motorist who's defending against a charge might point to facts showing it would have taken significant time or effort to put the car in motion—circumstances like not having the keys or being asleep in the back seat.

With impairment DUI charges (as opposed to per se DUI charges), the prosecution focuses on the actual effects that the drugs or alcohol had on the driver.

State laws differ on the level of impairment prosecutors must show to get a DUI conviction. In some states, like in Maine, the required showing is minimal: The prosecution just needs to prove that drugs or alcohol had some effect on the driver. In other states, such as California, the substances ingested must have had a "substantial" effect or rendered the driver "incapable" of driving safely. But regardless of the impairment standard, the prosecution generally must link the driver's condition to drugs or alcohol.

Challenging the Level of Impairment

It can be tricky to convince a jury that you weren't impaired enough to meet the legal standard for a DUI—especially in states that just require proof of "some" effect. Evidence of impairment often comes from the arresting officer's observations. For example, the officer might testify that you:

To counter the officer's testimony, the best tact is usually to provide an innocuous explanation for the officer's observations rather than directly dispute what the officer observed. For example, allergies and certain medical conditions can cause bloodshot eyes. And a disability or an injury might explain less-than-perfect FST performance. Of course, for some observations (like the odor of alcohol), there may not be an innocent explanation.

Disputing the Link to an Intoxicating Substance

In some cases, an officer might wrongly assume a driver's odd behavior is related to drug or alcohol use. For instance, mental illness can be difficult to distinguish from intoxication. If there are no test results showing an amount of alcohol or drugs in the driver's system sufficient to cause impairment, the driver might be able to persuade the jury that the apparent intoxication was actually related to something other than drugs or alcohol.

Prescription Drugs

Having a prescription for a drug or medication generally won't get you off the hook for a DUI. However, some states, like Minnesota, allow a limited defense for drivers who can show they took a prescription medication as prescribed by their doctor.

With a per se DUI charge (as opposed to an impairment charge), the focus is generally on the amount of alcohol in the driver's system. Drivers with a BAC of .08% or more are deemed per se under the influence.

Proving a per se DUI charge involves producing chemical test results showing the driver was above the legal limit. Implied consent laws generally require all drivers lawfully arrested for driving under the influence to submit to testing. So, in many cases, the driver will have taken a blood or breath test.

Challenging Breath Test Results

Modern breathalyzers are fairly accurate. However, they aren't perfect, and not following proper procedures when using a breathalyzer can lead to significant error.

All breath-test devices have an inherent margin of error. With many devices, the results can be off by as much as .01%. So, a driver who's just barely above the legal limit may have a good argument—based on the margin of error—that the prosecution can't prove the charge beyond a reasonable doubt. In other words, there's no way to know whether the excessive BAC was a true result or attributable to the inaccuracy of the machine.

A tainted breath sample can also cause an artificially high BAC reading. Breathalyzers are designed to measure the concentration of alcohol from the vapor in a person's lungs. However, to get an accurate measurement, no alcohol (or substances registering as alcohol) can be in the person's mouth. Otherwise, the breath test results will reflect a combination of the person's lung and mouth alcohol, which can be significantly higher than the person's actual BAC. To avoid this problem, officers are supposed to observe the driver for a 15-to-20-minute waiting period prior to administering a breath test. In cases where the officer doesn't adhere to the waiting period procedure, the defendant might be able to raise questions about the accuracy of breath-test results.

Challenging Blood Test Results

Generally, blood tests are more accurate than breath tests. But a skilled defense attorney might still be able to cast doubt on blood-test results or find a way to get the results excluded from evidence.

Fermentation of Blood Samples

One of the more common sources of blood-test errors is improper storage. Blood that isn't stored properly after collection can ferment. Fermentation can increase the alcohol concentration in the sample and lead to a misleadingly high BAC measurement. With a good understanding of blood-testing procedures, an attorney can hone in on any errors that may have led to inflated BAC results.

Warrantless Blood Draws

Implied consent laws impose administrative consequences (license suspension) for refusing to submit to DUI testing. However, the Supreme Court has said that because blood tests are so invasive they implicate a criminal defendant's constitutional rights. Therefore, police generally need a warrant to require a blood test. If police obtain your blood without a warrant, you might have a legal argument that the results should be thrown out.

Rising Blood Alcohol Defense

Even with a BAC that's above the legal limit and no good way to attack the accuracy or legality of the testing, a defendant may still have a line of defense. With the "rising-blood-alcohol" defense, the defendant tries to establish that his or her BAC was below the legal limit while driving but rose above the legal limit by the time the testing was conducted.

This defense is based on how the human body metabolizes alcohol. Basically, it takes time for alcohol to absorb into your system and affect your BAC. And there's always going to be some delay between when a motorist was driving and BAC testing takes place. So, if you get pulled over while alcohol is still absorbing into your body, chances are your BAC will be higher when it's measured than it was when you were actually behind the wheel.

Like most other BAC-related defenses, the rising-blood-alcohol defense tends to work best when your BAC is only marginally above the legal limit. And the defense typically requires the testimony of an expert with knowledge of alcohol metabolization.

Talk to a DUI Attorney

Some DUI defenses might be obvious while others are more subtle. For a thorough assessment of the available defenses in your case, get in contact with an experienced DUI lawyer. A qualified DUI attorney can tell you how the law applies to the facts of your case and help you decide how best to handle your situation.

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