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Challenging DWI Evidence Based on Junk Science

Many people arrested for DWI in the Twin Cities area do not hire a private defense attorney because they assume the evidence gathered by police and used by prosecutors is based on sound science. Despite this common assumption, much of the evidence used in DWI cases is based on unfounded assumptions and junk science.  Challenging DWI evidence can succeed in the hands of an experienced DWI attorney.

Below, we provide an overview of the weaknesses inherent in some of the typical types of evidence used to support probable cause for a DWI arrest and/or by a prosecutor at trial. 

Challenging DWI evidence based on ‘bad driving’ (which does not necessarily constitute intoxicated driving)

When stopping motorists suspected of driving while impaired, officers often rely on so-called ‘erratic driving’ practices that are considered to provide an indication of drug or alcohol impairment to justify screening procedures, or ‘field sobriety tests.’ These types of unsafe or illegal driving include but are not limited to the following:

  • Drifting into an adjacent lane
  • Straddling the yellow line
  • Exercising unreasonably wide turns
  • Driving too slowly
  • Running red lights or stop signs

While these types of driving can be a sign that a driver is impaired, there are many other reasons that a driver might engage in unsafe driving practices. Reasons such as fatigue, distraction, or others not related to intoxication.

Further, the officer will often focus on one example of poor driving while ignoring dozens of properly executed traffic maneuvers. An experienced Minnesota DWI lawyer understands how to attack a police officer’s assumptions regarding the interpretation of a motorist’s driving maneuvers.

Challenging DWI evidence based on field sobriety tests (which can be unreliable)

Although only three field sobriety tests (FST) have been approved for providing a reliable indication of alcohol impaired driving, these ‘standardized field sobriety tests‘ (SFST) each run a high risk of falsely indicating a sober driver is intoxicated.

The three SFSTs that have been approved by the National Highway Traffic Safety Administration (NHTSA) include:

If the officer administers other tests like asking a motorist to recite the alphabet backward, these tests do not provide accepted evidence of intoxication.

Even the SFSTs have a significant error rate when it comes to identifying drivers with a BAC over the legal limit of .08 percent. A study conducted by the Southern California Research Institute at the behest of the NHTSA found that the SFSTs had the following accuracy rates:

  • Walk and Turn: 68%
  • One-Leg-Stand: 65%
  • Horizontal Gaze Nystagmus: 77%

The lack of scientific reliability of these tests is magnified by the many factors that can increase the risk that a sober driver is misidentified as being alcohol impaired, such as:

  • Inadequate training of officer
  • Illness, injury or disability of the motorist
  • Failure of officer to comply with proper procedures for the administration and evaluation of STSTs
  • Environmental factors (e.g. proximity to traffic, darkness)
  • Constricting clothing or inappropriate footwear

When our Minnesota DWI defense law firm represents individuals charged with an alcohol or drug-related driving offense, the judge and/or jury is made aware of the inherent inaccuracy of SFSTs and aggravating factors that make this screening method even less reliable.

Challenging DWI evidence based on chemical testing of breath of blood

Chemical testing of blood alcohol concentration (BAC) via breath, blood, or urine (drug cases) carries the most persuasive aura of reliable scientific evidence; however, this type of evidence also constitutes a far from infallible basis for determining a driver is intoxicated.

Problems with the breath test

Breath testing does not even directly measure the percentage of alcohol in a motorist’s blood. Breath testing devices measure the concentration of alcohol in breath and use a mathematical assumption referred to as the ‘partition ratio‘ to estimate the concentration of alcohol in the blood.

Breath test accuracy can be compromised by gender, low-carb diets, medical conditions (e.g. GERD, acid reflux, and regurgitation), individual body chemistry, and a multitude of other factors. Further, breath testing reliability can be even less accurate when proper procedures are not followed. For example, he officer might fail to properly conduct a 15 minute observation period to mitigate the risk of artificially elevated BAC results caused by the presence of ‘mouth alcohol.’

Problems with the blood test

While blood testing is more accurate than a breath test, this form of chemical testing can still erroneously indicate a sober driver is over the legal limit. Chemicals can be expired or improperly mixed with the blood sample. Similarly, the blood specimen might be improperly drawn, stored, or transported. The bottom line is that chemical BAC testing and other evidence used in Minnesota DWI prosecutions is not based on infallible hard science.

If you have been arrested for DWI in the Twin Cities area, we invite you to speak to a Minnesota DWI Lawyer at Gerald Miller, P.A. as soon as possible. We are committed to defending the rights of our clients, challenging DWI evidence and exploiting weaknesses in the prosecutor’s case. Contact us today at (612) 440-3212 to schedule your free and confidential case evaluation.

About the author

Gerald Miller

Gerald Miller is a top-notch and experienced DWI/DUI lawyer at Gerald Miller P.A. in Minneapolis, MN. He has more than 35 years of experience in Criminal Defense practice. He has also been a mentor to numerous DUI/DWI defense attorneys.

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