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What Evidence Do Prosecutors Need for a DUI Conviction?

Many people assume that a DUI charge is an open-and-shut case, but the reality is often more complex. Even if you feel sober, you may be over the legal limit. And while a field sobriety test might seem like definitive proof of impairment, the evidence is rarely straightforward.

It’s important to remember that in any criminal case, including DUIs, the prosecution must prove guilt beyond a reasonable doubt. This is a high bar that requires an abiding conviction that the evidence presented supports a finding of guilt.

If you’re facing a DUI charge, a criminal defense lawyer can guide you through the legal process, examine the evidence against you, and work to reduce or dismiss your charges.

Elements of a DUI in California

First, let’s talk about what prosecutors must prove to convict you of a DUI. Under California Vehicle Code Section 23152(a), it’s unlawful for a person who is under the influence of any alcoholic beverage to drive a vehicle.

This means prosecutors must show:

  1. You were driving a vehicle, and
  2. You were under the influence of alcohol at the time of driving.

Seems simple enough, right? But both of these elements come with some nuance.

“Driving” doesn’t just mean actively operating a moving vehicle. In California, you can be considered “driving” if you’re in actual physical control of the vehicle. This means if you’re sitting in the driver’s seat with the engine running, even if you’re parked, you could potentially be charged with a DUI.

Being “under the influence” means your physical or mental abilities are impaired to the point that you can no longer drive like a cautious, sober person. This is a subjective standard that requires evidence of actual impairment.

In addition to these two elements, California has a second DUI law, Vehicle Code Section 23152(b). This law makes it illegal to drive with a blood alcohol concentration (BAC) of 0.08% or higher. This is known as a “per se” DUI because you can be convicted based on your BAC level alone, even without additional evidence of impairment.

Proving Drug vs. Alcohol-Related DUIs in California

Most DUI cases are charged under Penal Code 23152, which has several subsections. For an alcohol DUI under 23152(b), the prosecution only needs to prove that you were driving with a blood alcohol concentration of 0.08% or higher. This evidence makes for a relatively straightforward conviction.

However, drug DUIs, whether involving marijuana, cocaine, methamphetamines, or other substances, are charged under 23152(f) for being under the influence of drugs or 23152(g) for combined drug and alcohol impairment. Unlike with alcohol, there is no definitive quantitative measure that automatically indicates impairment.

The law states that for a drug DUI conviction, the prosecution must prove your “mental or physical abilities are so impaired that you can no longer drive with the caution of a sober person using ordinary care under similar circumstances.” Demonstrating that level of impairment, especially if there was no erratic driving, presents more challenges for the prosecution compared to an alcohol DUI case.

Types of Evidence Prosecutors Rely On

So, what kind of evidence do prosecutors use to prove these elements? Let’s break it down.

1. Chemical Tests

Chemical tests, like breathalyzers and blood tests, are often the cornerstone of a DUI prosecution. These tests measure the amount of alcohol or drugs in your system and can provide powerful evidence of a DUI violation.

However, these tests are not infallible. Breathalyzers must be regularly calibrated and maintained to ensure accuracy. Blood samples can be mishandled or contaminated. And there are various medical conditions and circumstances that can lead to falsely high BAC readings.

2. Field Sobriety Tests (FSTs)

Field sobriety tests are a series of physical and cognitive exercises that officers use to assess impairment. The standard tests include the Horizontal Gaze Nystagmus (following a penlight with your eyes), the Walk-and-Turn (walking heel-to-toe in a straight line), and the One-Leg Stand (balancing on one foot while counting).

While these tests can provide clues of impairment, they are not conclusive. Many sober people struggle with balance and coordination, especially under the stress of a police encounter. Medical conditions, age, weight, and even footwear can impact FST performance.

3. Police Observations

Police officers are trained to look for signs of impairment during DUI stops. They’ll note things like:

  • The smell of alcohol on your breath or in your car
  • Slurred speech or difficulty communicating
  • Bloodshot, watery eyes
  • Flushed face
  • Fumbling with your wallet or documents
  • Difficulty exiting your vehicle
  • Swaying, staggering, or leaning for balance

While these observations can be compelling, they’re also subjective. A skilled defense attorney can often provide alternative explanations for these symptoms.

4. Driving Patterns

The reason for the initial police stop can also serve as evidence of impairment. Officers will testify about any erratic or illegal driving maneuvers they observed, such as:

  • Weaving within or across lanes
  • Straddling the center line
  • Nearly hitting objects or vehicles
  • Driving on the wrong side of the road
  • Excessively wide turns
  • Stopping well before or after a stop line
  • Inconsistent speeds or very slow speeds

Again, there may be defenses to this evidence. Poor road conditions, vehicle malfunctions, or simple distractions could explain some of these driving patterns.

5. Statements and Admissions

Anything you say during a DUI stop can be used as evidence against you. This includes admissions of drinking, statements about where you were coming from, and even apologies or expressions of remorse.

This is why it’s so crucial to exercise your right to remain silent during a DUI investigation. Politely decline to answer questions about your activities or alcohol consumption.

How to Challenge DUI Evidence in California

Prosecutors rely on a wide range of evidence in DUI cases. But, an experienced DUI defense attorney knows how to challenge this evidence.

At the Nieves Law Firm, we leave no stone unturned in examining the prosecution’s case. We look at issues like:

  • Whether the initial police stop was legal
  • If field sobriety tests were conducted properly
  • Whether breathalyzers were calibrated and maintained correctly
  • The chain of custody for blood samples
  • If the police violated your rights during the investigation
  • Whether any statements were coerced or obtained illegally

We also gather evidence to support your defense, including witness testimony, expert opinions, and surveillance footage when available. Our goal is to cast doubt on the prosecution’s evidence and narrative.

Contact The Nieves Law Firm For DUI Defense Today

The stakes in a DUI case are high, but a strong defense can make all the difference. If you’re facing DUI charges in California, don’t face them alone. Contact the Nieves Law Firm today for a consultation.

Even in cases where the evidence gathered is enough to get a DUI conviction, it could be possible to negotiate lesser charges, protect your driving privileges, or keep you out of jail.

We have helped many Californians charged with DUIs. Don’t let this experience have a permanent impact on your life. Reach out today.

Author Bio

Jo-Anna Nieves is the Founder and Managing Attorney of The Nieves Law Firm, an Oakland criminal defense law firm she created in 2012. With more than 11 years of experience in criminal defense, she has zealously represented clients in a wide range of legal matters, including DUIs, domestic violence, expungement, federal crimes, juvenile law, motions to vacate, sex crimes, violent crimes, and other criminal charges.

Jo-Anna received her Juris Doctor from the Florida State University College of Law and is a member of the State Bar of California. She has received numerous accolades for her work, including being named a Super Lawyer Rising Star the past 8 years, the #12 Fastest Growing Law Firm in the U.S. by Law Firm 500 in 2019, and one of the fastest growing companies in the U.S. by Inc 5000 in 2023.

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