Understanding a DUI Traffic Stop — Before and After It Happens
A DUI traffic stop follows a predictable sequence that law enforcement is trained to execute and document carefully. Understanding how that sequence works — what officers are looking for, what your rights are at each stage, and how the evidence gathered affects your case — is essential whether you are trying to protect yourself in the future or understand what happened during an arrest.
Updated September 2026
Ms. Williams has over 33 years of legal experience and has defended DUI cases throughout Central Illinois. As a member of the National College for DUI Defense, she has specialized training in the science and procedure behind DUI stops, field sobriety testing, and chemical testing — and she knows exactly how to identify where law enforcement got it wrong.
◆ Illinois Law
An officer must have reasonable suspicion of a traffic violation or criminal activity to lawfully initiate a traffic stop. Once stopped, a DUI investigation and any resulting arrest are governed by 625 ILCS 5/11-501 (the DUI offense statute) and 625 ILCS 5/11-501.1 (Implied Consent). Evidence obtained during an unlawful stop may be suppressed under the exclusionary rule, potentially eliminating the prosecution\'s case.
Sources: 625 ILCS 5/11-501 — Driving Under the Influence; 625 ILCS 5/11-501.1 — Implied Consent and Chemical Testing; U.S. Constitution, Fourth Amendment
Phase 1: The Initial Stop
Before an officer can pull you over, they must have reasonable suspicion that a traffic violation has occurred or that criminal activity is afoot. This is a legal threshold that matters enormously to your defense. Common reasons for DUI-related stops include swerving or weaving between lanes, failing to maintain lane position, wide turns, stopping abruptly or unusually, driving significantly below the speed limit, running a red light or stop sign, or equipment violations like a broken taillight.
If the stop lacked a legally sufficient basis, any evidence gathered as a result — field sobriety test performance, breathalyzer results, admissions — may be suppressible. This is one of the first things Ms. Williams examines in every DUI case.
Phase 2: The Initial Contact
The moment the officer approaches your window, the DUI investigation has already begun. Officers are trained to observe everything during this phase: the smell of alcohol or cannabis from the vehicle, your speech patterns, the condition of your eyes, how easily you locate your documents, and your general demeanor and coordination.
Everything the officer observes is documented in their report and forms part of the prosecution's case. You are legally required to provide your driver's license, registration, and proof of insurance. Beyond that, you have the right to remain silent. You do not need to answer questions about where you were going, where you came from, or whether you have been drinking.
Phase 3: Field Sobriety Tests
If the officer suspects impairment, they will typically ask you to step out of the vehicle and perform standardized field sobriety tests (SFSTs). In Illinois, field sobriety tests are voluntary — you may decline. The three standardized tests recognized by NHTSA are the Horizontal Gaze Nystagmus (HGN) test, the Walk-and-Turn test, and the One-Leg Stand test.
These tests are highly subjective and sensitive to conditions that have nothing to do with alcohol. Uneven or sloped pavement, poor lighting, wind, medical conditions including inner ear issues or leg injuries, certain footwear, age, weight, and nervousness can all affect performance. Officers must administer these tests precisely according to NHTSA protocols — deviations form the basis for legal challenges.
Ms. Williams's NCDD training includes in-depth study of field sobriety test administration standards. She knows how to identify improper administration, environmental problems, and physical factors that undermine the reliability of these results.
Phase 4: Preliminary Breath Test
After field sobriety tests, an officer may request a Preliminary Breath Test using a handheld roadside device. This test is voluntary in Illinois, and the results are generally not admissible as evidence of BAC in court — but they can be used to establish probable cause for arrest. Declining the roadside PBT does not trigger the same consequences as refusing the official evidentiary breath test after arrest.
Phase 5: Arrest and Official Chemical Testing
If the officer determines probable cause exists, you will be placed under arrest. Illinois's Implied Consent Law then applies — you are required to submit to an evidentiary chemical test. Refusing after arrest triggers a statutory summary suspension under 625 ILCS 5/6-208.1: 12 months for a first offender who was not in a crash that caused personal injury or death, and 3 years if the person is not a first offender. A refusal after a crash that caused personal injury or death can instead be a statutory summary revocation under 625 ILCS 5/11-501.1. Refusal can also be presented as evidence at trial. If a breath test is administered, the equipment must be properly certified, calibrated, and operated following specific protocols. Failures at any point can be grounds for challenging the results.
If you were already on probation or court supervision, that arrest can also lead to a petition alleging a violation. See probation violation defense.
How Traffic Stop Evidence Shapes Your Defense
Every phase of a DUI stop generates evidence — the officer's observations, dashcam and bodycam footage, field sobriety test documentation, and chemical test records. Ms. Williams examines all of it. The goal is to identify every point where the law was not followed, where the science does not hold up, or where the evidence is weaker than it appears on the surface.
Frequently Asked Questions: DUI Traffic Stops in Illinois
What if the officer didn't have a valid reason to stop me?
An illegal stop is a significant defense. If the officer lacked reasonable suspicion, a motion to suppress can be filed. If granted, the prosecution loses the evidence gathered during the stop — including chemical test results — and the case often cannot proceed. Ms. Williams analyzes the circumstances of every stop carefully.
Are field sobriety tests accurate?
Even properly administered, standardized field sobriety tests have meaningful error rates. The HGN test, considered the most reliable, still carries a documented error margin. The Walk-and-Turn and One-Leg Stand are particularly affected by physical conditions and environmental factors unrelated to alcohol.
Can the breathalyzer results be wrong?
Yes. Breathalyzer equipment must be maintained, calibrated, and operated according to strict standards. Calibration failures, mouth alcohol contamination, radio frequency interference, operator error, and medical conditions can all produce inaccurate readings. Ms. Williams reviews all maintenance and calibration records as a standard part of every DUI defense.
What if I refused the breathalyzer?
A refusal triggers a statutory summary suspension under 625 ILCS 5/6-208.1: 12 months for a first offender who was not in a crash that caused personal injury or death, and 3 years if the person is not a first offender. A refusal after a crash that caused personal injury or death can instead be a statutory summary revocation. Refusal can be used as evidence at trial. It also eliminates a potentially damaging BAC reading from the prosecution case. The strategic implications are case-specific. Ms. Williams can evaluate exactly where your case stands.
What if I performed poorly on field sobriety tests because I was nervous?
Nervousness, medical conditions, fatigue, physical limitations, and environmental conditions are all legitimate factors affecting field sobriety test performance. These factors are documented and presented as part of your defense.
Ms. Williams defends clients following DUI stops across Central Illinois including Peoria County, McLean County, Tazewell County, LaSalle County, and Macon County.