A Tennessee traffic stop can begin with something that seems minor: drifting near the fog line, failing to signal, speeding, or having a brake light out. But once an officer turns on the blue lights, that brief roadside encounter can quickly become much more serious. A routine stop may lead to DUI questioning, field sobriety tests, a vehicle search, drug or firearm allegations, or an arrest.
Whether the officer’s actions were lawful often depends on two important legal standards: reasonable suspicion and probable cause. These terms are related, but they are not the same. Below, we’ll break down what reasonable suspicion and probable cause mean, how they apply to Tennessee traffic stops, when a legal stop can become an illegal detention, and how a defense attorney may challenge evidence obtained after an unlawful stop.
What Is Reasonable Suspicion?
Reasonable suspicion is the lower of the two standards. It may allow an officer to briefly stop a vehicle and investigate whether a traffic violation or criminal activity is occurring.
Reasonable suspicion requires more than a guess or vague feeling. The officer must be able to point to specific facts that made the stop objectively reasonable. In traffic stop cases, those facts may involve driving behavior, a suspected equipment violation, or observations suggesting possible impairment.
Importantly, the officer does not always have to prove that the driver actually violated the law before making the stop. The question is whether the officer had a reasonable basis to investigate based on what was known at the time.
For example, an officer may argue that reasonable suspicion existed because the vehicle repeatedly drifted across lane markings, was being driven erratically, or appeared to have a defective brake light. Whether that explanation holds up depends on the facts, the video evidence, and the officer’s testimony.
Reasonable suspicion can justify a brief investigation, but it has limits. It does not automatically justify an arrest, a full vehicle search, or a prolonged detention.
What Is Probable Cause?
Probable cause is a higher standard than reasonable suspicion. It generally exists when the facts known to the officer would lead a reasonable person to believe that a law has been violated, a crime has been committed, or evidence of a crime may be found.
In a DUI case, reasonable suspicion may justify the initial stop, but probable cause is usually required before the officer can make an arrest. Probable cause for DUI may be based on the totality of the circumstances, including the driver’s manner of driving, odor of alcohol, slurred speech, bloodshot eyes, admissions to drinking, field sobriety test performance, and other roadside observations.
Probable cause also matters in vehicle search cases. A traffic stop alone does not automatically allow police to search a car. Officers generally need consent, a warrant, probable cause, or another recognized exception to the warrant requirement.
Reasonable Suspicion vs. Probable Cause: Key Differences
The simplest way to understand the difference is this: reasonable suspicion may justify a brief stop or investigation, while probable cause may justify stronger police action, such as an arrest or search.
Stage | Standard Usually at Issue | Example |
Initial stop | Reasonable suspicion or probable cause | Officer observes possible impaired driving or a traffic violation |
DUI investigation | Reasonable suspicion | Officer observes odor of alcohol, slurred speech, or unusual behavior |
DUI arrest | Probable cause | Officer believes the driver is impaired based on the total facts |
Vehicle search | Probable cause, consent, warrant, or exception | Officer claims there is evidence of a crime in the vehicle |
A stop that starts legally can still become unlawful if the officer goes beyond what the law allows. For that reason, Tennessee traffic-stop cases often come down to timing: what the officer knew, when the officer knew it, and whether those facts justified each step of the encounter.
Common Tennessee Traffic Stop Examples
Traffic stops in Tennessee often begin with ordinary driving conduct. The legal issue is not simply whether the officer found evidence later. The question is whether the officer had the required legal justification at the time of the stop and, later, whether additional facts justified expanding the investigation.
Fog Line, Center Line, and Lane-Departure Stops
One of the most common reasons officers give for a DUI stop is that the driver crossed or touched the fog line, center line, or another lane marking. Tennessee law requires a vehicle to be driven “as nearly as practicable” within a single lane and not moved from that lane until the driver first determines the movement can be made safely.
That phrase matters. A brief lane movement may have an innocent explanation, such as road conditions, avoiding debris, wind, construction, or another vehicle. But Tennessee courts have upheld stops where an officer observed lane-line movement that created reasonable suspicion for an investigatory stop. In State v. Linzey Danielle Smith, the Tennessee Supreme Court held that a stop was supported by reasonable suspicion where the officer observed the driver once cross and twice touch the fog line.
For the defense, the focus is often on the details: How many times did the vehicle move? Did it actually cross the line? Were there road conditions that explained the movement? Does the video match the officer’s report?
DUI Suspicion
A stop may also be based on driving behavior that the officer believes suggests impairment. Examples may include weaving, driving too slowly, sudden braking, delayed reactions at traffic signals, or driving the wrong way.
Tennessee’s DUI statute prohibits driving or being in physical control of a vehicle while impaired by alcohol, marijuana, controlled substances, prescription drugs, or other substances affecting the central nervous system. The statute also prohibits driving with a blood or breath alcohol concentration of 0.08% or more for most drivers.
Once a DUI investigation begins, the officer may look for additional facts, such as odor of alcohol, slurred speech, bloodshot eyes, admissions to drinking, open containers, or performance on field sobriety tests. Those facts may become important later when determining whether the officer had probable cause to make a DUI arrest. In State v. Bell, for example, the Tennessee Supreme Court found probable cause where the driver had been driving the wrong way on a divided highway, smelled of alcohol, and admitted drinking.
Equipment Violations
Some stops begin with an alleged equipment issue, such as a broken brake light, defective headlight, expired registration, dark window tint, or a license plate problem. These cases can turn on whether the officer could actually see the alleged defect and whether the video supports the stated reason for the stop.
A defense attorney may review the lighting, distance, camera angle, weather, and officer testimony. If the alleged equipment issue does not appear on video, that does not automatically make the stop unlawful, but it may create an important factual dispute.
Turn Signal, Stop Sign, and Speeding Stops
Many DUI and criminal cases begin with a basic traffic allegation: speeding, rolling through a stop sign, failing to signal, making an unsafe lane change, or following too closely. These stops may appear straightforward, but the details still matter.
For example, the defense may look at whether the officer had a clear view, whether the roadway markings were visible, whether the driver’s movement was actually unsafe, or whether the officer’s report matches the bodycam or dashcam footage.
Anonymous Tips and Calls from Other Drivers
Sometimes police stop a vehicle after receiving a report from another driver or a 911 caller. These cases can be more complicated because the officer may not personally observe the alleged bad driving before initiating the stop.
The key questions usually include: Who made the report? Was the caller identifiable or anonymous? What details did the caller provide? Did the officer corroborate the vehicle description, location, or driving behavior before making the stop?
These examples all come back to the same principle: a Tennessee traffic stop must be justified by facts, not a hunch. A minor traffic allegation may justify a stop in some cases, but it does not automatically justify a DUI arrest, vehicle search, or prolonged detention. The legality of the stop depends on what the officer knew, what the officer observed, and whether each step of the encounter was supported by the proper legal standard.
These examples all come back to the same principle: a Tennessee traffic stop must be justified by facts, not a hunch. A minor traffic allegation may justify a stop in some cases, but it does not automatically justify a DUI arrest, vehicle search, or prolonged detention. The legality of the stop depends on what the officer knew, what the officer observed, and whether each step of the encounter was supported by the proper legal standard.
When a Legal Stop Becomes an Illegal Detention
Even when a Tennessee traffic stop starts legally, it can become unlawful if the officer keeps the driver longer than necessary without additional justification.
A traffic stop has a limited purpose. Usually, that purpose is to address the reason for the stop by checking the driver’s license, registration, proof of insurance, and warrant status, then deciding whether to issue a warning or citation. In Rodriguez v. United States, the United States Supreme Court explained that police generally may not prolong a traffic stop beyond that traffic-related mission to investigate unrelated criminal activity unless they have independent reasonable suspicion to do so.
For example, an officer may stop a driver for speeding or an improper lane change. During that stop, the officer may handle the ordinary tasks connected to the traffic violation. But the officer generally cannot delay the driver to investigate unrelated drug activity, wait for a K-9 unit, or ask unrelated questions in a way that adds time to the stop unless new facts support reasonable suspicion.
This issue often comes up when police use a routine traffic stop as the starting point for a broader investigation. A stop may begin with a minor traffic violation, but then the officer starts asking about unrelated matters, including travel plans, criminal history, probation status, outstanding warrants, or whether there is anything illegal in the vehicle. Those questions are not automatically unlawful. The constitutional issue is whether they prolonged the detention beyond the time reasonably needed to complete the traffic stop without independent reasonable suspicion.
In State v. Graves, the Tennessee Court of Criminal Appeals addressed this kind of issue after a traffic stop led to drug and firearm charges. The defendant challenged the delay between the initial stop and the arrival of a canine unit. The court held that the State failed to establish that the otherwise lawful traffic stop was not unreasonably prolonged in time and scope, reversed the convictions, vacated the judgments, and dismissed the case.
The timing matters. Courts may look at questions such as:
- How long did the stop last?
- What was the original reason for the stop?
- Was the officer actively working on the citation or warning?
- When did the officer begin investigating something unrelated to the traffic violation?
- What facts, if any, supported expanding the investigation?
- Did the officer wait for a K-9 unit, backup officer, or additional investigation after the traffic tasks should have been completed?
For drivers, the key point is that a lawful traffic stop does not give police unlimited time or authority. If the original reason for the stop has been addressed, the officer generally needs a new legal basis to continue detaining the driver. Without that additional reasonable suspicion, evidence found after the stop was prolonged may be vulnerable to a motion to suppress.
How Defense Attorneys Challenge Traffic Stops in Tennessee
Challenging a Tennessee traffic stop usually begins with the timeline. A defense attorney will look at what the officer claimed to know, when the officer claimed to know it, and whether those facts justified each step of the encounter.
The review often starts with the officer’s stated reason for the stop. If the officer claimed the driver crossed a lane line, failed to signal, rolled through a stop sign, or had an equipment violation, the defense will compare that explanation with the available video, reports, and witness testimony.
Key evidence may include:
- dashcam and bodycam footage;
- police reports and citations;
- dispatch logs and timestamps;
- roadside statements;
- field sobriety test instructions and performance;
- breath or blood testing records;
- K-9 deployment records, if applicable; and
- officer testimony at a suppression hearing.
In DUI cases, the defense may examine whether the officer had reasonable suspicion to begin the stop and whether additional facts developed before the officer expanded the investigation. For example, if the stop was based on speeding, what facts justified asking DUI-related questions or requesting field sobriety tests? If the officer arrested the driver, what facts supported probable cause at that moment?
In drug or firearm cases, the focus may shift to whether the officer had a lawful basis to prolong the stop or search the vehicle. A minor traffic violation does not automatically justify a broader criminal investigation. If the officer delayed the stop to wait for a K-9 unit, ask unrelated questions, or seek consent to search, the defense may challenge whether that delay was supported by independent reasonable suspicion.
Video evidence can be especially important. Sometimes the footage supports the officer’s account. Other times, it may show that the alleged violation was unclear, that the driver’s behavior was less suspicious than described, or that the officer extended the stop after the traffic-related purpose had been completed.
If the defense believes the stop, detention, search, or arrest was unlawful, the attorney may file a motion to suppress. At that hearing, the prosecution must justify the officer’s actions under the applicable legal standard. If the judge agrees that police violated the driver’s constitutional rights, evidence obtained as a result of the unlawful conduct may be excluded from the case.
Talk to a Knoxville Criminal Defense Attorney About Your Charges
If you were arrested after a traffic stop in Knoxville or East Tennessee, the legality of that stop may be one of the most important issues in your case. The officer’s stated reason for the stop, the video evidence, the timeline of the detention, and the facts used to justify any search or arrest can all affect your defense.
At Barnes & Fersten, our criminal defense lawyers carefully review traffic-stop cases to determine whether law enforcement had reasonable suspicion, probable cause, or another lawful basis for each step of the encounter. If police exceeded constitutional limits, we may be able to challenge the evidence through a motion to suppress.
To speak with a Knoxville criminal defense attorney about your case, contact Barnes & Fersten today by calling 865-805-5703 or filling out our contact form for a free, confidential consultation.
Attorney At Law, Managing Partner
Brandon D. Fersten is an esteemed Knoxville attorney practicing DUI, criminal defense, and juvenile law. Known for his empathetic approach and commitment to his clients, he brings a record of favorable case outcomes including dismissals and not guilty verdicts at jury trials resulting in Brandon being recognized as one of the “Top 40 Under 40” in Criminal Defense, U.S. News’ Best Lawyers: “Ones to Watch,” and Super Lawyers’ “Rising Stars”. Brandon’s professional accolades, combined with his passion for justice, position him as a reliable criminal defense advocate in the East Tennessee legal landscape, including Knox County, Blount County, Sevier County, Loudon County, Roane County, Anderson County, Cumberland County, Hamblen County, Monroe County, and McMinn County.
