What Do Officers Watch for During a DWI Traffic Stop?

In 1966, Congress passed the National Traffic and Motor Vehicle Safety Act, which created the National Highway Traffic Safety Administration (NHTSA). Since that year, more than a million people have died in traffic accidents in the U.S. But the rate of death has decreased dramatically. In the 1960s, the fatality rate in traffic accidents was nearly four times what it is today. This reduction is due in part to a reduction in the number of crashes involving alcohol.

An NHTSA-created DWI detection guide contributed to this reduction, and law enforcement agencies across the U.S. use the guide to detect and arrest drivers suspected of DWI. However, if you're facing a DWI arrest, it's a good idea to know and understand what Texas police officers are looking for during a traffic stop to fight any unjust charges against you. A lawyer who is an expert in Texas DWI law can help guide you along this path and zealously defend your rights.

What is a DWI in Texas?

In Texas, you can face arrest for DWI if you drive a vehicle in a public place while intoxicated. “Intoxicated” means:

(A) not having the normal use of mental or physical faculties by reason of the introduction of alcohol, a controlled substance, a drug, a dangerous drug, a combination of two or more of those substances, or any other substance into the body; or

(B) having an alcohol concentration of 0.08 or more.

Tex. Penal Code § 49.01(2) (2001). Under the statute, there are two ways you can face DWI charges in Texas:

1. Blood Alcohol Content Over .08%

Under Texas law, if your BAC is over.08%, you are legally “intoxicated.” For those with a commercial driver's license, the legal limit is .04%. But it's essential to understand that BAC testing isn't infallible. Test results can be inaccurate for many reasons, making it even more important to hire an expert in DWI defense to represent you.

2. Not Having the Normal Use of Mental and Physical Faculties

Even if your BAC is below the legal limit, if law enforcement officers decide that you don't have the “normal use of mental or physical faculties” after using drugs or alcohol, they could also decide you're intoxicated. Deciding that you don't have the normal use of your faculties is a subjective judgment made by a police officer that may not be accurate.


Determining whether to pull a driver over for a suspected DWI is also highly subjective. That's why the NHTSA, a division of the U.S. Department of Transportation, developed a guide in 1979 for detecting drivers who are driving while intoxicated. In 2015, the NHTSA updated its guide to include 24 cues broken down into categories of driving performance, including:

  • Speed and braking issues
  • Maintaining proper lane position
  • Vigilance problems
  • Judgment issues

The NHTSA sponsored a study interviewing law enforcement officers across the country to develop the DWI detection guide, compiling a list of more than 100 cues that signal a driver may have been drinking. From its initial list of 100, the NHTSA developed a final list of 24 cues broken down into the four categories listed above. The NHTSA also intended the DWI guide to help officers:

  • Detect impaired drivers
  • Articulate the behaviors they observe on arrest reports
  • Support officers' expert testimony in court

According to the NHTSA, if a police officer observes one of the 24 cues, the list can predict whether a driver is intoxicated about 35% of the time. They claim the number can be higher depending on the type of infraction and how many an officer observes. For example, the DWI guide claims that a driver weaving in traffic is 50% likely to be intoxicated, while some cues such as driving on the wrong side of the road increase the probability to 70%. Even accuracy 70% of the time leads to a high number of possible observation and police errors.


Within each category of cues in the NHTSA DWI guide, there are several cues that police watch for in a suspected DWI.

Maintaining Proper Lane Position

In the category involving maintaining proper lane position, the NHTSA DWI guide identifies seven cues police should watch for to identify possible impaired drivers. These cues include:

  • Weaving: Weaving happens when a car moves from one side of the lane to the other. Maintaining proper lane positions can be difficult for impaired drivers.
  • Weaving across lane lines: Impaired drivers may also move between lanes while swerving or have one or more tires veer into another lane.
  • Straddling a lane line: In some cases, impaired drivers will even straddle two lanes, driving straight or weaving, with tires in both lanes.
  • Swerving: When a car makes an abrupt move away from its current course, it's called swerving. It often happens when an impaired driver is about to collide with something or realizes they are drifting into another lane or off the road.
  • Turning with a wide radius: When a car appears to drift into another lane or to the outside of a lane while turning, this is another cue that police watch for possible DWIs. Many alcohol-involved crashes happen with an “expanded turning radius” or when a turning vehicle drifts into another lane.
  • Drifting: A vehicle drifts when it moves in a straight line with a slight angle, carrying the vehicle into another lane or across the center line. A driver correcting course can do so gradually or swiftly.
  • Almost striking a vehicle or other object: If a vehicle almost hits another car or an object on or beside the road, whether through drifting, swerving, or weaving, the police consider this a strong cue for a possible DWI.

Speed and Braking

Within the speed and braking category, the NHTSA identifies certain behaviors police should watch for, including:

  • Stopping or braking problems: Stopping without jerking or stopping too quickly or slowly can be difficult for intoxicated people. Police will watch for stopping too far away from a curb, stopping at an inappropriate angle, or stopping over a line or before a line.
  • Problems maintaining consistent speed: Impaired drivers often have difficulty maintaining a consistent speed on the road. Police will watch for accelerating or decelerating without cause, driving at varying speeds, and driving more than ten miles per hour below the speed limit.

Vigilance Problems

The NHTSA DWI guide identifies six vigilance cues for police officers to watch for in the DWI guide. Impaired or intoxicated drivers often have difficulty paying attention to the details of driving. Possible vigilance clues indicating an intoxicated driver include:

  • Driving in the wrong lane or driving the wrong way on a one-way street: Intoxicated drivers can fail to pay attention to signs and roadways and end up driving the wrong way or in the wrong lane.
  • Not responding to traffic signals quickly: An impaired driver may not notice when the light changes.
  • Slowly responding or failing to respond to an officer's signals: Officers will watch for drivers who don't notice their flashing lights or signals to pull over.
  • Stopping in a lane for no apparent reason: An impaired driver may stop for no apparent reason in traffic because they're having difficulty dividing their attention, they're confused, or they're even asleep at the wheel.
  • Driving with no headlights at night: Impaired drivers can often forget to turn on their lights at night or in the rain.
  • Not signaling a turn or lane change or inconsistently signaling: The police will watch for drivers who forget to signal before turning or changing lanes or for drivers who signal that they're turning one way and turn the other way instead.

Judgment Problems

Operating a car or other vehicle requires constant decision-making from the driver. If a driver is drunk, this constant judgment-making process is also impaired. As a result, the DWI Guide from NHTSA identifies six judgment-related cues that may indicate a drunk driver. These judgment problems include:

  • Following others too closely: Drunk drivers can also make choices that endanger other drivers, like following too closely to allow a safe stopping distance.
  • Conducting unsafe or illegal lane changes: Research shows that drunk drivers are less inhibited about making illegal or unsafe turns.
  • Driving on something that isn't a road: A drunk driver is also more likely to drive on something that isn't a roadway, like a sidewalk, median, or shoulder.
  • Stopping inappropriately when signaled by an officer: When an officer signals, we all know to signal and move over when it's safe to do so. But an impaired driver may make a poor judgment call and stop immediately in a lane or continue driving and fail to pull over.
  • Inappropriate behavior with an officer or unusual behavior: If a driver is belligerent, rude, argumentative, overly affectionate, throws things from the car, urinates on the side of the road, or displays other unusual or inappropriate behavior with an officer, this can be a cue that a driver is intoxicated.
  • A driver appears to be impaired: Officers will watch for physical signs that a driver is intoxicated, like gripping the steering wheel tightly, driving with their face close to the window, slouching in the seat, or staring fixedly ahead while driving.

According to the NHSTA guide, some driving behaviors, if combined with another cue, can predict a drunk driver more than 50% of the time, including:

  • Failing to signal or signaling inconsistently with a turning action
  • Driving without headlights at night

Post-Stop DWI Cues

The NHTSA's DWI guide also includes post-stop cues that may predict intoxicated drivers, including:

  • A driver is having a hard time with their vehicle's controls. For example, if they can't open the glove compartment or roll the window down.
  • A driver is having a hard time getting out of the vehicle. They may stumble, fall, or have difficulty opening the door.
  • Fumbling with their license or registration. The driver may struggle to open the glove compartment, locate their wallet, or drop their credentials.
  • Repeating questions a police officer asks or repeating comments.
  • An unsteady driver who is swaying or having balance issues.
  • A driver with slurred speech.
  • A driver that is slow to respond or the officer must repeat themselves.
  • A driver that provides incorrect information or changes their answers.
  • An odor of alcohol coming from the driver.

Problems With the NHTSA DWI Guide Cues

While the NHSTA guide is based on research from analyzing more than 12,000 DUI stops, many of these cues are based on law enforcement interpretation of a person's actions during a time of high stress. An expert in DWI defense will know how to identify the problems with law enforcement's justifications for your DWI stop and know how best to defend your actions to a prosecutor and a court.

Speed and Braking Issues:

Aside from speed clocked by radar or a police officer following a driver, if a police officer believes that a driver is driving with inconsistent speed, this belief is largely based on their perception of a driver's speed. It isn't typically based on information recorded objectively. Concerning braking issues, the police officer's observations or assumptions can be incorrect. For example, you may jerk to a stop if there's something in the road, a potential hazard on the side of the road, or even dangerous conditions inside your car. While a police officer may assume under the DWI guide that you are intoxicated, the reason for your driving may be completely benign.

Maintaining Proper Lane Position:

Failure to maintain proper lane positions can also be somewhat subjective. The driver may have many reasons for swerving, straddling a lane, changing lanes without signaling, and other “unsafe” maneuvers identified as DWI cues under the NHSTA Guide. For example, other drivers, pedestrians, road hazards, weather conditions, poorly marked lanes, and poorly marked or missing signs can all impact your ability to maintain lane position. Moreover, sometimes even the best of us is distracted while driving by kids, passengers, or cell phones. While distracted driving is never a good idea, a swerve doesn't necessarily equal an impaired driver.

Vigilance Problems:

The NHSTA DWI Guide also identifies vigilance problems as a DWI “cue.” However, driving conditions that a police officer may interpret as a cue that you are drunk can also have simple explanations. For example, it's easier than you'd think to end up driving the wrong way on a one-way road at night, in an unfamiliar area, with poor lighting and poorly marked signs. If your lights are off at night, you may have an electrical issue with your car or, if your headlights normally come on automatically, maybe the valet at the restaurant turned your headlights off. If you fail to respond to a police car with lights and sirens immediately, you may not see or hear it, particularly if your passengers are noisy or distracting you. Again, driving while distracted is never ideal, but it isn't always a sign of a drunk driver.

Judgment Issues:

Judgment issues while driving can also be subjective. If you're driving on a median, it may be a better indication that the road needs better signs, better lighting, and better markings. If a police officer thinks you are belligerent during a stop, but you're just annoyed at being pulled over by someone who thinks you've been drinking, the police may still interpret this as a sign that you're impaired.

What Happens During a DWI Traffic Stop?

The police can't pull you over whenever they feel like it, no matter what you see on TV. To avoid violating your constitutional rights, the police need a “reasonable suspicion” that you've committed a crime. By understanding what should happen during a routine traffic stop and how police incorporate the NHSTA DWI Guide cues into a stop, you can see how an experienced DWI defense attorney builds a defense. A DWI arrest often relies on a police officer's subjective interpretation of these DWI “cues.” Police officers can make mistakes and misinterpret cues.

The Traffic Stop

To have “reasonable suspicion” to pull you over, the police need a reasonable belief that you committed a crime. Traffic violations, including speeding, driving too slowly, swerving, and many of the “cues” listed in the NHSTA DWI Guide, qualify as “reasonable suspicion” for a traffic stop. After pulling you over, the police will continue to watch for the cues listed in the NHSTA DWI Guide, and they will observe your coordination, behavior, and conduct to decide if they suspect you of driving while impaired. The police will watch for the cues discussed above, as well as:

  • Whether you have bloodshot eyes
  • Whether you have dilated pupils
  • An odor of alcohol or drugs coming from you or your car
  • Your cooperation
  • Any aggressive speech or behavior
  • Your physical coordination
  • Whether you have drugs or open containers of alcohol visible in the car

If the police believe you may be intoxicated based on all of the circumstances, they may ask you to perform several field sobriety tests.

Field Sobriety Tests

If the police ask you to complete a field sobriety test, they could ask you to do standard or non-standard tests. Non-standard tests include tests such as:

  • Reciting the alphabet from the middle or backward
  • Counting by threes or asking you to do math in your head
  • Having you touch your nose with your eyes closed

The judge won't generally allow non-standardized tests in court as evidence of intoxication. The standardized tests the police may ask you to complete can include:

  • Horizontal Gaze Nystagmus (HGN): The HGN test includes watching the involuntary jerking motion your eye naturally makes when you look to the side. The police officer will watch whether your eye can smoothly follow an object, whether you have distinct eye jerking when looking fully to the side, and eye jerking within 45 degrees of the center.
  • Walk and Turn: The walk and turn test involves walking a straight line, heel to toe, like a tightrope walker. The police will watch to see whether you can finish tasks with your attention divided while taking nine steps.
  • One-leg Stand: During a one-leg stand test, the police officer will have you stand while holding one leg six inches above the ground for 30 seconds. While you balance, the officer will ask you to count by 1000ths.

Many field sobriety tests can be challenging if you are disabled, have hearing problems, or have other health issues. The weather, the traffic, the road conditions, and what you're wearing can all affect your performance on a field sobriety test. Moreover, don't forget that your “performance” on these tests is based almost entirely on the subjective judgments of a police officer. An experienced DWI attorney will understand how to craft a defense in your case.

BAC Testing

If the police believe you've “failed” your field sobriety tests, they'll ask if you'll submit to a BAC test – either a blood or breath test. Unfortunately, human errors and equipment errors can affect BAC test results. As a result, BAC tests can be inaccurate. Errors during BAC testing can include:

  • Breaking the chain of custody
  • Failing to properly train police or lab techs performing your test
  • Improperly storing blood samples
  • Failing to calibrate or properly clean or reset the testing equipment
  • Failing to follow sample collection or testing protocols
  • Improperly collecting blood samples
  • Using up the entire blood sample without leaving samples for retesting

An expert in DWI law can examine every aspect of your case, including your BAC testing, processes, and results, and develop the best possible defense.

The Arrest

If your BAC testing results show your levels are over the legal limit, or if the police have “probable cause” to believe you're impaired, they will arrest you. If you've refused BAC testing, the police will likely seek a warrant for a blood test or arrest you anyway if they believe they already have probable cause to do so. “Probable cause” means that law enforcement officers reasonably believe that you committed a crime, considering the facts and circumstances of the case. The U.S. Supreme Court stated:

"[E]xists where the facts and circumstances within the officers' knowledge, and of which they have reasonably trustworthy information, are sufficient in themselves to warrant a belief by a man of reasonable caution that a crime is being committed." Brinegar v. U.S., 338 U.S. 160 (1949).

After evaluating your case, if your attorney believes the police didn't have probable cause for your arrest or BAC warrant, you may have grounds to attack the arrest or warrant as an unreasonable search or seizure.

Challenging a DWI Arrest

Your attorney may have the basis to challenge your DWI arrest at pretty much every stage of your DWI stop. It's important to remember that a DWI arrest and even a high BAC are not DWI convictions. You are innocent until proven guilty, and you should consult a DWI expert about developing a well-rounded defense.

Police Officer Stop

Your attorney may be able to use eyewitness testimony, dashboard cam evidence, or even cameras in the area to show that the police did not have reasonable suspicion for your stop. If other evidence can show you didn't commit the traffic law the police say you violated, your attorney may be able to show that the police didn't have reasonable suspicion, making your stop invalid.

Officer Observations of Behavior

If the police relied on their observations of your behavior to arrest you or to obtain a BAC warrant, your attorney might be able to exploit this as a weakness in the state's case. Officer observations of the “cues” developed by the NHSTA can be largely subjective. Or, as we discussed earlier, something a police officer interprets as a problem with judgment, attention, or vigilance can have an innocent explanation or be a natural result of a health emergency, road conditions, or other drivers.

Field Sobriety Tests

Field sobriety tests rely almost entirely on the observations and judgment calls of your arresting officer. Bad weather, heavy or fast traffic zooming by, gravel or rocky shoulders, and inappropriate footwear can all affect your ability to perform well on a field sobriety test. Moreover, health problems that affect your balance can affect how you perform, as can hearing problems that affect your ability to hear the police officer, especially traffic noise. Even low blood sugar can affect your ability to perform well on a field sobriety test. Moreover, if there is a dashcam video of the field sobriety tests, your attorney may be able to show that the officer's interpretations of your performance were incorrect.

BAC Testing

It's important to remember that BAC tests can be wrong. While testing for your blood alcohol level is more objective than field sobriety tests, your lawyer may be able to challenge the results in court in certain situations. For example, BAC tests may be inaccurate if:

  • The tech drawing your blood sample didn't follow protocols or procedures
  • The police officer administering your breath BAC test didn't have the proper training or failed to follow procedures
  • The breathalyzer hadn't been properly calibrated, maintained, or cleaned between tests
  • The blood analysis equipment wasn't properly maintained or calibrated
  • The techs and police officers involved failed to maintain the chain of custody
  • The lab techs used up your entire blood sample, failing to leave enough to repeat testing
  • The tech collecting your blood didn't add the proper anticoagulant to keep the blood sample from degrading
  • The techs failed to store your blood sample properly

Moreover, a health condition and the foods you eat can also affect your BAC blood or breath test. Your BAC can also change dramatically if the testing happened too long after your initial stop. Drugs and alcohol can continue to enter your system while you wait, resulting in a much higher BAC than immediate testing may have revealed. An attorney who is an expert in DWI defense will have intimate knowledge of the proper collection and testing procedures and can identify any problems in your BAC results.

DWI Arrest or Search

To arrest you or to obtain a warrant for BAC testing, the police must have probable cause to do so. If the police have “probable cause,” it means they have a reasonable belief you committed a crime considering all the facts and circumstances. The Fourth Amendment protects all of us from “unreasonable searches and seizures,” stating that “no Warrants shall issue but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.”

The “facts and circumstances” the police can use to support probable cause can include many of the NHSTA DWI Guide cues such as:

  • Reckless driving
  • How you behave or respond during a stop
  • Your performance on field sobriety tests
  • The results of a BAC or drug recognition expert (DRE) tests
  • Visible drugs or alcohol in your car

An expert in DWI defense can challenge whether the police reasonably relied on these facts and circumstances to support probable cause for your arrest or warrant. Your attorney may also be able to raise additional constitutional defenses on your behalf.

Hire a DWI Board Certified Texas Expert

When facing a DWI charge, it's important to remember that you are innocent until proven guilty. You need to consult an expert in DWI defense to determine your best path forward. Don't let anyone railroad you into pleading guilty until you understand all your options.

Attorney Doug Murphy is Board Certified in both DWI Defense and Criminal Law. He is only one of two Texas attorneys Board Certified in both these specialties, making him a recognized specialist in DWI and criminal defense. Doug had aggressively defended thousands of Texans facing DWIs and other criminal charges through the years and has more than two decades of experience in complex DWI defense. Doug's peers also recognize him as one of the best in his field.

In 2021, Best Lawyers in America named Doug a “Lawyer of the Year” in Houston DWI defense. Find out why the Houston media calls Doug “the drinking driver's best friend.” Call the Doug Murphy Law Firm today at 713-229-8333 to set up a consultation.

Contact Us Today

If you are facing DWI or other criminal charges in Texas, contact our office today to discuss your case, so we can begin working on your defense. Please provide only your personal email and cell phone number so that we can immediately and confidentially communicate with you.