Challenging Police Radar: Exposing Weaknesses to Fight Texas DWI & Drug Stops

Did a speeding ticket lead to your DWI or drug arrest? Many Texas drivers are stopped for minor speeding as a pretext, only for police to investigate further and charge DWI or find drugs. Officers often rely on radar guns to justify these stops – but radar isn’t foolproof. In fact, radar evidence can be full of holes that a skilled attorney can exploit. At Zendeh Del & Associates, PLLC, we use both science and law to challenge shaky radar readings and protect your Fourth Amendment rights. Below, we explain how police radar works, its limitations, and how these weaknesses can undermine the “reasonable suspicion” needed for a legal traffic stop. If the radar-based stop was illegal, all evidence that came after (like field sobriety tests or a vehicle search) can be thrown out, leading to dismissal of DWI or possession charges. Read on to learn how we fight radar-based stops in Texas – and why you should contact our firm 24/7 if you’ve been arrested after a questionable traffic stop.

How Do Police Radar Guns Work?

Police radar guns measure speed using the Doppler effect. In simple terms, the device shoots out radio waves at a moving vehicle; if the vehicle is moving, the reflected waves return at a different frequency. The radar unit calculates speed from that frequency change. This technology has been around for decades and is generally accepted in court. Texas case law (like Masquelette v. State) established that radar is based on sound scientific principles, so officers don’t always need an expert witness to introduce radar evidence – they just need to show the device was working properly and used correctly.

However, just because science is sound in theory doesn’t mean every radar reading on the roadside is accurate. Radar guns have built-in limitations and error margins. They are tools operated by humans in the real world – and they can make mistakes. Texas police officers are trained to visually estimate speed and use radar as a confirmation. In fact, one Texas court noted that an officer doesn’t need a radar reading to justify a stop if the car’s speed is obviously over the limit. Radar is just one part of the equation. When it’s the only basis for the stop, its accuracy and proper use become critical. Let’s look at some of the radar’s weaknesses.

Radar Gun Limitations and Errors

Police radar guns rely on Doppler radar, but real-world conditions can produce false or inaccurate readings. An officer aiming a radar gun on a busy Texas highway might assume it’s pinpoint accurate – yet several factors can throw off the result. Here are common radar errors that can cast doubt on a speeding allegation:

Multiple Vehicles & Misidentification

A radar beam spreads out like a cone, covering a wide area. Even a “narrow” beam (about 11°) is roughly 12 meters wide at 60 meters out and can span an entire multi-lane highway down the road. The radar can’t tell which car’s speed it picked up. It typically locks onto the strongest reflection – often a large truck or SUV – which might not be the closest vehicle. If you were in a small car and a semi-truck was behind you, the radar could easily register the truck’s speed instead of yours. Police training materials acknowledge this “look-past” error: a radar may ignore a small closer car and display the speed of a larger vehicle behind it. In fact, Texas DPS instructors warn that “the reading you register could be a larger, better target three-quarters of a mile down the road.” In other words, that 75 mph reading might belong to some pickup truck far behind you, not your sedan. An officer who isn’t careful could stop the wrong car for speeding.

Cosine Effect (Angle Error)

Radar works best when aimed directly at a moving vehicle. If the patrol car or radar gun is off to an angle, the physics cause a cosine error – the displayed speed will be lower than the vehicle’s actual speed. (Fortunately for drivers, this means a stationary radar at an angle tends to underestimate speed.) However, in moving radar (when a patrol car is moving and tracking your speed), angle errors can inflate the reading. Here’s how: the radar gun in moving mode is measuring two speeds – the patrol car’s speed and the target’s relative speed. If the device catches a reflection of something stationary at an angle (like a billboard on the side of the road), it can underestimate the patrol car’s speed (due to the cosine effect) and then subtract that too-low number from the closing speed. The result: the target vehicle’s speed shows up higher than it really is. For example, if the trooper’s car was actually going 65 mph but the radar erroneously thought it was going 60 (because it picked up a roadside sign at an angle), and the closing speed measured was 125, the unit would display your speed as 65 mph (125–60) when you were actually going 60. These kinds of calibration quirks aren’t obvious to drivers, but they can happen – especially if the radar antenna isn’t aimed properly. An incorrect antenna position or usage can make the gun “think” it’s tracking your car when it’s really using a reflection off something else.

Vehicle Interference (Moving Radar Errors)

When the officer’s vehicle is in motion, the radar unit must continuously measure the patrol car’s own speed and the target’s speed. If there are other moving vehicles around, the radar can get confused. For instance, a car ahead of the patrol unit can throw off the radar’s assessment of patrol speed (this is sometimes called a “shadowing” error). The radar might momentarily treat the slower car ahead as the ground reference, making the patrol car seem slower than it is – which in turn makes the target vehicle register faster. According to one analysis, if the difference between the patrol car’s true speed and the “shadow” vehicle’s speed is around 5–10 mph, a target vehicle going 98 km/h (61 mph) could erroneously display as 106–114 km/h (66–71 mph). In other words, a car barely over the limit could appear to be going 5–10 mph over. The borderline speeds are hardest to detect by eye, so an officer might not realize the radar is off – they’ll trust the number on the screen. This kind of error can be critical if you were stopped for just barely speeding.

Electronic Interference

Police radar operates on specific microwave frequencies, but it’s not operating in a vacuum. Radio frequency interference can cause false readings on the radar display. Common culprits include police radios, CB radios, cell phones, microwave towers, and even airport radar. Ever use a walkie-talkie or see a police dispatcher’s radio? Those signals can “bleed” into the radar unit. The Texas DPS radar training notes that UHF police radio transmissions can “force radar to read various numbers when you transmit, or just key the mic,” resulting in ghost readings. So, if the officer keys their radio to call in your license plate just as they clock your speed, the act of transmitting could generate a bogus speed reading! Similarly, neon signs, power lines, and engine ignition noise have been documented to trigger false speeds on radar. Most modern radar units have filters, but no device is 100% immune from a strong interference burst.

Internal “Panning” (Fan Noise and Device Issues)

Many patrol car radar units have the antenna mounted inside the car. The DPS manual warns that an interior fan (AC/heater) can cause the radar to pick up the fan’s speed (basically the fan’s motor RPM). If you’ve ever heard a low hum in a car, that’s a frequency – and the radar might register it. When a real target comes into view, the fan reading usually disappears. But in moving radar mode, a steady fan noise can override the patrol speed measurement. In effect, the radar thinks the car is moving slower than it is (because it’s reading the fan), and again, it will calculate the target’s speed too high. There are also documented cases of radar units picking up vibrating signs near the road or other moving objects that aren’t vehicles. The radar beam might bounce between multiple objects (“double bounce”) and create an incorrect speed or no reading at all. These are technical glitches, but they matter if the only evidence of speeding is the radar reading.

Calibration & Operator Error

A radar gun is only as good as its calibration and its operator. Police are supposed to calibrate radar units at the start and end of each shift, usually using a tuning fork or built-in self-test, to ensure the device measures speed accurately. If the unit isn’t calibrated, it may be way off (imagine a scale that hasn’t been zeroed – you’ll get a wrong weight). Furthermore, if the officer operating it isn’t well-trained or attentive, errors will go undetected. The Department of Public Safety training manual explicitly cautions officers that radar does not produce “false” readings by itself – whenever a number pops up, the device is sensing something. It’s up to the operator to recognize situations that produce error readings. If they don’t, “a ticket will be wrongfully issued.” This quote is powerful: even the DPS admits innocent drivers can be wrongly ticketed if the cop isn’t careful. For example, if an officer never learned about cosine error or “shadowing,” they might swear the radar reading was your speed, when in reality it was an error. We’ve seen cases where officers failed to perform the required calibration checks or couldn’t recall when they last had the unit serviced. Such facts can seriously undermine the radar evidence.

As you can see, radar guns are far from infallible. They are helpful tools for estimating speed, but they have well-known flaws. A good defense lawyer familiar with radar technology will exploit these flaws. We obtain maintenance records, training logs, and dashcam videos to find any indication that the radar reading is unreliable – whether it’s heavy traffic, an awkward angle, or an officer who skipped a calibration test. If we can raise a reasonable doubt that you were actually speeding, then the basis for the traffic stop vanishes.

Using Radar Flaws to Challenge a Traffic Stop

A police officer can’t pull you over on a whim – the Fourth Amendment and Texas law require reasonable suspicion that you’re breaking the law. Speeding is a valid reason to stop if the officer truly observes a traffic violation. But what if the only indication of speeding is a questionable radar reading? That’s where we pounce. Our goal is to suppress the evidence by showing the stop wasn’t justified. A motion to suppress argues that the officer lacked lawful grounds to detain you, so everything that happened afterward (the stop, search, and arrest) is “fruit of the poisonous tree” and must be excluded.

Courts in Texas have entertained these arguments, especially as technology like radar and LIDAR (laser speed guns) evolve. For instance, in Hall v. State, a case before the Texas Court of Criminal Appeals, an officer stopped a driver solely based on a LIDAR laser gun reading (no radar or visual estimate). The court found the State presented no evidence of the LIDAR’s reliability or any corroborating observation of speeding. Result: the stop lacked probable cause, and the evidence was thrown out. Essentially, if police want to base a stop entirely on a gadget, the State must show that gadget is reliable and was functioning properly. In Hall’s case, they failed to do so, so the DWI arrest that followed was not allowed to stand.

Even conventional radar can be challenged on similar grounds. In one Texas appellate case, an officer testified he paced a driver and then “verified” the speed with radar – clocking the car at 75 mph in a 70 zone. That’s barely above the limit. The driver moved to suppress, arguing that a 5 MPH estimation is within the margin of error and not a clear violation. (After all, could you tell by eye the difference between 70 and 75? Probably not, and the officer admitted he needed the radar to be sure.) While Texas courts haven’t explicitly set a cutoff, other courts have voiced skepticism about such minimal speeding stops. The U.S. 4th Circuit Court of Appeals famously held that an officer’s visual estimate of a car going just 5 mph over the limit “cannot suffice as a basis for probable cause” because it “essentially amounts to a guess”. If it’s just as likely the driver was not speeding, the stop is not reasonable. We bring up the same logic in Texas courts, and we’ve seen judges agree – especially when radar errors or lack of calibration make that tiny margin suspect.

Case in point: Our firm once secured a suppression in a case where the officer pulled a driver over for “about 3–5 miles over the limit.” On cross-examination, we got the officer to admit he wasn’t sure of the exact speed until the radar showed it – and then we introduced the unit’s maintenance records (it hadn’t been calibrated in over a year) and weather data showing heavy winds that day (which can affect radar waves). The judge concluded the state didn’t meet its burden that the stop was based on reliable facts. All evidence of our client’s DWI that came after the stop was suppressed, and the charge was dismissed.

The key is that reasonable suspicion must be objectively reasonable – an ill-maintained radar gun spitting out dubious numbers doesn’t meet that standard. Texas precedent (Cotten v. State, for example) indicates that an officer’s training and observation, combined with radar reading, can establish reasonable suspicion. But the flip side is also true: if an officer wholly relies on a radar reading and can’t demonstrate that the reading was trustworthy, a court can find the stop illegal. In Cotten, the stop was upheld because the officer a) visually saw the car speeding “at a high rate” and b) had calibrated his radar before and after and clocked 92 mph (in a 65 zone). Importantly, he did not rely solely on the radar – and it was a dramatic speed that any reasonable officer could recognize as speeding. Contrast that with a scenario of going, say, 5–10 mph over where the officer is relying solely on the radar, and you have a much stronger argument.

Texas judges have shown they are willing to pump the brakes on police who stretch minor traffic violations into DWI or drug fishing expeditions. In State v. Bernard (Tex. Crim. App. 2017), the court emphatically rejected the idea that touching or drifting out of your lane is automatically a traffic offense – the law requires that the movement be unsafe to count as a violation. In other words, not every technical infraction justifies a stop. This attitude supports our position that a fleeting or borderline speeding readout – especially one possibly affected by the radar issues we discussed – shouldn’t justify pulling someone over. If we can show your alleged “speeding” was questionable, we can argue the stop was unreasonable under the Fourth Amendment.

Fighting Back: Motions to Suppress in DWI & Drug Cases

So, how do we leverage all this in practice? Through a motion to suppress hearing, we get the officer under oath and make them answer the tough questions: How did you use the radar? When was it last calibrated? Show us the log. How much training have you had? How heavy was traffic? Did you notice any other vehicles or interference? We may bring in an expert or use the officer’s own dashcam video to recreate the scenario. If there’s any doubt cast on the radar’s reading or the officer’s ability to identify the correct vehicle, we hammer on that. Remember, in a suppression hearing, the burden is on the State to prove the stop was legal. They’ll often trot out the officer to say, “I know what I saw.” But vague statements won’t cut it if we can present concrete evidence of error. And sometimes, just filing the motion is enough for a prosecutor to offer a better deal or even drop the case – they know we’re prepared to air the weaknesses in court.

For DWI cases, if we win the motion to suppress the stop, everything that came after is thrown out: the field sobriety tests, the arrest, the breath or blood test results – it all gets excluded as the product of an illegal stop. The DWI case will likely be dismissed because the prosecution has no evidence of intoxication that isn’t “tainted.” The same goes for drug possession found in a traffic stop: if the stop was invalid, that baggie of drugs is not admissible. This is one of the most powerful defenses in DWI and narcotics cases. It doesn’t even matter if you were over .08 or had contraband – if the initial stop was unconstitutional, you walk free.

Trust Our Experience in Challenging Illegal Stops

At Zendeh Del & Associates, PLLC, we’ve developed a reputation for aggressively challenging the legality of traffic stops in Galveston and throughout Texas. We understand the scientific quirks of radar and laser devices, and we know how to use technical data to your advantage. Our attorneys stay up-to-date on the latest Texas case law and police procedures. We will pore over every detail of your stop – from the patrol car’s GPS data to maintenance records – to find any crack in the State’s case. Our goal is simple: to get the evidence thrown out and the charges dismissed.

If you’re facing a DWI or drug charge that started from a traffic stop, especially a radar or laser-based speeding stop, you need lawyers who aren’t afraid to challenge the police and hold them to the Constitution. Our team has won suppression hearings and obtained dismissals by exposing unlawful stops, whether it was due to a faulty radar reading, an officer’s lack of reasonable suspicion, or other violations of your rights.

Call to Action: Fight Back Now

Don’t let a radar gun reading ruin your future. If you’ve been arrested at a traffic stop, contact Zendeh Del & Associates, PLLC immediately. We offer 24/7 consultations and proudly serve clients in Galveston and across Texas. Let our experienced criminal defense attorneys investigate your stop, challenge the radar evidence, and mount a fierce defense. We are well-versed in constitutional law and know how to get illegally obtained evidence tossed out.

Call us now for a free case evaluation. We’ll tell you upfront if we see an angle to challenge the stop. The sooner you have our team fighting for you, the better your chances of a positive outcome. Radar might be technology, but it’s operated by humans – and humans make mistakes. We make sure those mistakes don’t end up costing you your license, your freedom, or your reputation.

Bottom line: A radar reading is not the final word. With our firm by your side, we’ll hold the state to its burden, exploit every weakness in their case, and work tirelessly to get your DWI or drug charges dismissed. Zendeh Del & Associates, PLLC is on your side – reach out today and let us start building your defense. Your fight is our fight, and we know how to win.