Why do officers touch car taillights during a traffic stop? The answer involves surprise tactics, hidden risks and something you didn’t know they were leaving behind.
If you have a habit of binge-watching police bodycam videos on YouTube or if you’ve ever received a traffic violation, you may be familiar with this scenario: Through a blare of sirens, a car is pulled over by law enforcement. Both vehicles park on the side of the road. After a tense few moments, a police officer steps out of the car and calmly walks toward the driver. Approaching the rear of the vehicle, the officer taps the left-hand taillight. What you’re probably not familiar with is the reason why cops touch taillights when pulling someone over.
During a traffic stop, cops have to follow a whole host of procedures, not only for their safety but also for the safety of the person they’re pulling over. While tapping the taillight may be a lesser-known step (compared with asking for your license and registration, for example), it was standard procedure across the police force at one time. This practice isn’t rooted in superstition, and it isn’t a policing secret. As it turns out, it has to do with the officer’s well-being.
We spoke to two state police personnel and a criminal defense attorney to find out the reason behind this behavior—and whether you should be concerned if it happens to you. Read on to learn exactly why cops touch taillights during traffic stops.
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What’s the reason police tap taillights during traffic stops?
There are two primary reasons why cops touch taillights, according to Sgt. Logan T. Brouse, the director of the Pennsylvania State Police communications office. And both boil down to safety.
To leave a fingerprint
The first reason police tap taillights? “To leave the officer’s latent fingerprint on the vehicle as physical evidence of the encounter in case the vehicle flees,” Brouse says.
Criminal defense attorney Joe Hoelscher says that this routine maneuver serves as a sort of breadcrumb left to prove that the police officer had approached that particular vehicle. “Leaving a thumbprint on the brake light is an old-school way to tag a car with a fingerprint, so it can be identified conclusively as the vehicle involved in a stop should the officer become incapacitated,” explains Hoelscher.
While taillight taps aren’t as ubiquitous today (more on that below), they can still be a simple and worthwhile procedure for law enforcement. Depending on how the interaction unfolds, even a routine speeding ticket can unexpectedly escalate into something far more serious. In such cases, proving that a certain police officer was at the scene is important. And a taillight tap is a way to do exactly that.
To check the trunk
Brouse says there’s one more reason why officers touch car taillights, and it too relates to safety protocol. Offers tap a car’s taillight “to ensure the trunk was securely closed, reducing the risk of someone hiding inside and posing a threat,” Brouse says.
This move is usually reserved for situations in which two officers are present since it can be risky for a single officer to do alone. But when cops sense danger, one officer may check if the trunk is securely latched, just in case someone’s hiding inside, waiting to ambush.
Why was this safety step established?
Law-enforcement officers pull over more than 50,000 drivers on a typical day and more than 20 million motorists every year, according to the Stanford Open Policing Project. So following protocol is vital for public and officer safety.
But just how important is it in today’s world, where tech can track everything? Let’s find out.
Is the taillight tap still necessary?
Leaving fingerprints on a car is still very common, but it’s not a procedure every police officer learns. With the proliferation of CCTV on street corners, dashcams on cars and surveillance cameras worn by law enforcement, cops can establish their presence near a vehicle without tapping the taillight during a traffic stop.
“Although modern technology—such as body cameras and dashcams—has largely diminished the necessity of this tactic, some officers continue to do it out of habit or as an added measure of caution,” Brouse tells us.
Elena Russo, a spokesperson for the Maryland State Police, believes in the need for the taillight tap even in today’s age of modern technology. “Leaving an identifying mark on a vehicle while conducting a traffic stop provides our troopers an extra layer of safety, evidence and accountability,” she says. “This common practice is taught to candidates at the Maryland State Police Academy during their six months of training to become a Maryland State Trooper.”
Is there a legal reason to touch a car’s taillight?
While a fingerprint provides a layer of security by proving a cop was next to a car, it might not hold up as evidence in court. According to a report by the American Association for the Advancement of Science, examiners “no longer claim the ‘100% accuracy’ of a fingerprint analysis.” There’s a growing understanding of error rates in fingerprinting procedures, questioning the reliability of fingerprint analysis itself.
But that doesn’t mean the process of touching a car’s taillight at a traffic stop is pointless. “It is less important now that video is ubiquitous,” Hoelscher says, “but it requires so little effort that some officers are still trained to do it.”
What other reason might an officer tap a taillight?
Officers don’t just tap a car’s taillight for safety reasons; they may do it to gain the upper hand during a traffic stop. That small knock can startle the car’s occupants.
This instantly raises the pressure, setting the tone before the officer asks a single question. According to The Law Dictionary, this surprise tactic has worked well when it comes to arresting “intoxicated drivers” and “sellers of prohibited substances.” When startled, suspects may scramble to hide illegal items (often carelessly). And that’s exactly the kind of slipup an officer is trained to spot.
While it can be effective, tapping the taillight isn’t without risk. Startling the occupants might just backfire, causing panic—or worse, provoking an attack. A cop needs to weigh all safety concerns before taking action.
What should you do if a cop touches your taillight?
As the experts have explained, there’s more than one reason why cops touch taillights. And if you’re reading this from the comfort of home, chances are, you’re not in any danger. Still, a cop touching your taillight can carry consequences down the line.
No matter the reason, those fingerprints could have a negative effect on future law-enforcement interactions, says Hoelscher. When a car’s taillights are dusty, the officer’s fingerprint can stand out, he says. If this is the case for your car, “the next officer might be more aggressive, realizing you’ve had another law-enforcement interaction recently,” he says. “So my advice is to wash your car after getting a ticket or the next time you get stopped.”
At the end of the day, a taillight tap is just one piece of a much bigger picture. If you find yourself pulled over with a cop who just touched your taillights, a little calm, cooperation (and maybe a post-ticket car wash) can go a long way.
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Sources:
- Sgt. Logan T. Brouse, director of the communications office for the Pennsylvania State Police; email interview, Aug. 19, 2025
- Elena Russo, spokesperson for the Maryland State Police; email interview, Aug. 18, 2025
- Joe Hoelscher, trial lawyer based in San Antonio; interview, August 2019
- The Stanford Open Policing Project: “Findings”
- American Association for the Advancement of Science: “Fingerprint Source Identity Lacks Scientific Basis for Legal Certainty”
- The Law Dictionary: “Why Do Policemen Touch Your Tail Light When They Pull You Over?”
