Do the Police Need Probable Cause to Conduct a Traffic Stop?

Police officers need a legal basis to stop a vehicle—but when exactly do they need reasonable suspicion versus probable cause? This question often arises in DUI and other traffic-related cases in Pennsylvania.

The Pennsylvania Supreme Court addressed this issue in Commonwealth v. Chase, explaining the standards required for traffic stops depending on the circumstances.

When Can Police Stop Your Vehicle in Pennsylvania?

According to Commonwealth v. Chase, a traffic stop is permitted under two different legal standards:

✅ Reasonable Suspicion (Lower Standard)

An officer can stop a vehicle based on reasonable suspicion if the stop serves an investigative purpose. That means the officer believes stopping the vehicle may help determine whether a violation or criminal activity has occurred.

✅ Probable Cause (Higher Standard)

If there is no need for further investigation—meaning the violation is clear and undisputed—the officer must have probable cause to justify the stop.

What is an Investigatable Offense?

Certain traffic violations are non-investigatable because the facts are immediately clear. In these cases, police need probable cause to conduct a stop. Examples include:

In contrast, DUI-related stops often fall under the category of investigatable offenses. For instance, if an officer observes:

These observations give the officer reasonable suspicion that the driver may be impaired. A traffic stop can then be initiated to investigate whether the driver is under the influence or if there’s an innocent explanation, such as adjusting the radio or avoiding a road hazard.

Why Does the Legal Standard Matter?

If an officer stops a vehicle without the required level of suspicion—whether that’s reasonable suspicion or probable cause—the stop is considered illegal and violates the driver’s constitutional protections against unreasonable searches and seizures.

For example, in a recent Centre County DUI case, a State College police officer conducted a traffic stop without sufficient probable cause. The defense attorney successfully filed a motion to suppress the evidence, resulting in all evidence from the illegal stop being excluded from the case.

Facing DUI Charges? Your Rights May Have Been Violated

Anyone charged with DUI in Pennsylvania should have their case reviewed by an experienced defense attorney. Challenging the legality of the traffic stop is often a critical part of an effective DUI defense strategy. If the stop was illegal, any evidence collected—such as breath tests, field sobriety results, or officer observations—may be suppressed in court.

Speak to an Experienced State College DUI Lawyer

The attorneys at JD Law have successfully defended thousands of DUI cases by challenging unlawful traffic stops and protecting clients’ constitutional rights.

For a free case review, contact JD Law by email or leave a message at (814) 689-9139. Email is the preferred method of communication, as phone calls are directed to voicemail due to the high volume of spam and unsolicited calls.