What is the primary offense for which law enforcement can stop a vehicle in Pennsylvania?

The primary offense that law enforcement can use as a basis for stopping a vehicle in Pennsylvania is driving without a valid driver's license. This offense is clear-cut and directly relates to a driver's legal capacity to operate a vehicle. If a driver cannot present a valid driver's license during a traffic stop, law enforcement has probable cause to initiate the stop.

While the other options could potentially lead to a traffic stop, they are not as universally recognized as a primary reason for stopping a vehicle. For instance, speaking on a cellphone while driving is subject to specific regulations, but it may not be immediately visible or as easily enforceable as driving without a license. Similarly, driving too slowly is not necessarily an offense unless it creates a safety hazard or obstructs traffic, and not using headlights in the rain may lead to a stop, but it is dependent on visibility conditions and the specific circumstances on the road. Ultimately, driving without a valid license is a more straightforward violation that justifies law enforcement action.