Which action may result in a traffic violation?

Using a cell phone while driving without a hands-free device can indeed result in a traffic violation. Many jurisdictions have laws that prohibit the use of handheld devices while operating a vehicle due to the distraction it causes, which increases the risk of accidents. This action not only diverts the driver's attention away from the road but also impairs their ability to react swiftly to changing traffic conditions, leading to potential dangers for both the driver and others on the road.

On the other hand, the other options describe actions that are generally compliant with traffic laws. Driving with headlights on at night is a required practice for visibility and safety. Maintaining a reasonable speed in good conditions aligns with safe driving practices and obeying speed limits, while using your turn signal when changing lanes is a legal requirement aimed at ensuring clear communication among drivers to prevent accidents. Thus, option B stands out as the only action that could lead to a traffic violation under most driving regulations.