In which scenario is it mandatory to use turn signals?

Using turn signals is a critical aspect of safe driving practices and is mandated by traffic laws in many regions. The requirement to signal when entering or leaving a lane serves several important purposes. It communicates to other motorists your intentions, thereby enhancing safety and reducing the likelihood of accidents. When a driver signals before changing lanes, it allows other drivers and pedestrians to anticipate the movement, facilitating smoother traffic flow.

In the context of turn signal usage, entering or leaving a lane specifically refers to instances where a driver changes from one lane to another or merges into traffic, and signaling is essential in these situations to alert others of your planned maneuver. This not only adheres to traffic regulations but is also a crucial aspect of defensive driving strategies, aiming to prevent collisions and promote overall road safety.

Other scenarios, such as those mentioned in the other options, do not universally require the use of turn signals under all traffic laws. For instance, signaling during heavy traffic is not explicitly mandated in all situations since it may not improve communication in congested conditions as clearly as when changing lanes. Similarly, the requirement to signal when reversing is less clear-cut and may depend on local regulations or specific circumstances. Thus, the most definitive answer regarding mandatory signal usage is when entering or leaving a lane.