What must a rider do before changing lanes?

The requirement for a rider to signal their intentions, check mirrors, and look over their shoulder before changing lanes is crucial for ensuring safety on the road. This process involves signaling to inform other road users of the rider’s intention to change lanes, which helps prevent misunderstandings and potential accidents.

Checking mirrors is important for gaining awareness of neighboring vehicles and their positions, while looking over the shoulder ensures that the rider checks the blind spot, an area that mirrors may not cover adequately. This combination of signaling and checking creates a comprehensive approach to lane changing, significantly enhancing the rider's ability to perform the maneuver safely and responsibly.

The other choices do not promote safe riding practices. Simply checking the blind spot doesn't encompass the important aspect of signaling, which is vital for communication with other road users. Only signaling when other vehicles are present might lead to lapses in communication when the road is less crowded, and changing lanes without signaling disregards the requirement for informing others, potentially leading to dangerous situations.