What must a driver do before changing lanes?

Before changing lanes, it is essential for a driver to check mirrors and blind spots to ensure that the lane is clear and safe for the maneuver. This practice helps to confirm that other vehicles or obstacles are not present that could lead to a collision when merging into the adjacent lane.

Blind spots are areas not visible in the mirrors, and ignoring them can increase the risk of accidents with vehicles that may be approaching quickly or are already in the lane you intend to enter. Checking mirrors ensures that the driver is aware of the current traffic conditions and the speed of the vehicles nearby. This step is critical for safe driving and is a fundamental part of defensive driving techniques, which emphasize awareness of one's surroundings.

Choosing to speed up to match traffic, signaling only if necessary, or relying on a horn would not provide the same level of safety or situational awareness required when changing lanes. These options do not encompass the comprehensive visual checks that are vital to prevent accidents.