Before making a lane change to your left, you must signal for at least three seconds. What else must you do?

Before changing lanes, you must not only signal your intention to turn at least three seconds in advance but also check your vehicle's blind spots. This rule is crucial because blind spots are areas that cannot be directly seen through the rearview mirrors while driving, and other vehicles may be present in those areas. Changing lanes without checking these blind spots can lead to collisions with those vehicles, resulting in traffic accidents.