What must be done before changing lanes?

Before changing lanes, it is essential to signal your intent, check your mirrors, and check your blind spots to ensure it is safe to do so. Signaling alerts other drivers of your intention to change lanes, promoting safety and reducing the likelihood of accidents.

Checking your mirrors provides a view of the traffic around you, allowing you to gauge the position and speed of vehicles that may be approaching from behind or alongside. Additionally, checking your blind spots—areas that are not visible in your mirrors—is crucial because vehicles can occupy those spaces without being seen. This comprehensive approach to lane changing not only enhances your own safety but also the safety of other road users.

The other options lack important aspects of safe lane changing. Relying solely on the rear-view mirror or just looking over your shoulder does not give a complete view of the surrounding traffic, while signaling only after changing lanes does not inform other drivers about your intentions ahead of time. Hence, the combination of signaling, checking mirrors, and checking blind spots is the correct and safest method before making a lane change.