Which of the following is required before changing lanes?

Before changing lanes, it is essential to check mirrors and blind spots. This practice ensures that you are fully aware of your surroundings and any other vehicles or obstacles that may be in the adjacent lane. Checking mirrors gives you a general view of what's behind and to the sides of your vehicle, while also being vigilant in monitoring your blind spots—areas that are not visible through your mirrors.

Using just your turn signal does not provide you with the information needed to gauge safety when changing lanes. While signaling is a crucial aspect of lane changes to communicate your intentions to other road users, it doesn't replace the necessity of visual checks. Looking back over your shoulder alone may not provide adequate peripheral awareness and can also distract from other important views, such as checking mirrors. Relying solely on the rearview mirror is insufficient as it may not show vehicles that are in your blind spots. Therefore, a combination of checking mirrors along with blind spot awareness is critical for safe lane changes.