What action must you take when you enter a highway?

When entering a highway, it is essential to yield to traffic already on the highway. This means you should be aware of the vehicles that are currently traveling at higher speeds and make sure they have the right of way as you merge onto the road. The primary goal is to ensure a safe entry without disrupting the flow of traffic that is already established on the highway.

Yielding allows you to gauge the speed and distance of oncoming vehicles, which is crucial for entering safely. It gives you the opportunity to merge smoothly into the traffic stream without causing collisions or abrupt changes in speed for other drivers.

In terms of other actions, stopping completely before entering the highway is generally not necessary or safe, as it could create confusion and increase the risk of rear-end collisions. While accelerating to match the speed of traffic is important, it's not the only action you should take; yielding is a critical step prior to merging. Lastly, changing lanes immediately without checking can be extremely dangerous, as it disregards the need to assess your surroundings and could lead to accidents.