This page of The Last of Us game guide contains a walkthrough of the Highway Exit stage as part of the Bus Depot chapter. This is a calmer level, during which the player explores the title highway and visits, among others, a bus station. The heroes will meet with giraffes. Do not forget to explore and look for collectibles.
When you start, keep going ahead while searching the car wrecks thoroughly.
Eventually, you will reach a closed gate. You can walk over it by climbing onto the bus.
A bit ahead, you should notice the bus depot.
Walk inside and run down the stairs. Then, look at the ladder to the right.
As you look at it, you need to press the triangle three times before Ellie climbs up and pushes it down.
Lean the ladder against the wall and follow the girl.
This way, you will reach a feeding giraffe. After you stroke the animal (the triangle), go ahead.
When Ellie stops, talk to her after you stand to her left.
After a short cutscene, walk through the door to the left and go ahead until you reach a depot platform.
You will find numerous tents there, which you should search through.
Once you are done with the tents, go to the right edge of this location and walk through the destroyed bus.
At the other side, you will find an entrance to an underground tunnel.