Chapter 3: Freeman Pontifex 7
Zombies will start crawling out of the sewage as you reach new islands. Focus first on the aggressive, dark-dressed zombies as Alyx will take out most of the slower-moving guys. Eventually you'll reach a landing with a ladder that leads to the top of a hill. Watch for a few headcrabs in the building to your right (and grab the items inside) before going left toward the bridge.
As you step onto the bridge, it starts to collapse. Face down the length of the bridge and look for a small catwalk that runs along the left side of the main structure of the bridge. Climb the short ladder at the end of the catwalk to get on top of the bridge's railing. From here you can cross the railing to reach the other half of the falling bridge.
Note the myriad of ruined cars on the second half of the bridge. Use the Gravity Gun to move at least three of these cars to the end of the bridge that's currently facing up. The weight of the cars will push that end down, forming a ramp of sorts you can use to get your target car back across the bridge. Backtrack to the hillside and onto the first half of the bridge where you'll find a car you can drive (it's the dope-looking one). Get a running start and use your turbo to jump onto the second half of the bridge. Continue up the ramp you've created and you'll reach the landing at the other side of the bridge.
Hop out of the car and go left. There's a short ladder nearby that'll lead you to a room with a switch that opens a door for Alyx and the Vort. Once reunited with the two, sit through a brief scene and then hop into the car. The Vort powers a generator that opens up a massive door you can drive through. Head into the dark tunnel to complete the chapter.