Exit the safe room and climb up the spiral staircase to reach the surface, then climb the bridge. To the left will be some ammunition, but you need to turn right in order to cross the large river.
Follow the left-hand side of the bridge until the concrete barricades. After passing the concrete, climb over the fuel truck and other vehicles; you should see a burning truck nearby. It will explode when approached, attracting the horde. To cross the hole that's left behind, walk along the two iron diagonal beams.
After the destroyed section, you can't go farther with the path blocked by a large bus. Walk onto the side platform on the right (following the arrows), and walk to the lowest platform to collect grenades and ammo. The lowest platform is attached to a pole that zombies will climb up, which will need to be killed in order for the team to climb down. Once they stop pressing the attack, you can climb down the mast.
You can now reach the other shoreline, where you can change weapons if required. When you apporach the junk yard, a bomber will destroy a section of the bridge, setting off a car alarm and attracting the horde. However, the safe room is not too far, being at the end of the yard on the right.