Crossing the broken bridge with vehicles on "Tsavo Highway"
When you reach a bridge with a Warthog with barriers, usually you cannot cross this area (at least not easily). However, back in one of the buildings (before the bridge) is a Grav Lift. Grab it and throw it a bit before the break in the bridge, then run the Warthog or any other vehicle over it. This will make the next area much easier, especially under the Legendary difficulty setting.
When you reach the bridge where you have to leave your vehicle, slow down and look at the barriers. They can be flipped over to create ramps. Hit a loose barrier so that the top of it falls over away from the immovable barriers and position it as close to the immovable barriers as possible. Then, get a good running start before you hit it. It is easiest to push it from the end of it, then turning it to get it into position. You can also use a Warthog to push them. The Warthogs usually flip in the air, but they usually make it across the gap.
When you reach the point on the bridge where you cannot pass with a vehicle, notice a steel girder that you can walk across, but cannot drive your Warthog over. To get across, get into a Chopper and drive through the hole in the cement fence. Then, drive the Chopper across the steel girder and you can use it to kill the Brutes.