An Overpass Encampment or The Overpass Gambit is a type of base that exploits the height, durability, and tactical inaccessibility of highway or freeway overpasses.
It is a common enough occurrence to see motorcyclist sheltering under overpasses during unexpected rainstorms. The reasons for sheltering under a bridge is pretty obvious, it is the most convenient place to get out of the rain. But one of the best places to take shelter from zombies is above an overpass. Even with no modification, an overpass distinctly limits the directions that zombies may approach from, but with some work and dynamite or cement saws, a team of survivors can easily make an overpass into a place of safety. Most bridges in the United States are held by the pillars alone. Because no structural support comes from the decking, parts can be removed without compromising the integrity of the structure at large.
A prime example of an Overpass base is Arefu, a town in Fallout 3.
Any bridge can be used, but obviously longer/larger bridges will hold more people. The jersey barriers on
the sides will offer protection from gunfire and from falling off.
Abandoned Vehicles left on an overpass can be used for improvised walls to control anything that moves across the overpass.
The decking will support the weight of one or more recreational vehicles or mobile homes. Before one destroys both sides of the decking, park on the opposite end in hopes of protecting it from shrapnel. Use shaped charges on the decking, or heat the concrete with a fire, then pour cold water on the super heated section to break the concrete into chunks. Proceed to smash the weakened concrete with sledgehammers or similar objects.
There are downsides to using an overpass as a base. During the mass evacuation, most overpasses will be filled with vehicles as people try to escape. This can lead to panicked people en masse, or lots of abandoned cars that can restrict your safety. There is also the chance that the overpass is destroyed in hopes to slow the infected, so many may be bombed if servicing an evacuation route. There are millions of the bridges in America though, so if one destroys one in the process of reconditioning it or if it is unusable due to parked traffic, it may be best to simply find another one.
Overpasses are often very exposed to the elements of wind, rain and snow. The elements can also degrade the overpass over time, especially if you have already begun the process by making a base out of it, so without maintenance, it may collapse.
Zombies will likely swarm underneath, meaning that you may have found a place to hold out, but you have no way of getting supplies or even escaping as hundreds or thousands of zombies swarm underneath.
There is also another disadvantage to the overpass that turns its advantage into a double-edged sword, particularly if walls are constructed: if a zombie were to break through one side of the defenses, then it would be very difficult to stem the tide of undead that pour through that hole in the defense, and you'd be cut off from escape if that were to happen. Thus, you have to keep an extra careful eye on both sides to make sure that zombies have not breached the defenses, and to keep said defenses well-maintained.