MotorStorm: Apocalypse is not post-apocalyptic, no sir. There's still a semblance of order as you start the race, but the apocalypse kicks in while you're busy trying to get pole position. This results in more obstacles, unpredictable racing conditions, and your foot flooring the gas.
The game is set in a fictional city oh-so-creatively called, The City, which was modeled after the California Bay Area. Evolution Studios also notes that this game will be more story driven that its predecessors, but they haven't detailed as to how that unfolds.
Gameplay-wise, you can still ride anything from bikes to monster trucks, but this time, you're going to have five more vehicles in between. In addition to the old MotorStorm lineup, you also get supercars, superbikes, muscle cars, choppers, and the hot hatch.