The Residential area is the 4th area you'll encounter, and it has a Halloween theme. Some buses will be driving toward you, some will be jutting out from side streets and not moving. Despite the achievement saying you must be run over, the stationary buses still count for this. You might actually need to grind this one just a bit, because while you'll definitely crash 15 times before completing the Residential area, the buses aren't likely to be the main obstacle you struggle with. No sense wasting effort though, so the best approach is to work your way through the area normally, and if you still haven't hit 15 by the time you've unlocked the 5th area, just load up any level and repeatedly find a bus to run into.