If you look at the screenshot, you'll see my car which dramatically killed itself after crossing over the edge of the grid, which is the wall in the distance. You don't fall over the edge, you just bug out and stuff.
But mine had quite a travel behind it returning after his space trip.. i guess it just clipped the ground
No map in the game is infinite, they all have a set size that the creator decided to use. At the edges of every map there is a wall of sorts that will destroy your car.
This is just how the engine works, there is no way to create a map that does not have a set size. You can create a map that is larger then the grid map, but at a point you start running into other issues.
Give this thread a read: http://www.beamng.com/threads/5489-Idea-for-airplane-maps?highlight=airplane