Oh, boy, a L4d map! I've been waiting to see one-a these.

Sever's chapter thing is an important thing to think about, if you're going to do it that way. Either way, one thing that somewhat bugged me is that there were no objectives other than the obvious: gtfo of zombieland. Between the start and the very end, it seemed like you were aimlessly running from place to place, with no significant goal for a single chapter. A good way to get ideas for chapters/stuff in the middle is to think of the obvious objective, then think of things they'd have to do before being able to get to that finale. This, I think, would have a great potential for a crescendo if one of the objectives ended up attracting a lot of zombies, or something like that. It also gives players something new to have in L4D other than just simply "run!"

As for the reason that the zombies are in mechanical places... well, perhaps in a previous objective (see above if you TL;DR'd it), something was done to gain the zombies' attention, taking them to your location.