Hmmmm
Pilot: HT, we need bridges.
HT: Ok,..we can do that visual, no problem.  Only a few polys needs really.
Pilot: HT, we want to be able to blow them up.
HT:  Ok,...more code for the collision system,..we need object status too, and we will need some flags to control what has been blown up.
Pilot:  HT, what good are bridges unless we can drive across them?
HT:  Dang,..well,..uhmmm...ok,..special terrain object.  
Pilot:  And HT,..we need roads over the bridge too.
HT:  Ok,..need to modify the road system so it knows it can go across a bridge object.
Pilot:  HT,..why can't we fly under the bridge?  Every time I try I blow up.
HT:  
 Modify the bridges so we do not have any polys underneath them.  Yes, I know we have to redesign them for that, but thats what they want.  Yes, I know I have to redo the collision system again for it.
2 weeks later:
Pilot:  HT, why dont we have that new plane we asked for 3 weeks ago?
HT:    I need a drink.