With my Ac3D approach i make a mesh that's rectangular and big enough for all polys. The AC3D trick is to call it something__airport_outside (hmm i think, i'm away from my pc) and then it gets a one level of smoothing. Cut out of that mesh using boolean knife and cut away are the runways with a stronger level of smoothing as these have __airport_runway (again, i think)
Thanks, I remember seeing a screenshot of doing things that way. Seems like a good technique without the complexity of having a single mesh, quadifying it, then vertex painting etc, like in the Max tutorial.