sry i didn't reply erlier. I've bin kind of busy. I admit that complicated models have bad collision detection. It will make the game slow to! I found a program on inet that can export GTA (3 and Vice) buildings\models to 3D studio MX models. Which can be exported to gamestudio models. With a little editing, you will have prefabed buildings, bridges and alike (just give them a new skin and edit a bit)

Note: The models in GTA aren't that complicated, and if they are, it are multiple models (like the broken bridge in GTA 3).