There's nothing wrong with you doing the tiles within the one surface. Just assign different texture co-ords. For instance split the texture up into different things, such as a road part, grass... etc etc, and assign the tiles to different parts of the same texture.
There is a balancing act in this though. Try and keep the surface count low-ish. The more surfaces (Meshes basically), the slower things will run. If you create your tiles out of the one surface, it will get rendered quicker.
I would suggest processing the cars within a certain range. Probably wouldn't be a great idea keeping tracking moving, checking collisions... etc, on everything in the city. The way GTA does it, is it keeps track of cars and people within a certain area. If they go out this area, they disappear.
Pretty ambitious project :o)
There is a balancing act in this though. Try and keep the surface count low-ish. The more surfaces (Meshes basically), the slower things will run. If you create your tiles out of the one surface, it will get rendered quicker.
I would suggest processing the cars within a certain range. Probably wouldn't be a great idea keeping tracking moving, checking collisions... etc, on everything in the city. The way GTA does it, is it keeps track of cars and people within a certain area. If they go out this area, they disappear.
Pretty ambitious project :o)