Ok, tutorial on how to make an overlay/decal:
http://www.interlopers.net/tutorials/5296 ... Seriously, use that for your road markings, it'll look much nicer and take way less resources. Also, knowing how to make your own decals is a very useful skill. The textures you have now aren't the best, either. If you really want some sexiness, you should consider making custom concrete textures.
The detailing probably will have to be done with models/textures if you want to be efficient. Look up a program called "Propper" for turning things from Hammer into models... that way you don't need to sink vertices into things like a billion metal bars, as seen in the second pic of your map.
13 levels is probably too many anyway, since it's so huge already. And optimization... oh man. It's a dozen levels and it's all open, hope it runs ok. That said, it looks massive and very cool, makes me want to race in it.