Hi!
Do you want to drive on the bridge, or only see it from a distance? If you want to drive on it then make it as 3D as possible, otherwise I think you could make it more simple.
To drive on the blue bridge:
If we are looking for a true replica I think the main structure of the bridge would best be done in simple 3D, while the top and the white railings should be done with flat, transparent 2D panels.
I would make a texture that looks a bit like this:
<center><img src=http://mtm2.com/~trackville/pics/bluex.gif></center>
The idea being that you apply 3D shadows on the texture so the square beams look more like structural steel. Then you could make simple rectangular beams, map them to your texture, then assemble a bunch of them in BinEdit, joining the vertices. Hey, I hope you are using the latest version on BinEdit, it's amazing.
Somewhat like this:
<center><img src=http://mtm2.com/~trackville/pics/bluex.jpg></center>
The steel beams in that example are not placed correctly, but I think you get the idea. A single upright beam is basically a classic "parallelogram" geometric shape. I think by using such shape and joining them together you might end up using less vertices for the whole bridge. Note how the upright pieces are narrower than the top and bottom beams, that way they can fit inside the bigger beams and still look good. I think this design is highly optimised while still providing a proper look.
Overall, the idea is that you make one or two beams, then just keep inserting them over and over, rotating and translating them as needed, which BinEdit does very nicely now (applying operations only to selected vertices). You can then move key vertices and "merge" them to optimise the model.
The texture you make might look something like this:
<center><img src=http://mtm2.com/~trackville/pics/bluex2.jpg></center>
The piece at the upper left is used on the beams. You would probably have a bigger one, in a couple of different shapes perhaps, with rivits and seams, perhaps. It's just an idea.
The cross piece at the bottom (my example is too big) could be used for flat transparent panels for the top of the bridge (black can be made transparent). These panels would be stretched across the top vertices of the upper beams.
Likewise the white railings could be used as flat, transparent panels. While the long white section could provide the texture for an upright 3D beam that the railings connect to.
I think I would make the bottom sections of the bridge seperate from the top, using yet another texture. Then insert them in sections, end to end, to provide a driving surface.
These are just some ideas that I hope will help you plan this out. I also hope someone else will come along and share their ideas and help. It was very easy to make my examples thanks to the new BinEdit, and Paint Shop Pro, but making a whole bridge would certainly take some time. You could even check out some scrapyards to find some existing models to edit. Good luck!
Bye for now.