Perhaps you can replace some of the girders I marked in red with 2D textures using transparent faces, and leave the vertical columns that sit close to the road as 3D models. That way, it will still look good from afar and close up without requiring so many vertices
(you most likely won't notice the area I marked in red when you are crossing the bridge anyway, but you do see the vertical beams, so that portion of the bridge I marked can be done in 2D with little loss of quality)
I had to make a similar compromise when I added sparkplug cables to one of my cars....I noticed that the cables were only visible from certain angles,so I was able to make them successfully using flat 2D transparent textures, while leaving the rest of the engine model in 3D form. Making the cables as 3D models would have added hundreds of polys/vertices to the total.
I also used flat surfaces and transparent textures to make an ultra-low vertex frame...the same frame done with 3D "tubes" was more than double the vertex count of the 2D model (136 vert. versus 388 for the 3D frame)
