Here should be some usefull information:
You don't have to model the whole bridge, since I have done the Venice end of the bridge. The part you do is exactly 3 kilometers long and 44 meters wide.
This image shows the part to be modelled
The bridge consists of 4 lanes street and four railway tracks, the are seperated by a small wall (about 1m) on both sides there is also a wall likt that one.
In this image you can see the basic structure of the bridge - five archs then one big column and again five archs. I think we should simplify it like this, so actually you only have to make one section and repeat it untill you reached 3 km. (The measurements are in the image above)
Here you can see the bridge from the side - it is very low, but I'm sorry I have no measurements for the height.
I hope this is enough info for the moment
Joey