Here's some in-game screen shots:
No.3 Bridge without vehicles
No.3 Bridge with vehicles
No.4 Bridge without vehicles
No.4 Bridge with vehicles
I don't know
The bridge with trusses looks better without any units on it, and the bridge without trusses looks better with units on it.
Personally, I don't like to stop vehicles on a bridge if I can help it, because they're too vulnerable. But in some battles this is unavoidable.
Cross