Been placing bridges in Visitor today, had problems at first, but here is what I come up with. First of all Ignore what they look like in Buldozer(they look fine in the game) and don't position them with Buldozer either. Use Ctrl-R and Ctrl-P to rotate and position them in Visitor.
This diagram and the two formulas should give you an idea how I went about things. (diagram might be scrambled, but you should get the basic idea)
Top(19)__ _______
\ /
\__Bottom(-10.035)_______/
Formulas
End sections
Bridge Height - ((Top + Bottom)/2) + 0.75 = ERS
example :33 - ((19 + 10.035)/2) + 0.75 + -19.2325
Middle sections
Bridge Height - (Top + Bottom) + 0.75 = ERS
example: 33 - (19 + 10.035) + 0.75 + -4.715
ERS = Elevation Relative to Surface(in the object position popup, Ctrl-P)
Notice I've removed the minus(-) from 10.035 and added it to the answer, it's just to make thing easier, I'm no mathematician.
My terrain surface, and the road surface on the bridge match up perfectly. I guess you could use your own values. Hope it helps