i've managed to do this crossing, i deleted the control points from both sides of the bridge, top and bottom. Then the corridors dropped to right place. Once i made a very small "hole" in the bridge just drop that one control point down, duplicate texture, assigned it as not driveable, no BTB ground...Ugly but it worked. If you don't touch the AIW button in BTB after that, you can assign the "hole" back as "drivable". But if you touch that AIW button, BTB calculates everything back again. Not a solution, more a workaround.
ow that i read that, it's simple, make a material change in the road at the problem place, assign it as not driveable material, do the AIW and revert changes..
What i never solved was the small brake/turn that AI cars made at that spot, nothing showed on BTB but a straight line. But who cares about AI, it has no feelings..