The problem is that there are quircks with how OFP handles bridges. This is kind of preliminary, but it's the explanation of what you need to properly build OFP compatible bridges.
From a posting by Midship:
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bridges are from class road but behave like class house, concerning 'land contact'. bridges cannot be lifted higher then 1/2 of their overall height extend, relative to their origin, but be burried to ground, until they become invisible. The overall model height for instance, one can get from the nice ODOL Explorer. From the highest LOD resolution, MinPos and MaxPos are giving this value. For the most_stred30 model, these are -20.756 m and 20.755 m, which as said, allows the bridge to be shifted upwards for not more then about 20.755 m, relative to the O2 origin.
Also, from the ODOL Explorer one gets an offset of 12.983 m for the Roadway/Memory LOD, on how much the road is above the surface (actually the roadway levitates). Therefore, the most_stred30 road strip, in total cannot be higher then about 33 m above the terrain. From this, one may get a rule, to calculate bridges in general. Y = Ty - Ly + Ry
Y: the height value to be put in the OPRW wrp matrix (perhaps Dschulle may see if the Y value from the object inspector is the same, then from the objects O2 origins. To be on the safe reference side, here I introduced the terrain height).
Ty: the terrain level, where the bridge pole (O2 XZ origin coordinate) is.
Ly: the offset from the Roadway/Memory LOD, away from the origin line.
Ry: the desired road strip level (constraint: Ry<Ly+My/2 (My: overall model height).
If one makes a road, then from the last segment, the bridge pole coordinate and the height from at least one open road corner Memory vertex are known (also the bridge parameters from the XML road list (ODOL Explorer). This allows to place and connect a bridge segment automatically to the previous road segment. If the first bridge segment is calculated and for the reason that bridges are straight, the following bridge segments will keep level automatically. As seen, if one appends another road segment at the end of a bridge, it's properly connected, but in dependence of the terrain slope, somewhat above or below the bridge end, the only work one has to do finally with the Elevation Tool and on behalf of the rebuild function.
What you probably need to do is make your bridge piers taller so they don't 'float'.
