Elevation
Elevation is one of the attributes of all Road types.
In most cases the elevation should be defined "as is" in the real world. Standard "Ground level" or 0 or in case of a cross-over / fly-over the upper road should be defnied with an elevation one higher. In case of a tunnel the elevation could be -1 or -2.
One exception: the elevation of railroads should be set at -5.