Elevation is a less important attribute of Road types: it had no affect on routing.
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.
See also the global Wiki page on Overpasses, Underpasses and Bridges.
Don't split segment to change the elevation for bridges. It is better to give the whole segment the same elevation.
On a grade separated Crossing of segments, both segments must have a different Elevation.