Hello:
Try setting the FS Menu GUI Terrain Mesh Complexity slider to 100 %, and Terrain Mesh Resolution slider to 1 Meter.
Then, in a FS flight at the area in question, if the terrain mesh data for the roads does not display a flat and level road surface, either you have a low resolution terrain mesh data set that is being aliased to nearby elevation data point elevations, and/or you may need to use a higher resolution terrain mesh source code elevation data set (if available).
You could try making the roads using BGLComp-compiled 'airport' Taxiway / Apron paths as hardened terrain flatten objects ( that also eliminate ground roll dust effects behind ground vehicle wheels

) below the draped custom photo-real aerial imagery.
Alternatively, you may wish to create a flat and level road surface as SHP2VEC-compiled CVX vector poly-line or polygon objects at assigned elevations which will align with and over-ride the underlying terrain mesh data for the roads.
Yet another alternative work-flow is, to 3D model the vector poly-line or polygon objects for the roads, then convert / export them into a BGL containing CVX vector "sloped flatten" objects that will align with and over-ride the underlying terrain mesh data for the roads using Arno's feature for that in ModelConverterX (aka "MCX").
If you have further questions on any of these ideas / options, feel free to inquire further.
BTW: What is the Geographic coordinates of the project location, and would you post a screenshot of the area in question ?
GaryGB