Here are the introductory paragraphs I've written for the 'user guide'  of our soon-to-be 'construction kit' for Cornucopia3d. I hoping that these paragraphs might help to explain the technical nature of the problem in simple, easily understandable terms.

"In the ideal world, to make a photo-realistic scene or animation with rural roads, it should be possible to first construct a terrain that one likes, and then lay out a road or set of roads along the contours of that terrain. A person then should be able to apply a high quality road bed and road surface to the contours of the road and add objects such as bridges, culverts, elevated roadways or other objects where needed, while having the road surface flow easily over those objects.

Unfortunately, it is not yet possible to build a convincing, photo-realistic road across a Vue terrain in this manner. The ability to do so in this seemingly intuitive manner presents a number of formidable challenges. Without going into the technical details, several kinds of additional technologies would be required that don’t yet exist in Vue. First, there would have to be a separate computer program capable of analyzing any given Vue terrain and determining a more or less level path across the contours of the terrain, in the desired direction that neither rises nor falls too sharply. These kinds of programs (“network analyses”) exist, but even when they are used, natural terrains still present some mounds and some dips that require “cut banks” (incisions into the mounds) and “fills” (areas of built of rock and soil) to create a road bed that is relatively level. So, a second technology would need to be available for the Vue program to automatically construct cut banks and fills along the roadway laid out by the network analysis program.

Finally, there is the matter of a realistic roadbed and road pavement. This might be made by using a Vue material, but a photo-realistic road surface is slightly convex, and a gravel or coarse asphalt roadbed typically lies under the pavement, and extends beyond it on both sides. The road surface must be continuous and must be capable of extending across bridges and culverts or other structures, implying that it must be an “object”, rather than a simple material.

It is possible to construct these features into the Vue program in the future, but for the present, the needed supporting functions do not exist. Therefore, we are taking a different approach to the matter of “rural roads” for the present.

To make it possible to easily build a scene or an animation-capable scene with rural roads, we take the approach of first building a road segment or system using a set of building-block objects, and then building a terrain around the “road” using one or more discrete Vue terrains."

Without going further into the details of our project (unnecessary for your project), essentially, we are building a reasonably extensive set of objects that can be assembled in different ways to form a long segment of rural roads that could climb up hills, go across valleys and across streams and around waterbodies. The building blocks of the system consist of various kinds of rural road elements, each of which has some portions of a “terrain mesh” attached to it. Normal Vue terrains in the Vue program can be merged into the road segment on either side of it so that when all the terrain pieces are given the same materials, the final assembly gives the appearance of a rural road laid naturally into a normal landscape.

I think this is the approach you might find that you need to adopt to accomplish your stated goal. That is, build  your road scene with road segments that you construct somehow, merge and butt-up chunks of your chosen type of terrain into those road segments, and then build out the terrain surrounding the road.  Basically, this is the only approach we've found that will lead to photo-realism, and also to a decent animation possibility.