I was confused that I could not find the details. Then remembered that spoilers do not show at all with JavaScript disabled. And now I have actual non-joking doubts on the roads.
How rough is the surface? All inclines are going to be a source of carriage pileups. At the very least, on-ramps. Take a corner too fast on a rainy day, off you go. The surface has good traction? Either have an army of dung cleaners, or lose it. Do not expect sheep herders to clean up after themselves. Forbid animals on the road? Now you gotta police it, with increased personell costs.
You're basically making a giant slab of rock without gaps. Thermal dilation is going to be an issue, cracking, and ice will slowly eat away at the surface, creating ever-growing pockmarks on the surface (which will become perenially filled with stagnant water and animal excretions) and allowing cracks to grow on tensioned parts of the structure.
Make it in gaps, and unless you're touching bedrock, each section will slowly sink on the terrain, tipping every which way, at a median rate of 1cm per year on clay soil. Soil humidity is a factor too: if it rains, even "settled" sections will rise cms in days. Unevenly, of course.
Aside from material and construction costs, this is why elevated roads are so expensive: maintenance. And when maintenance is dependant on your magical low-birthrate subspecies, you're just making another future crisis.
On tolling: it pushes the poor people even further down. As normal intervillage roads fall in disrepair because the fancy new ones are so much better for commerce, the ability for poor people to relocate falls further. The tolls also increase the needed startup costs for commerce ventures, increasing monopolization of the market.
I would propose two alternatives: normal, ground level road development that is civilian-maintainable, or a high capacity standardized vehicle that uses the costly elevated infrastructure at maximum capacity, run by the same entity that taxes the commerce, with an x% capacity reserved for passenger use, where the passenger only pays for luggage, if carrying any.
I was joking earlier with the monorail, but a railway system is the most bang for your buck in transport infrastructure, after ports.