That's pretty cool. Personally, I'd prefer small roads and outdoor parking lots to be au naturel unpaved. Zero construction cost, and I imagine the maintenance cost is just trimming grass.

Only works if the natural soil is remarkably solid or the traffic negligible. Unpaved roads with more than very modest traffic generally need routine grading (repairing corrugations by scraping it over with a levelling blade) and re-graveling

Accurate. And, if there is significant rain and/or snowmelt, roads can quickly become impassable (see: "mud season" in New England, and surely many other places)

Won't they become unusable when it rains?