I'd be interested in calculating just a subset of roads and comparing to the overall average. Dual carriageways are the most likely to adapt the terrain to the road rather than building the road to the terrain. If I recall correctly, Germany has the strictest requirements for the straightness, and the States has a rule that every X miles must have a section that can be converted to an impromptu landing strip. The website is completely crushed right now, but once it's back I may play with the data.
It's not as ridiculous an idea as Snopes makes it sound - there are plenty of places which do put those large expanses of tarmac to dual use. I remember seeing a stretch of highway in Switzerland which was also a military airstrip (flat stretches of ground being at something of a premium in the Alps). There are plenty of real examples: https://en.wikipedia.org/wiki/Highway_strip
