I learned that there were several "S" bridges on the original route and they are unique in the USA. There is an explanation below of exactly what "S" bridges are and why they were built.
While listening to my tour guide at the National Rd. Museum I also learned that the eastern part of the National Road in Ohio(Zanesville to WV line) was made of brick because because of all the turns and hilss made it a better method of creating a good surface , while from Zanesville west concrete was primarily used.
It is interesting to see how the bricklayers did their thing on curves and "S" bridges. Sometimes there were several of these lines in a pretty short length of road.
There are still some sections of the original brick National Road that have not been paved over to see and actually drive on (slowly). These sections are not part of Rt. 40 but are very close and marked.
This part of Ohio is knows as the home of the "alphabet" bridges given the "Y" bridge in Zanesville and all the "S"bridges.