To correct the statement below, there are in fact more than two S bridges remaining in the United States.
For example,the "S" Bridge in Washington County, Pennsylvania http://www.historicbridges.org/pennsylvania/sbridge/index.htm is also a stone S Bridge on the National Road.
There is another in Ohio: http://en.wikipedia.org/wiki/%22S%22_Bridge_II_%28Muskingum_County,_Ohio%29
There may be one or two more on the National Road.
Stone arch bridges were built with the S shape to cross rivers not perpendicular to the roadway because the engineering knowledge of a bridge skew was not available at the time.
There are only two "S" Bridges in the USA. The Tyrone "S" Bridge (US 62 Bridge) over the Kentucky river in Tyrone, KY and this one. See the Tyrone "S" Bridge photos/info page on this website.
I LOVE these kind of bridges!! And stone arch too! If anyone has any S bridge photos I would love to see them here or feel free to email them to me if you don't mind sharing. S bridges are a rare find and my state don't have any :(. THANKS GUYS!