The clear span (abutment face to abutment face) for the Medora Covered Bridge is 430'4" while the clear span for the Cornish-Windsor Covered Bridge is 422'.
The CW Bridge has one pier and two long spans. Because of the long spans, the arches were so high in the original design that they were found to be impractical. The original length was 422' because it was contained between the abutments. After the third failure, it was decided that a new design was necessary. Instead of removing the pier and creating three spans with two piers, a lattice truss design was developed . In this design, because the truss is above the pier and abutments it was necessary to extend the truss beyond the faces of the abutments for stability. This makes the truss of the CWB longer. The claim is that extending the truss somehow extends the span that is being crossed.
In the picture below, the top rope is attached to the trees and represents the MCB. The bottom rope is wrapped around the trees for support and represents the CWB. The question is: Is the span the distance between the trees or does it include the part of the rope around the trees?
The MCB is a Burr truss design and the arches are seated on the abutments. It was not necessary to extend the truss beyond the abutments for stability. The truss can be extended any distance, but the span does not change.
Another way to measure is the siding at the floor. The most consistent measurements are: MCB 459'4" and the CWB 449'5".
For most people, the length of the bridge is the roof length. According to David Fischetti, the Architect for the 1988 rebuild of the CWB, and the sign in front of the bridge, the roof length is 460'.
Jim Barker, the engineer who designed the plans for the 2010 rehab of the MCB cites the roof length of the MCB at 461'. Independent measurements for the MCB by Bill Cotterman, an Archchitect from Indianapolis, the Jackson County Sheriff's department and at least three others, show the roof length at 461'4".
The Smolen-Gulf Covered Bridge in Ashtabula County Ohio is the Longest Covered Bridge in The U.S. It was opened in 2008 and is not Historic. The Cornish Windsor Covered Bridge over the Conneticut River between New Hampshire and Vermont and the Medora Covered Bridge over the East Fork of the White River in Southern Indiana each claim the be the longest historic covered bridge in the U.S.
Medora Covered Bridge