Re: Monday answer
Frederick get credit today. He came closest for one of the sources. Small gold star.
Reason one: The bridge was named for Ebenezer Zane, a worthy pioneer, who, in fact, after 1796 Congress authorized to cut trees and opened the way as far as Kentucky, but this thought was foisted on us by a docent who was from Zanesville, Ohio. It could be true but one of the bridges I saw was near Washington, PA thirty miles east from Wheeling, W.Va (but dated circa 1810), where ole� Eb started his work.
I favor one or both of the other reasons.
Reason two: A zig in the bridge assured that traffic slowed down (usually in a valley or swale) and could not be run easily at high speed�and impossible with a wagon and team. Bridge surfaces could be dirt or plank. In all weather it paid to be careful and go slowly. Japanese gardens have bridges with offset stones too. They are placed in such a way that one must slow down and side step or change direction and thus allow a moment for thought on the beauty of the garden. That wasn�t the idea for the National Road bridge, but it did have the same effect. A wagon team did about 15 miles a day. A freight wagon could get Baltimore shellfish to Pittsburgh before the recipient was poisoned, but care on the bridges was better safety.
Reason three: At the time there was the consideration by the teamsters of using 40 foot wagons or even tandem wagons. The National road was hilly and while not so awfully difficult for a team of four to six, the roadbed had to be maintained. A typical wagon was fourteen feet at the bottom of the bed and sixteen feet at the top and carried six to eight tons--half that if the trip was long and team food had to be carried. The bigger wagons would have carried much more. If a loaded 40 foot �super� wagon used a bridge the bridge could be overloaded. The zig/zag prevented the larger wagon from passing without the wheels hitting the bridge sides. If a tandem was considered, the process of unhooking one wagon, taking one across, parking wagon one, disconnecting the team, getting the second wagon, reconnecting, crossing, reconnecting both wagons, was considered a sufficient enough pain to prevent a wagon train. The offset prevented overloading of the bridge and it also kept the surface in better shape.
As an aside, in the early days, it was the responsibility of the road builder to provide stepping stones and a shallow crossing point with access to the road without using the bridge. It also allowed for the team to get some water while on the move. Some say this was the start of calling a person �a stick in the mud�. Around here similar fords are still around in some places, and on the weekends I�ve seen cars pull off the highway drive down to the stream and get washed. I never thought that these old well worn turnoffs might have been part of an original road plan.
The horses used to pull these heavy conveyances were a cross between a Flemish draft horse and an English breed chosen for size and endurance. They were most often black, and occasionally a bay or dappled gray, stood five feet tall, were remarkably docile, and cost between $170 and $200 in early 1800�s dollars. They were decorated with fancy pom poms and other elaborate head and forelock gear and put on quite a show as they marched through towns and villages. If the wagon made it safely without help from another driver he arrived �with his bells on�. Conestoga horses were finely bred and named Conestogas after the area in Pennsylvania where they originated. Sadly the breed is now considered extinct.
What eventually did the �Z� bridges in were the winters, a lot more people traveling west of Pittsburgh, and railroads. In winter snow built up over the sides in the center of the bridge and made them impassable with wheel or sled. Too many people were stranded in the winter owing in part to the bridge design and that was not good for horse, wagon, freight, people or politics..
Some info came from the docents between Uniontown, PA, Wheeling, W.VA., and Zanesville, Ohio, on Route 40 with some support from books by Eric Sloane and Edwin Tunis. The web, with the subject of �Conestoga� also gave or confirmed info.