PENNSYLVANIA (WHTM) — Throughout Pennsylvania, there are many covered bridges. Some of them you can still drive over to this day. Many of them, however, are only able to be walked across.

But why do these bridges exist and how many of them are scattered around Pennsylvania?

According to Uncovering Pennsylvania, there are approximately 209 covered bridges in Pennsylvania. Thirty-four out of the 67 counties in the commonwealth are home to these structures.

The county with the most covered bridges is right here in the Midstate. Lancaster County has a total of 29 of these bridges. Other Midstate counties that have covered bridges are Adams, Cumberland, Dauphin, Franklin, Perry, and Juniata County.

But why were these structures built in the first place?

Uncovering PA says that while many theories are out there as to why covered bridges exist, the most accepted theory is that it was to extend the life of the bridges, some of which can be extended to up to 100 years.

Most covered bridges throughout the state use the Burr truss method. This was designed by Theodore Burr in 1804. The design uses two large arches resting on abutments at either end of the bridge. The covered structure of the bridge was then built around the arches.

Another truss system used on covered bridges is the queen post truss. Usually used to span long distances, this design places a rectangle panel in the center of two triangles. Around 38 bridges in Pennsylvania were built with this truss system, according to the Theodore Burr Covered Bridge Society of PA.

At one point, it is estimated that Pennsylvania had over 1,500 covered bridges, but because of vehicle types and advances in technology, the number has largely decreased to the number we have today.