"At one point, the Keystone State had at least 1,500 covered bridges, and today more than 200 have stood the test of time. Found in 40 of Pennsylvania's 67 counties, more covered bridges exist in Pennsylvania than in any other state, according to the Theodore Burr Covered Bridges Society of Pennsylvania, based in Lancaster, as well as several websites, including Wikipedia."
Somerset County 10
Westmoreland County 1
Washington County 23
Greene County 7 (Ranked by Martha Stewart as one of the best places in America for fall foliage)
Bedford County 14
"Mr. Aldom said that in the early 1800s, agriculture and industrialization were growing rapidly throughout the Northeast and rivers and estuaries were lifelines of both trade and communication. Farmers and industrialists needed bridges to efficiently move commerce, animals and people over the often rugged landscape.
Traditional European building methods (think: the stone bridge of sighs in Venice) wouldn't work in the comparatively harsh North American climate. The hot summers and frigid winters created freeze/thaw cycles that would overturn stone paving. So wood -- especially in a state full of timber -- became the building material of choice.
Pioneer builders chose to cover the bridges because less exposure to the elements meant the wooden superstructures would last longer."
A few counties have sugested tour maps and online guides--all listed in the article.