Caitlin, there are three East-West routes and 4 North-South ones.
http://www.dot.state.pa.us/BIKE/WEB/tour_routes.htm
The "Crush the Commonwealth Easy Leisure Ride" (yeah, right) took Rt S almost all the way to Philadelphia. Rt S joins the GAP between West Newton and Rockwood, then takes off NE from Rockwood on the appealingly named "Water Level Rd."
The PennDOT bike route maps, although detailed, leave a little to be desired: they don't show nearby rail-trails and the map for one of the routes won't even show where it crosses other PA bike routes.
I took Rt A to WV once. Between Houston and I-70, it was like other heavily trafficked roads, but south of I-70, it was a nice route.
I've been meaning to write the state legislators to tell them what more visible "Bicycle Route" signs and a wide, clean shoulder would do for the bad parts. At least having the routes gives us an easy vocabulary to talk to the government about bike travel.
Mick