For my next thesis, can I mention the Montour Trail transition? When you're in Boston PA, about 24 miles from Point State Park if you ride via the GAP, you can deviate west and join the Montour Trail. (see MontourTrail.org for details).
There is a gap of about six miles between Boston and when the Montour Trail is actually a trail, and those six miles of "future trail" have been sources of great frustration for the unprepared. From Boston PA, you can ride the detour and the Montour Trail and the Montour-Airport connector, and in about 54 miles you'll be at the Pittsburgh Airport.
The Montour Trail is beautiful. It's not developed, it's not commercial, it's very pretty and an excellent surface. It's just not complete to Boston PA yet, it's still a work in progress.
Using the Airport Connector, which lets bikes on an old joint-use airport service road, you can ride your bike right up to the terminal entrance, it's pretty cool.
Last year I encountered several groups of non-Pgh bicyclists who were starting their Pgh-DC ride at the airport, and a few of them were pretty unhappy at the six-mile interruption at the Montour-GAP connection. It's just not finished yet, and these folks were misinformed.
So if you're riding from DC to Pgh and you get to Boston, you've got 24 miles to ride to PointStatePark, or about 54 miles to ride to get to the Pittsburgh Airport via the Montour Trail.