Just an FYI; don't know if this has been posted before.
I rode from NJ to MD and back on Saturday and Sunday via I-78 across PA. Near the area of Shartlesville, PA I ran into construction (paving?) on I-78 where both Eastbound and Westbound squeezes down to 1 lane in both directions. Traffic was backed up for a few miles and was completely stopped at times before getting to the merge points. I don't know how long this work is supposed to last, but if this area of I-78 is in your plans you might want to map your way around it for the time being.
I haven't searched for any PA DOT website that might list the exact mile markers/exits that are involved and suggest alternate routes since it's too late to do me any good, but I'm sure the info must be available somewhere online if you are planning to ride through that area.

 

I used to drive that way a long time ago going to college and the road's been a constant mess. If it is not there, then someone has decided to "improve" the road through Allentown area. Be glad you didn't have to ride I-78 headed toward NYC as the express lanes are torn up for the next year or more.

 

I have been taking I-78 four to five times a year for the past ten years and can't recall when that highway HAS NOT been under construction. If not for the constant road work and crazy truck traffic it is actually a pretty scenic ride once you get past Allentown. Took a ride to Gettysburg last weekend and had to exit at Shartlesburg to detour around a serious truck accident right past that exit. The rural roads that run parallel to 78 are a gorgeous ride. I'm gonna get a good PA map a plan a ride along 78 without actually getting on it. Will probably make better time too.

 

You're right, once you're past Allentown. Riding from up here in NY down to York, down Rt. 33 to 78 is a real trip. 33 is a pretty laid back road, but 78 from Easton through Allentown is like an all hell broke loose speed contest. Beyond the major wide spots in the road, it is a pretty road, if not a little boring over to 81 and then south. The side roads, 501 and the like down to Lancaster are really gorgeous if you have the time. You can take them down to Rt. 30 and go across to York also.

 

Old 22 follows 78. You probably can get a free PA map if you goto the website,

http://www.visitpa.com/visitpa/home.pa

Ridin' 78 is a pain in the kidneys!
When I go up to Easton I take 222 to 22.

Watch out for the leaves & bambi