When the Interstate Highway System was built, it largely followed previously well-worn footpaths, wagon tracks, farm-to-market roads, and the US Highway System. They all followed the contours of the Earth - through mountain passes, along rivers, and straight as an arrow on the Great Plains. The result of all these 'roads' following the same routes is that you end up with many highways becoming 'duplexed', that is sharing the same physical roadbed. Sometimes this is for a short or modest distance, but in at least one instance the Interstates (I-55/I-44/I-40) have completely replaced a legendary highway (US-66).
But US-11, while running basically right next to I-81 for mile after mile, is still there in all its relaxed-paced, stop and smell the roses glory. One can drive it up through the Great Valley of the Appalachians and never be more than a few miles from I-81 and its truck stops. Around Scranton/Wilkes-Barre PA, however, US-11 and I-81 turn more northwest rather than northeast towards Boston. My suggestion would be to look at a couple of other US highways that also run parallel to some Interstates. Those would be US-6 (I-84), US-209 (I-81) and US-20 (I-90). Use the US labeled roads in the countryside and the Interstate labeled roads to get through urban areas.