Trunk tags aren't primarily based on the road's physical construction (e.g. lane counts, speed limit, whether the road is divided or not, etc.), rather how it's used in the network. Though Pottstown and West Chester are nowhere near as large as Allentown is, Route 100 serves as a very busy commuter corridor, seeing much more traffic over a long distance than other nearby primary routes, per AADT data. And while Route 100 is nowhere near as congested as 662 between Reading and Allentown—considering its an important enough route that an expressway was built along it through Pottstown and that its the most direct route linking commuters and long-distance travelers between Allentown and West Chester and past Philly—I think trunk classification here would still make sense.