The motorway classification is built on the grounds of physical criteria, unlike the minor-trunk hierarchy which is based on how regionally important the roads are and what places they connect.

The motorway section of GWMP between I 495 and the airport is primarily accessed by ramps and interchanges and is grade-separated from other roads and paths, with the only exceptions being a gated driveway to a US Park Police station at Turkey Run Park and a very narrow, low-speed section with an at-grade crosswalk for the Mount Vernon Trail on Columbia Island (tagged as trunk for the sake of filling the gap). Once GWMP passes the maintenance facility near the airport, the only other junctions with the road are at-grade intersections with Marina Drive, Slaters Lane, and Bashford Lane, which would collectively disqualify this segment from being a motorway.

As for Suitland Parkway, in my opinion the trunk route should be shifted from the parkway over to Pennsylvania Avenue as Suitland Parkway is another scenic byway maintained by the NPS that disallows trucks and similarly lacks an all-directional junction with the Beltway like GWMP does. However, I think some additional highway reclassification should be done in the area so that might deserve its own discussion.