For many of the NPS routes, It is a shame that there are restrictions on putting in place traffic signals. There are many points along Beach Road or the GW Pkwy exit to Memorial Bridge that would be so much safer with traffic signals.
Though there are signals on at several places on the NPS-maintained parkway system in the Washington area.
Including these (list is not all-inclusive):
George Washington Memorial Parkway at Slaters Lane (Alexandria)
Clara Barton Parkway at Chain Bridge/Canal Road, N.W.
Ohio Drive and Independence Avenue, S.W.
17th Street and Independence Avenue, S.W.
15th Street and Independence Avenue, S.W.
Several along Constitution Avenue, N.W. between 23rd Street and 17th Street (some are maintained by DDOT)
Naylor Road and Suitland Parkway
Forestville Road and Suitland Parkway
Beach Drive, N.W. at Park Road