Looking thru all the All-American Roads for possible additions:
A1A Scenic & Historic Coastal Byway: not covered by existing systems on
an old alignment at St. Augustine BeachAcadia All-American Road: all covered
Alaska Marine Highway: lol
Beartooth Highway: all covered
Blue Ridge Parkway: already in usanp
Chesapeake Country Scenic Byway: all covered
Chinook Scenic Byway: all covered
Colonial Parkway: already in usanp
Creole Nature Trail: not covered by existing systems on
a piece west of Oak GroveDoor County Coastal Byway: all covered
Flaming Gorge - Green River Basin Scenic Byway: all covered
Florida Keys Scenic Highway: all covered
George Washington Memorial Parkway: already in usanp
Great River Road: mapped
Harriet Tubman Underground Railroad Byway: not covered by existing systems on some branches, which are
at least partially signedHells Canyon Scenic Byway: not covered by existing systems on
NF 39Historic Columbia River Highway: already in usaush and usaus
Historic National Road: mapped
Historic Route 66: already in usaush
International Selkirk Loop: mapped
Lakes to Locks Passage: not covered by existing systems on
a piece south of Rouses Point and former NY 314 (unsigned branch?)
Las Vegas Strip: not covered by existing systems; may be unsigned
Lincoln Highway Scenic & History Byway: Nebraska needs to be checked if
any of this is signedNatchez Trace Parkway: already in usanp
Newfound Gap Road Byway: all covered
North Shore Scenic Drive: all covered
Northwest Passage Scenic Byway: all covered
Pacific Coast Scenic Byway - Oregon: all covered
Red Rock Scenic Byway: all covered
Route 1 - Big Sur Coast Highway: all covered
Route 1 - San Luis Obispo North Coast Byway: all covered
San Juan Skyway: all covered
Scenic Byway 12: all covered
Selma to Montgomery March Byway: not covered by existing systems in Montgomery;
it is signedSeward Highway: all covered
Trail Ridge Road/Beaver Meadow Road: all covered
Volcanic Legacy Scenic Byway: mapped
Woodward Avenue (M-1) - Automotive Heritage Trail: not covered by existing systems
in downtown DetroitNone of these jump out at me as must-haves.