I've been looking into finding the national headquarters for the various auto trail associations, and I'm posting my initial findings below:
Dixie Highway - Chattanooga, Tennessee (Patten Hotel)
Dixie Overland Highway - Columbus, Georgia
Florida Short Route - Columbus, Georgia
Lincoln Highway - Detroit, Michigan
National Old Trails Road - Kansas City, Missouri
National Park-to-Park Highway - Cody, Wyoming
National Parks Highway - Spokane, Washington
Ocean-to-Ocean Highway - Redlands, California
Old Spanish Trail - San Antonio, Texas
Pikes Peak Ocean to Ocean Highway - Colorado Springs, Colorado.
Victory Highway - Topeka, Kansas
Yellowstone Highway - Douglas, Wyoming; Cody, Wyoming
Yellowstone Trail - Minneapolis, Minnesota
Parsa
Another for the list:
Custer Battlefield Highway - Mitchell, South Dakota
Cannon Ball Trail - Leon, IA
River Road Association - Des Moines, IA
Rocky Mountain Highway Association - Denver, CO
The Sunflower Trail - Beloit, KS