Mather had concerns over letting the Office of Public Roads develop highway systems within the national parks.

The National Park-to-Park Highway Association was formed in 1916 and began promoting roads and roadway improvements in the Northwest and Rocky Mountain states.

[2] In 1915, a Denver group of motorists took off on a 500 miles (800 km) journey from Rocky Mountain National Park to Yellowstone.

The Wonderland Trail Association was already promoting the next segment of the journey from Yellowstone to Glacier and then westward to Mount Rainier.

[2] In 1917, the Parks Highway Association began marking the route from Glacier to Mount Rainier and added a southern segment to Crater Lake.