An early map of the route that appeared in multiple newspapers in the spring of 1915 takes it along the Yellowstone Trail alignment in South Dakota,. But at the very same time boosters in North Dakota were describing it as running across their state via Fargo, Bismarck and into Montana at Glendive. (The routeof the Red Trail)

The newspaper map above shows the spring 1915 route of the National Parks Highway (going through South Dakota) and the color map below shows the 1917 route of both roads. The 1915 map had to be prepared by a common source, and my vote is for the National Parks Highway boosters in Spokane. There is some conjecture here, but the 1915 map was certainly not prepared by folks in North Dakota, as the route shown didn't go there! The color maps shows the National Parks Highway as it actuall was in 1917, and as it appeared in Association publications.

RED = National Parks Highway, Yellow = Yellowstone Trail, Orange = Where both overlap.
At least one other auto trail played an important part in the National Parks Highway story. The Red Trail was the creation of a group based in Dickinson. It was a North Dakota route, not the transcontinental highway that the PBS Old Red Trail TV program suggests. I have read several stories in the Bismarck newspapers that confirm my view, and any of several atlases of transcontinental routes from the period are equally clear. But at least in North Dakota the Red Trail and the National Parks Highway were often used interchangeable to describe a common route in that state.
The National Parks Highway was the brain child of Spokane boosters, but it seems to me that the North Dakota people had more at stake in its success. Spokane was going to benefit from almost any route from the east via Yellowstone to the west coast. I don't see why Spokane folks would want to sour relationships with the Yellowstone Trail leadership, and it doesn't really seem necessary to create a competing route. Virtually all the traffic went through Spokane and it shouldn't matter to Spokane boosters if tourists went through North Dakota or South Dakota! But the folks in North Dakota should care, and they did. They saw the Yellowstone Trail as their big competitor.
Eventually the headquarters of the National Parks Highway moved to Bismarck, but this was apparently late in its history. And the people in North Dakota took a greater interest in the routes in Minnesota, for the very obvious reason that the decision to follow the Yellowstone Trail or the National Parks Highway / Red Trail was made in Minnesota.
In 1924 the National Parks Highway created a "temporary" headquarters in Bismarck, with the intension to strengthen ties and attention to the route coming from the east. When you read the article in the Bismarck newspaper it seems that they slip from temporary to permanent easily. My guess is that Spokane folks were losing interest, perhaps for the reasons I have cited, and Bismarck folks got more active, so the organization naturally shifted to Bismarck.. Why they moved is conjecture.
My real purpose isn't to dwell on the politics and history of the highway, but to set tire on road. Thus I am in the midst of copying my 1917 - 21 strip maps of the route (I try not to use originals on a trip because they inevitably get damaged.). At the moment I have copied only North Dakota and Minnesota, but the work goes on.













