It’s August 1, 1915.
“Hooray! Today we can finally drive our new automobile into Yellowstone National Park.” Something like this must have been shouted between the open-topped cars lined up to pass through the North Entrance Arch on the first day it was legal to “motor” through the park. [Turns out we showed you that arch in our July 28th post.]
It’s true that a man named Henry G. Merry from nearby Horr, Montana had decided thirteen years before, in 1902, to “pilot the car [a Winton] to the fort and talk things over with the commandant,” according to Merry’s son’s account many years later. 
You see, the Secretary of the Interior and superintendent Colonel John Pitcher had agreed that year that automobiles must be banned from the park due to the terrible condition of the roads and the danger of frightening the horses. But Merry went anyway—and was told he was under arrest and would have to pay a penalty. However, according to son Edward T. Merry: “When my father asked what the penalty would be, the officer very seriously replied, ‘You will have to take me for a ride in this contraption.’” But soon Merry was ushered out with a warning never to try it again.
Officials knew they would eventually have to improve the roads enough for cars to use them, and eventually this was done. Exactly one hundred years ago today the new era began. Fifty Fords, Buicks, Wintons, Haynes, and others entered the park. Within a year it was obvious that horses and autos were incompatible on the bumpy, narrow roads, and of course, the horses lost the contest.
[My source for this story was The Yellowstone Story, Volume II, by Aubrey L. Haines, pages 264 to 269. The late 1890s Winton touring car is courtesy of Wikipedia commons.]