There was also increasing pressure to allow the use of a "new-fangled" form of transportation, known as the automobile! Park officials were certain that allowing these new machines into the park would spell trouble. For one thing, the roads were rough and very hazardous; for another, they were certain to cause chaos and conflict with the horses used so widely throughout the park.
Finally, on April 21, 1915, Secretary of the Interior Franklin K.
Lane authorized private automobiles to officially enter Yellowstone,
beginning later that same year. By the close of the first season,
958 automobiles, carrying 3,513 people had entered the park. The expected
conflict between horses and automobiles did indeed arise, eventually
resulting in the decision that horses would no longer be permitted
on the park roads.