This chart shows the very rapid growth in the number of automobiles in the U.S. from 1910 to 1930.  Notice that the graph is not showing yearly sales, but the number of cars registered.

   The rapid growth was possible because the price of cars like the Ford Model T was actually falling.  Large scale production and the development of the moving assembly line were the key reasons for this.
 
     Many new industries were created or expanded in these years to serve the needs of auto makers and car owners.  The economic benefits from this, of course, were enormous.
 
   By the mid-1920s, the average American family had a car.  Still, horse-drawn carriages and wagons did not disappear immediately, especially in smaller towns and rural areas.

Background for the chart question

   The automobile had many advantages over horses for city residents in the early 1900s.  A horse needs daily attention and grooming.  It must be fed and exercised whether it is used for a trip or not.  As any stable boy knew only too well, the job of cleaning up after a horse is no fun at all.  City streets, too, were fouled by horses.

   An automobile needs gas only when it is used.  It can stay garaged without care for weeks or months.  Although it produces some pollution, this was not nearly as bad as the problem left by horses on city streets.