Streetcars.

The electric streetcar was the most influential transportation alternative in the 1900s. Cities expanded along streetcar routes, and they enabled skilled workers to move further from their workplaces and out of slums and permitted middle-class families to move to new suburban neighborhoods. The streetcar network was so complete that it was possible to travel from New York City to Portland, Maine, via streetcars. The explosive growth of Los Angeles in the 1900s would have been impossible without the streetcar. Pacific Electric, a streetcar company founded by railroad magnate Henry E. Huntingtonin 1901, was by 1920 moving a quarter of a million riders a day across more than a thousand miles of track, making Los Angeles a suburban city, and a planner's nightmare, almost from its inception.

Automobiles.

Just as the train and steamboat revolutionized travel in mid-nineteenth-century America, the...