The greater availability of automobiles was one of the greatest changes in American life in the 1910s (and beyond). This change affected American life in a number of ways. Looking at your question, I assume that you are asking more about the changes to people's everyday lives as opposed to the changes that cars caused in the overall economy.
If so, the major impact of the automobile was to give people new freedom and to expand the horizons of their lives. With automobiles and good roads, people were now able to go out in their cars and do more things for fun. City people could go to the country to relax. Country people could come to the city for excitement. By allowing people to do more things for fun, cars really changed people's lives.
On another level, we should remember that a major impact of the automobile was on the cleanliness of cities. We don't often think about this, but before cars, horses were very important parts of transportation in cities. Having horses pulling wagons all over cities meant that cities were dirtier and smellier than they are now because of all the manure. This might seem like a silly thing, but it is a major change in people's lives.