With the number of U.S. residents age 65 and older increasing in coming years, a report from Transportation for America shows that millions of them will live in areas with no or limited public transportation service.

The report, Aging in Place: Stuck Without Options, describes how the trend toward “aging in place” in suburbs and exurbs may leave more than 15.5 million Americans 65 and older with limited mobility options as soon as 2015—and the situation will only increase over time.

“With rising life expectancies, America‘s largest generation also will be the oldest ever,” the report states. “Inevitably, aging experts note, a large share will find that their ability to navigate by vehicle diminishes or disappears over time. These millions of older adults will need affordable alternatives to driving alone in order to maintain their independence as long as possible.”

The report ranks 241 U.S. metro areas in five size categories by the percentage of seniors with poor access to public transportation, now and in the coming years, and presents other data on aging and transportation. The size categories are: less than 250,000; 250,000 to one million; one to three million; three million or more; and the New York metropolitan region, which was treated as its own category due to its size and uniquely robust transit system.

According to the report, in just four years 90 percent of seniors in metro Atlanta will live in neighborhoods with poor access to options other than driving—the largest in the population category of three million or more. Kansas City tops the list for metro areas of one to three million, while in the smaller categories, 100 percent of seniors in Hamilton-Middletown, OH, and Hagerstown, MD, will have to face poor access to public transportation in 2015.

“The baby boom generation grew up and reared their own children in communities that, for the first time in human history, were built on the assumption that everyone would be able to drive an automobile,” said John Robert Smith, president and chief executive officer of Reconnecting America and co-chair of Transportation for America. “What happens when people in this largest generation ever, with the longest predicted lifespan ever, outlive their ability to drive for everything? That’s one of the questions we set out to answer in this report.”

The text of the report is available here.