More than 700,000 American households do not have a car and lack access to public transit, making them less likely to find and keep jobs, transit advocates said.

“We knew there were pockets of households who are economically hampered by the fact that they own no car and have no access to transit, but we didn’t fully understand the true scope of the problem until now,” said Adie Tomer, author of a report compiled for the Brookings Institution’s Metropolitan Policy Program.

This is no surprise to public transit advocates, who have watched low-income families get priced out of transit-dense urban...