Transit ridership is declining nationwide, while driving miles are up. Recent research indicates that increased access to vehicles by lower-income residents is particularly responsible for the transit dip, as lower-income residents tend to ride transit more than higher-income earners. And land use policies that encourage or force low-income residents to move far from jobs may be a determining factor.

First off, as Governing reports, poor people are increasingly able to access vehicles:

Only 20 percent of adults living in poverty in 2016 reported that they had no access to a vehicle. That’s down from 22 percent in 2006, according to a Governing analysis of U.S. Census data. Meanwhile, the access rates among all Americans was virtually the same (6.6 percent) between those two years.

In addition, lower-income earners are now able to get easy access to car loan services, including “subprime” auto loans.

But while economic factors play a role, so does land use:

“The country is becoming more car-oriented, because the country is moving south. If you’re moving from transit-oriented cities in the Northeast and moving to Texas, you’re going to become more car-oriented,” [urban planner and transportation consultant Sarah Jo Peterson] says.

Urban planners who want to push for walkable neighborhoods and transit-oriented development can still make a compelling case for certain areas, particularly urban centers, she says. “What they don’t have is wind at their backs.”

This migration of poor families to the suburbs, where housing is cheaper but transit service is weak, requires them to purchase an automobile to get access to jobs.

In Los Angeles, the Southern California Association of Governments commissioned a study by UCLA researchers on the causes of the ridership decline. The numbers from the report [PDF] are striking in terms of how concentrated transit ridership is in L.A.:

Ten percent of all of the people who commuted to and from work on transit in 2015 lived in 1.4 percent of the region’s census tracts, which covered just 0.2 percent of the region’s land area; the average number of transit commuters in these few tracts was almost 12 times the regional average. Fully 60 percent of the region’s transit commuters lived in 21 percent of the region’s census tracts, which occupied 0.9 percent of the region’s land area. Overall, the most urban and transit-friendly neighborhoods in the SCAG region comprise less than one percent of the region’s land area.

So while researchers are still analyzing the various causes for the ridership decline, the fact that low-income residents are moving to exurban areas may be a prime reason. And that displacement is likely due to high housing costs and gentrification in our prime transit areas. Until we solve that problem, we may not be able to resuscitate the nation’s transit systems anytime soon.