As The Atlanta Journal-Constitution reported last week, fewer and fewer people are using MARTA and other local transit services. MARTA ridership fell 2.6 percent last year despite the collapse of the I-85 bridge and is down 22 percent since 2008.

In fact, transit ridership is falling across the region and nationwide. Experts say many factors have contributed to the decline. A recent Congressional Research Service report (PDF file) offers a good summary of recent research:

*Car ownership: The cost of owning a car – including licensing, parking fees and other costs – are relatively low in the United States. Over the last several decades, the share of U.S. households without a car has declined, and the number of vehicles per household has increased.

*Cheap gas: Cheap gas often suppresses transit ridership in the short run, while rising gas prices encourage many people to leave their cars behind.

*Ride-hailing services: Some studies show Uber, Lyft and similar services have cut into transit use. Others have shown they complement transit service – by, say, giving people rides to transit stations.

*Telecommuting: One study found 37 percent of workers telecommuted in 2015, up from 9 percent in 1995. With more people working at home, fewer require transit to get to work.

*Bicycling: Public bike-sharing programs and the construction of bike paths have led some people to skip transit for short rips.

*Service variables: Fares, hours of service, service frequency, safety and geographic coverage all affect transit use. In general, the greater the supply, the greater the demand for transit.

Check out the CRS report for a fuller discussion of ridership trends.

Declining transit use comes as MARTA and other local transit agencies are planning for metro Atlanta’s biggest transit-building binge in a generation. Critics say it’s a bad idea to bet on old transit technologies like light rail when new technologies like autonomous vehicles may transform transportation in coming decades. Supporters say public transportation is needed to address the region’s awful traffic congestion and to create urban, walkable communities.

You can read our full report on the debate here.