When it comes to mass transit, politicians never learn. Last month, Nashville Mayor Megan Berry announced a $5.2 billion proposal that involves building 26 miles of light rail and digging an expensive tunnel under the city’s downtown. Voters will be asked in May to approve a half-cent sales tax increase plus additions to hotel, car rental and business excise taxes to pay for the project.

San Antonio’s mayor, Ron Nirenberg, also wants to lay rail, even though his city’s voters blocked light-rail plans in 2000 and 2015. In 1933, San Antonio became the first major city in America to replace its streetcars with buses, which are faster, more flexible and cheaper to buy and operate. Nevertheless, Mr. Nirenberg has strongly supported rail construction on “high density corridors,” though he wants the transit agency to work out the specifics.

In the Tampa, Fla., area, transit planners are proposing a 35-mile light-rail line to St. Petersburg. They don’t know how to pay for it, especially since Tampa voters rejected a sales tax for light rail in 2010 and St. Petersburg voters rejected one in 2014.

These proposals are questionable at best and reckless at worst, given that transit ridership—including bus and what little rail these regions have—is down in all three jurisdictions. This is a nationwide trend: Data released this week by the Federal Transit Administration shows that ridership is falling in nearly every major urban area (with Seattle as a notable exception).

Some regions have seen catastrophic drops in ridership since 2010: 30% or more in Detroit, Sacramento and Memphis; 20% to 30% in Austin, Cleveland, Louisville, St. Louis and Virginia Beach-Norfolk ; and 15% to 20% in Atlanta, Charlotte, Los Angeles, Miami, San Antonio and Washington.

Adding rail service hasn’t helped. To pay for new light-rail lines that opened in 2012 and 2016, Los Angeles cut bus service. The city lost nearly four bus riders for every additional rail rider. Atlanta, Dallas, Sacramento and San Jose have seen similar results. The rail system in Portland, Ore., is often considered successful, but only 8% of commuters take transit of any kind to work. In 1980, before rail was constructed, buses alone were carrying 10% of commuters.

The main reason for this drop-off is that low gas prices and ride-sharing services have given people better options. Census data show that 96% of American workers live in households with at least one car, and anyone with a smartphone can summon an Uber or Lyft.

That said, transit ridership has been sliding for decades as jobs have become less highly concentrated in city centers. Since 1970, the number of transit trips taken per urban resident has fallen more than 20%. Outside the areas of New York, Boston, Chicago, Philadelphia, San Francisco and Washington, transit carries less than 1% of passenger travel. This belies the claim that mass transit is vital to urban economies.

Yet the subsides go on, seemingly forever. Since 1970, taxpayers have plowed more than $1.1 trillion (adjusted for inflation) into transit systems. Critics may reply that roads are also subsidized. But measured per passenger-mile, the subsidies for transit are more than 40 times as great as for driving.

The transit industry has compounded its problems by going heavily into debt, allowing unfunded pensions and health-care obligations to snowball, and failing to maintain the rail lines they already have. According to the Department of Transportation, the nationwide transit maintenance backlog is approaching $100 billion, causing exactly the problems you’d expect: derailments of New York City subways, slowdowns of Chicago’s elevated train, smoke in Washington metro tunnels, and other operational and safety issues. Even if all the money now spent on new construction were redirected to maintenance, according to the department, it would take 20 years to rehabilitate America’s rail transit systems.

Instead of spending billions on new rail lines, cities like Nashville, San Antonio and Tampa ought to use buses to move people faster, more safely, and for far less money. Rail is simply a bad investment.

That’s especially true given the bets being made by companies like Ford, Google and Uber on driverless cars. Some analysts predict that by the middle of the next decade, calling a driverless car will be as easy as hailing an Uber today. Why walk in the heat or cold for a bus or streetcar when you can hail a driverless car to your door for less money than the transit fare? Nashville’s first light-rail line won’t even open until 2026. By then, who’s going to want to use it?