Even as robotaxis and other automated vehicles multiply across American roads, nearly every mass transit system in the United States still requires a driver or operator on board. Two major cities are now trying to change that.

Officials in Washington, D.C., have advanced a plan that could let computers fully control one of its subway lines, and Los Angeles is seeking to build a new, completely automated route. Honolulu already runs one.

The Washington and Los Angeles projects—which would take years to implement and still face financial and technical hurdles—arrive amid a fierce debate over the safety risks and benefits of automated trains, and over whether the substantial upfront expense is worth the long-term savings in labor costs.

Supporters call the push long overdue, pointing out that American cities lag far behind numerous places around the world that operate unattended subways, metros, or buses, including several cities in Europe and Asia as well as major systems in Canada, Australia, South America, and the Middle East.

"It should be much harder to automate something that's on-road, that has all of these various obstacles coming at it, compared to a fixed-rail system," said Baruch Feigenbaum, senior managing director of transportation policy at the Reason Foundation, a libertarian think tank that has advocated for transit automation. "I don't know a nice way to say this, but basically the transit world is where innovation goes to die."

A half-century of automation, interrupted

Only in recent years have on-road vehicles from companies such as Waymo, Tesla, and Zoox taken the automation lead in the United States ahead of transit systems that run on tracks. The lag is notable because rail automation in the U.S. hardly new: operators have always been part of the Metro subway system in Washington, D.C., but its trains ran with some automated features when it opened 50 years ago. After a fatal crash in 2009, those features were switched off, then restored over the past two years.

Officials in the nation's capital have now renewed their push for automation on Metro's Red Line. In Los Angeles, an environmental review is underway for an automated track serving a new underground line of the city's heavy-rail system that would cut through the rough terrain of the Santa Monica Mountains.

Honolulu, meanwhile, is already running an elevated driverless line—the most expansive automated transit system in the country. It will stretch 19 miles when completed, surpassing the elevated driverless trains that Detroit and Miami opened as pilot projects in the 1980s, a monorail connecting hotels on the Las Vegas Strip, and smaller systems that carry passengers between airport terminals.

Not every experiment has endured. An automated monorail that once ran in Memphis, Tennessee, closed down years ago, and the driverless Jacksonville Skyway will soon halt operations indefinitely because of budget cuts, despite initial plans for an even larger automated system.

The shadow of Fort Totten

Nine people were killed and many others injured in a 2009 train collision near the Fort Totten station on the Red Line. The National Transportation Safety Board blamed the crash on a maintenance problem in the automated signaling system that separates trains from one another.

Randy Clarke, chief executive of the Washington Metropolitan Area Transit Authority, said in an interview with The Associated Press that the issue at Fort Totten had nothing to do with the cruise control-like system that was restored on the Red Line in 2024 and on Metro's other lines last year. Even so, he acknowledged that for many years the crash "paralyzed" Metro's plans for further automation.

"That was a big failure, there's no question. We do not do anything like that now," said Clarke, who called the current system "highly regulated, highly documented, very proficient."

Trains could conceivably run without a human worker inside the cab as early as 2033 under the nearly $1 billion plan that Metro's Board of Directors backed in April to upgrade the Red Line with safety features, including protective barriers that prevent passengers from getting onto the tracks. Clarke said that even if workers are not needed to physically run the trains, they will still be needed to maintain the tracks and respond to emergencies.

Union resistance

Unions have been persistent critics of automation. John Samuelsen, a former worker in the New York subway system who now serves as international president of the Transport Workers Union, said unions have fought off automation efforts in New York and elsewhere, calling them "a catastrophe waiting to happen."

"Passengers will be left dealing with a self-evacuation, which is a disaster in a system which runs on a third rail," Samuelsen said.

Honolulu's clean record

Honolulu opened its Skyline automated light metro transit system three years ago to relieve massive traffic bottlenecks for commuters, and expanded it last year. Vance Tsuda, the system's project director, said there has not been a single crash or injury associated with automation. Even hybrid systems are less safe than a fully automated one, he argued, because a human is far more likely to overlook something like an open door and prematurely clear a train for departure.

"If you have somebody making that decision, they could make a wrong decision," Tsuda said.

But David King, an associate professor of urban planning at Arizona State University who has researched automated transportation, said the success rate has to remain perfect to keep the public's trust. That, he said, is why the Fort Totten crash and other incidents involving automation left the technology so far behind.

"We allow a certain amount of failure with private industry that we don't tolerate when the state does it," said King, citing the several rockets blown up by Elon Musk's SpaceX—a record he argues would cripple NASA's budget if it happened in the public sector.

Los Angeles's mountain-crossing plan

Perhaps the most ambitious transit automation effort being pursued in the United States is the subway line Los Angeles officials voted to recommend beneath the Santa Monica Mountains. The area is one of the most notorious traffic headaches in the country, with the mountains dividing the bustling Westside employment hub from the San Fernando Valley.

A subway system already runs beneath the same mountain range near Hollywood, but that line has a driver onboard. The new proposal, which is undergoing an environmental review, would use shorter, more frequent trains, said Cecily Way, who oversees planning for the Sepulveda Project at LA Metro. Like the project in Washington, the new line in Los Angeles would feature station doors to protect passengers on the platform from oncoming trains.

Feigenbaum said that automation of a major transit line in Los Angeles or Washington could open the floodgates for other cities to follow.

"It sends a signal that this is technologically doable and this is something that maybe other systems are going to want to strive for," he said.

This story was originally featured on Fortune.com.