Instead of fixing New York City’s ailing subway system, let’s kill it and use the tunnels to shuttle autonomous cars around the city.

That sounds kind of crazy when you say it out loud, right? Yet that’s precisely the idea The Atlantic put forward in an op-ed last week when author Peter Wayner deemed a proposed plan to overhaul the New York City subway system that would cost an estimated $19 billion too expensive to consider. “Instead of fixing the old trains,” Wayner wrote, “let’s rip out the tracks and fill the tunnels with fleets of autonomous vehicles running on pavement.”

“It’s intriguing—and ludicrous,” Robert Cervero, professor emeritus at UC Berkeley’s city and regional planning department, told me on the phone from San Francisco. To Cervero, The Atlantic’s utopia promotes population decentralization and car dependency, two things that would dramatically alter New York City’s DNA.

“New York City exists as this kind of global hub for creative industries and globally important activities mainly because of density,” Cervero said. “You couldn’t have these tall buildings and this concentration of creative workers without public transit. The subway is the lynchpin.”

New York City desperately needs to alleviate surface congestion and make streets safer for cyclists and pedestrians. But it would be impossible to simply sub the subway for autonomous cars and have it be an efficient solution, Matti Siemiatycki, a professor of geography and planning at the University of Toronto, told me.

“People still need to get out of the vehicle—this is underestimated,” Siemiatycki said. Cars have to stop, he explained, and that makes all the other cars behind them stop. Meanwhile, the NYC subway has 472 stops and moves 5.6 million people in and out of those stations every day.

“To think that can be done with small vehicles, with people getting in and out of them, hasn’t been fully thought out,” he said. “It’s a lot more seductive to think about new technologies and new ideas, than it is to just maintain what we have.”

The idea of exchanging subways for autonomous vehicles—beyond the fact that AVs are still at least one or two decades away from perfection—ignores the sheer volume of cars still circulating above ground. In 2016 there were approximately 700,000 vehicle crossings a day via the Brooklyn, Manhattan, Williamsburg, and George Washington bridges and the Holland and Lincoln tunnels. It would be unrealistic to expect we could put all these cars, plus all the subway riders, in NYC’s existing subway tunnels.

One Atlantic reader, in a letter to the editor, asked whether the author had considered that replacing a single subway line with vehicles would move between 2,000 to 4,000 people an hour, when a subway train can move that number of people in a couple of minutes.

“To achieve that same throughput of people with autonomous vehicles would be very, very difficult,” Rick Geddes, a Cornell University professor and director of the Cornell Program in Infrastructure Policy, told me on the phone from Ithaca.

The better way, in his opinion, is for the US to finally embrace public-private partnerships (PPPs), which are used around the world to fund large infrastructure projects but which are strangely foreign to the US.

“I think it’s misguided to suggest New York City should abandon its subway system, that it’s beyond repair,” Geddes continued. “There are new and better ways New York City could deliver the renovation of the system.”

Geddes, who specializes in PPPs, explained that one option would be to sign multi-year contracts with operation and maintenance (O&M) companies to deliver efficiencies to the system. In New York City, those could look like modern signaling systems, LED lighting, energy-efficient heating and cooling, and so on. Any money saved through this leasing structure would be shared with the O&M companies and the city, the latter of which would put it in a trust to fund future renovations.

This concept is calling asset recycling. “That ensures that it’s gonna be used for the infrastructure,” said Geddes. PPPs aren’t perfect, but they’re a compromise between having governments own (and maintain) infrastructure and forking over entire transit systems to the private sector.

The subway is not merely an expenditure; it runs the majority of the city’s economy by shuttling people to work, bars, restaurants, and shopping malls. Abandoning it in favor of robot cars, said Geddes, “is basically walking away from an enormous amount of capital investment that’s been made over the decades.”

People like to dream big about infrastructure because it can literally change the world. We often forget, though, that we’ve still got to live in it.