In L.A.–where commuters each spent an average of 104 hours stuck in traffic in 2016–most people drive to work alone. But in 15 years, a new report estimates, more than 2 million of them may have given up their cars.

“We were very aware that the first time cities met cars, things went well for cars and somewhat less well for cities.”

Autonomous cars are likely to be on roads in three or four years. As adoption scales up, the cost of an Uber or Lyft (or whatever company replaces them) ride may drop roughly in half for consumers: not having to pay a driver will make the ride cost much less. The report, called Driverless Future, estimates how many car owners are likely to shift to hailing a driverless car because using an app is cheaper–and what that shift means for American cities.

“What we saw in the model–and we ran it a few different ways–is it’s going to be a monumental shift,” Joe Iacobucci, director of transit for Sam Schwartz, an engineering firm that partnered with Arcadis and HR&A to create the report, tells Fast Company. “Forty percent to 60% who are driving today will have an economic rationale to shift to those services.”

The report looked at three representative cities. In the New York metro area, the drop in car ownership could be as much as 60%, or 3.6 million vehicles, versus as much as a 44% drop in L.A. and a 31% drop in Dallas. (The differences in the economics are driven primarily by the cost of parking, which is very high in Manhattan and often free in Dallas).

With fewer personal vehicles, cities could radically change. “The potential reduction in space needed to store vehicles provides cities an incredible advantage or opportunity to rethink the way that our streets work, and our very pricey urban infrastructure,” says Iacobucci.

“The potential reduction in space needed to store vehicles provides cities an incredible advantage or opportunity.”

“When you look at an average street, in a lot of cases, you could say 25% of the street is actually dedicated to the storage of cars,” he says. “If we need one drop-off space that satisfies 20 parking spaces, we could essentially create on-street bus networks, separated bike lanes, and large pedestrian facilities to make our streets the best practice of a living street.”

Former parking garages could become housing or office space. In downtown Washington, D.C., 40% of land is dedicated to parking; in a less-dense downtown like Houston, that number is 65%. (The report recommends that any new parking garages built now should be designed for future conversion to another use). Related businesses, like gas stations and car washes, could also move out of densely-populated areas to free up space.