A 28-story building recently approved for downtown Bethesda will sit atop both a Metro Red Line station and a stop on the future light-rail Purple Line. A cycling and jogging trail will run beneath it, and dozens of shops and restaurants will be within walking distance.

The high-rise, which will have office space as well as apartments, is just the kind of high-density, transit-oriented development Montgomery County and other U.S. suburbs are planning on to attract younger residents as well as downsizing baby boomers looking for a more walkable, urban lifestyle.

But one detail of the building plan has drawn skepticism: a parking garage, with room for 700 cars.

“Why is it good to build vast quantities of parking for a building that is supposed to be transit-oriented development, literally on top of a Metro station and a Purple Line station?” one Bethesda Magazine reader recently asked in a back-and-forth exchange on the magazine’s website.

[Suburbs rethink transit to court millennials]

It’s a discussion taking place in suburbs and sprawling cities across the country, from Los Angeles to Atlanta, as they straddle a vast divide between their autocentric pasts and futures that hinge on more people getting around by foot, train and bus. Experts say the amount of parking in areas designed to be more transit-oriented will help determine how successful they are in curbing traffic congestion, making walking and biking attractive options and providing more affordable housing.

“It’s certainly turning out that one of the biggest factors that either inhibits or facilitates good transit-oriented development is whether you get the parking right,” said Stewart Schwartz, executive director of the Coalition for Smarter Growth. “It really looms very large in the quality of life in these walkable communities.”

Ample — and, traditionally, free — parking has been a staple of American suburbs and auto-dependent cities for more than 60 years. Vast parking lots and multilevel garages were the result of off-street parking requirements that assumed most people would reach new homes, office buildings and shopping centers by car.

But studies have shown a strong link between the amount of parking and how much people drive — and that becomes a problem in areas where local governments are investing millions in transit systems and encouraging high-density development with the potential to generate even more traffic.

When Fairfax County officials first envisioned transforming Tysons from a traffic-clogged sea of suburban office parks into a walkable, livable downtown clustered around Silver Line Metro stations, the Northern Virginia community was devoting more land to cars than to people, according to a county report.

[Silver Line jump-started Tysons boom, but some say it’s too much too soon]

The 2010 Tysons plan lowered minimum parking requirements for new residential buildings within a half-mile of Metro stations and scrapped those minimums altogether for new office buildings near transit. For the first time, the county also set maximum parking limits for both new residential and office buildings near Silver Line stations.

“The traditional way we’ve done parking in the suburbs is a minimum number of spaces required for each kind of use,” said Leonard Wolfenstein, a Fairfax transportation planner. “We didn’t want to do that in Tysons. It would just be counter to the behaviors we wanted to shape in the future — of people driving less and using transit more, and people living and working in Tysons.”

Montgomery recently began encouraging developers in some areas near transit stations to build less parking by giving them credit toward any traffic-mitigation projects they could be required to pay for. New maximum parking requirements also took effect countywide in 2014. Developers in some urban areas, all of which have a Metro station, have long been allowed to go below minimum parking requirements, and even provide no new parking, as long as they pay into a county fund that maintains nearby public parking garages.

“When projects come in, the first thing we ask is, how can we reduce the parking?” said Robert Kronenberg, the Montgomery planning chief for Bethesda, Friendship Heights, Silver Spring and other inner-Beltway urban areas. “The question is, how much is really necessary? That’s a tough nut to crack.”

Ironically, the 700 parking spaces that have attracted attention for the high-rise planned at Wisconsin Avenue and Elm Street in downtown Bethesda are almost half the minimum county zoning code required, planners said. They’re also less than one-quarter of the maximum 3,000 spaces allowed.

Austen Holderness, of Carr Properties, which is developing what’s known as the Apex site, said the building needs parking for the 50 to 60 percent of office workers expected to drive there. He said Carr was able to limit the parking to 700 spaces because some will be used by office workers during the day as well as by residents in the evenings and on weekends. The additional parking amounts to a net increase of 334 spaces, compared with those in the building there today, he said, and there will be relatively far less parking for the amount of density on the site.

With construction costs for underground parking running up to about $60,000 per space, Holderness said, developers have plenty of incentive to provide only the amount needed for future tenants.

“As long as we can build economically viable parking to meet the demands of the market,” Holderness said, “developers will be first in line to build less parking because of the massive costs it adds to our project.”

The stakes are high for finding the right balance, as the garages built today could be around for 40 to 50 years.

Buildings that include too much parking, experts say, encourage more driving, undercut public investments in transit, make areas around stations less walkable and drive up construction costs, which will be passed on to tenants through higher rents.

[Study: Cities, suburbs are requiring too much parking near transit stations]

Yet buildings that provide too little can create higher parking prices in the surrounding area and scare away stores and restaurants worried about having enough for customers. Surrounding communities concerned about traffic and parking spilling into their residential streets also can increase political opposition to high-density development, experts say.

Making an accurate prediction for parking needs might require a crystal ball. Rapid changes in technology, particularly the advent of Capital Bikeshare, car-sharing services like Zipcar and ride-hailing companies such as Uber and Lyft, have cut into car ownership. Meanwhile, the potential for driverless cars in the not-so-far-off future could require far smaller garages or even make them unnecessary.

When Washington Property began building the Solaire apartment high-rise near the Metro station in downtown Silver Spring just seven years ago, it included an average of one parking space per unit. Zipcar was becoming more popular, but ride-hailing companies were just getting started.

Today, the building’s parking garage is about 30 percent empty, and the developer is planning to incorporate that space into the garage for a second tower it plans to build next door. The total supply will give both buildings an average of about 0.7 parking spaces per unit, a big cut from the previous one-to-one ratio.

“The amount of disruption in the urban transportation sector is unlike anything we’ve seen in generations,” said McLean Quinn, a vice president for the developer EYA. The technological changes, he said, “have all of us scratching our heads about the demand for parking over the next several decades.”

Experts say it’s also still hard to determine how quickly — and how much — the demand for more walkable, urban lifestyles will translate into suburbanites giving up their cars. How many millennials, a generation still emerging into adulthood, will continue to shun driving? How many will be able to afford to live in the mostly “luxury” apartments and townhouses being built near Metro stations? Will baby boomers downsizing from their three-car garages continue to want at least two parking spaces in their Metro-friendly townhouses — one for the car they keep and another for their golf clubs and bicycles?

Some experts believe that private developers, more than urban planners, are in the best position to make those predictions. Developers, they say, have a financial incentive to build enough parking to clinch tenants and not waste money that they could spend on roof decks, eye-catching lobbies and other amenities their buildings need to compete.

“Cities are giving up on the idea that they know how much parking is needed,” said Donald Shoup, an urban planning professor at the University of California at Los Angeles and a national expert on government parking policies.

“They’re looking more to developers and the market to decide how much parking there should be. More and more people are beginning to be very dubious about where these [traditional] parking numbers come from.”