Garland and Christmas ornaments trim the outdoor walkway from the Court to the Plaza at the King of Prussia Mall, but the decor barely earned a glance from chilly shoppers Friday.

Whether clutching hot coffee or with hands jammed into pockets, most walked too quickly to notice the crane and construction crew or the artist's rendering of the indoor heated connector that could open in 2016, along with space for 50 more stores and restaurants.

"That would be awesome," Alyssa Wert of Allentown said as she, her sister, and a friend dashed through the cold. "We will probably be down here more often."

Already among the nation's largest shopping meccas, and one of the region's economic engines, King of Prussia is poised for its biggest burst of development in years.

What's coming?

More shopping.

More people.

And, inevitably, more traffic.

In addition to the mall expansion outlined last week, developers are continuing the transformation of the area's last big tract of open land - the former Valley Forge Golf Course - into a hospital, apartments, and more shopping. Several new box stores have been part of an ongoing makeover of the Route 202 corridor.

A decade from now, an elevated extension of the Norristown High Speed Line may wind over the chain restaurants and high-end clothing stores, shuttling workers and shoppers from around the region.

Amid the growth, King of Prussia is facing the reality that it must reinvent itself. The area is largely built out, a sprawl of big stores, parking lots, and not much room to expand.

The long-term future, officials hope, lies in a recently rezoned business park that will allow people to live near where they work, shop, and play - a development designed to feed young adults' appetites for urban-style convenience, where cars aren't necessary and home has a sense of community.

"I think that it is King of Prussia embracing its ultimate destiny as an edge city," said Matt Popek, a member of the Upper Merion Township Planning Commission.

Location, location

King of Prussia, which comprises a large portion of the township, borrowed its regal name from a local inn established in 1769. The area had the good fortune of being at the confluence of Southeastern Pennsylvania's major arteries as America fell in love with the automobile. Interstates 76, 276, and 476 now run through or near the area, as do Routes 202 and 422. Just by its geography, King of Prussia was fated to be a destination.

"The success came with the Pennsylvania Turnpike coming through King of Prussia," said Eric Goldstein of the King of Prussia Business Improvement District. "That led to the success of the mall in the early '60s."

The mall has more retail space than any other in America and attracts about 25,000 visitors daily, according to the township. It ballooned from an outdoor shopping plaza in the 1960s to home to more than 400 stores that generate $1 billion in sales a year, according to the owner, Simon Property Group.

This week marks the start of its high season.

But one week before Black Friday, the mall, bedecked for Christmas, was largely empty. A solitary mall Santa sat, Buddha-like, on his throne, as though readying himself for the hordes that will squirm on his lap in weeks to come. Shoppers spoke almost with dread of the coming retail explosion.

"I would never come here on Black Friday," said Molly Waterman of Malvern.

She was shopping with her 15-year-old daughter, Olivia. They agreed the diversity of stores at King of Prussia gave it an advantage over other malls. Every year, Olivia's dance troupe performs a holiday show there. Waterman said the size of the existing two structures, the Court and the Plaza, was already intimidating.

"It's so big, sometimes I park over here and shop in a few stores and then drive to the Plaza," she said.

An equally ambitious project underway at the 135-acre Village of Valley Forge, the former golf course barely a chip shot from the mall, is starting to take shape. A Wegmans opened on the tract two years ago, and the site is also slated to host a Children's Hospital of Philadelphia facility, 249,985 additional square feet of retail, and 365 residential units.

Those developments could yield $300 million in tax benefits over the next 25 years, township officials said.

Goldstein called the surge of retail development "probably unlike anything the area has seen in decades."

Boon and burden

Home to more than 57,000 jobs, Upper Merion Township has the most of any township in Montgomery County. It is the biggest submarket for office space in the Philadelphia region. And near the top of the list in the region in job creation.

Montgomery County receives about $3.7 million a year in tax revenue from the mall and the King of Prussia business park, said Jody Holton, executive director of the county Planning Commission.

Recently completed projects and proposed construction would add more than 600,000 square feet of commercial space by 2017. The county estimates, conservatively, 5,000 new jobs in the area over the next 25 years.

Because of all that activity and tax revenue, Upper Merion can pay for projects such as its $14 million community center and a $1.7 million extension to the Schuylkill Valley Trail without raising property taxes, Township Manager David Kraynik said. Business taxes, which account for almost half of the township's $28 million budget, helped the township have the lowest property tax rates in the county.

But being one of the 28,000 who live in Upper Merion means development is both a boon and a burden.

"I don't mind the tax benefits, but I do mind that there doesn't seem to be a lot of coordination," said Jeff Karpinski, a resident and software developer who complained about development's sprawl after a public hearing this month on SEPTA's rail-extension proposal.

Traffic is more than an irritant, said Kevin Gillen, a University of Pennsylvania professor with the Fels Institute. It depresses housing values and causes lost productivity while workers are stuck on the Schuylkill Expressway instead of at their desks.

And the development plans mean logjams on roads could increase. On some of the major arteries, daily traffic could grow by as little as two percent or as much as 34 percent, depending on the road, according to the Delaware Valley Regional Planning Commission.

"The worst part is the traffic," said Laurie Butera, a pharmacy technician in the township's Swedesburg neighborhood. She said she avoids King of Prussia for months at a time.

"I don't go there from November to January," she said.

Reversing the sprawl

The buzzword in planning today is infill, seeking to repurpose and increase density at existing sites rather than building on unspoiled ground. This fall, the township rezoned the King of Prussia Business Park - 672 acres of offices between the Pennsylvania Turnpike and Route 23 - to allow mixed-use development.

Ideally, existing structures will be replaced by buildings that house people, offices, shops, and restaurants under the same roof.

It's part of a sea change in America's urban development patterns, said Susan Wachter, co-director of Penn's Institute for Urban Research. Instead of flocking to greener suburbs, residents are heading "toward what is already developed."

Millennials, adults born after 1982, are driving mixed-use development. In general, they aren't interested in the backyards and car culture of suburban America. Instead, they want "connection and community," said Holton, who said there was an effort to shift development in that direction throughout Montgomery County.

One millennial at the rail-extension public meeting, 25-year-old Krista Guerrieri, currently commutes by bus from Philadelphia to King of Prussia.

"I like what they're trying to do with mixed-use development, but I wonder if some areas are too far gone into suburbia," she said.

In King of Prussia, the proposed five-mile extension of SEPTA's Norristown High Speed Line, which would include stops at the mall, the Valley Forge Casino that opened in 2012, and at least one stop in the business park, is essential to the success of mixed-use development.

The project has yet to settle on a final route for the rail, and still needs funding for 70 percent of its $500 million price tag before it can become a reality. If all goes according to plan, it could be finished in nine years.

Without it, millennials lose the ability to ditch their cars, and more density without public transportation would exacerbate road congestion.

"I'm not saying we have to turn our suburbs into urban areas, but you can create places that have public spaces, that have ground-floor retail," Holton said. "I see nothing but opportunity here for that."