Sun Valley Parkway divides America's past from its probable future. The road loops round the foot of the White Tank mountains, a craggy outcrop of rock that rises to 1,200 metres (4,000ft) from the Sonoran valley in Arizona. On one side of the road there is the America as it would have been experienced by Jack Swilling, the frontiersman who first spotted the area's potential for settlement in 1867. It is untouched desert, bristling with creosote bushes, palo verde and ironwood trees and the state's signature plant, saguaro cacti. At this time of year the desert looks wispy and brittle, but in a few weeks it will be carpeted with wild flowers and the ocotillo will be in bright red bloom.

On the other side of the road is Tartesso Homes, a rocky outcrop of a different sort. Its entrance is marked by a granite plinth that pronounces the community's name in large bronze letters, flanked by a lush green lawn and rows of palm trees. A two-lane road leads into the complex, past detached houses in various stages of completion, a playground and "community park". There are model houses to look around in styles to meet all tastes, from "trend homes" to Spanish haciendas, all arches and terracotta. The developers have replanted some of the saguaros, using wooden tripods to prop them up until their roots take, but despite this effort at blending in, the two sides of the road could not be more contrasting.

Tartesso Homes is the physical manifestation of a demographic trend felt across America but nowhere more keenly than in Arizona. The state has the fastest growing population in the country, recently overtaking Nevada, which held the title for 19 years. In 1950 there were 750,000 Arizonans; in 2000 there were almost 5 million and in 2020 there are projected to be 7.4 million.

Phoenix, the state capital that lies about 50 miles east of the White Tank mountains, is the fastest growing city in America. It covered 17 square miles in 1950; now it sprawls over almost 500, an area larger than Los Angeles.

A horse brought Jack Swilling to the area. But the town he helped to found, which later became known as Phoenix, took off only after the invention of the car and air conditioning, allowing newcomers to bear 46C (115F) temperatures.

Thousands descend every year on Arizona, led by baby-boomers reaching retirement. They come mainly from other parts of the US - from California where land is increasingly expensive, and the midwest, where the winters are severe. And each new arrival is looking to build a castle in the desert, the epitome of the detached, individualistic and car-dependent American dream.

Other cities build upwards. Phoenix spreads outwards, gobbling up the desert. "We have a default position in much of America, which if you do nothing about it will always be applied," said David Goldberg of the sustainable planning group Smart Growth America. "Single detached family homes on an acre or so where all the streets end in cul-de-sacs, there is only one way in and out and all needs are a drive away."

You can see the result along the Sun Valley Parkway, where at several points the desert has been stripped bare ready for new developments such as Tartesso Homes. All over Arizona the same pattern is repeating. Further west there are two new communities planned, each with 250,000 people. To the south-east of Phoenix a new town, Superstition Vistas, is envisaged for up to 900,000.

Thirty miles to the north of Phoenix is Anthem, a town that is relatively mature in Arizonan terms having been opened in 1998 and with a population of about 40,000. It is a kind of anti-city. It has no real centre, save a park with fancy water features and a "marketplace" full of superstores. The town continues to spread its tentacles into the surrounding desert, with the outskirts occupied by bulldozers and Mexican building gangs.

Kristina Jauch is one of the new residents on the edge of the town, having moved into a newly built home in December. She fled Illinois because of cold winters and mosquito-infested summers, and now runs a pet-sitting service. ("Relax, your babies are safe with us," says the blurb.)

"We feel like every day we are on vacation. We've got a smokin' view from our back window of the mountains, and the sunsets are breathtaking," she said.

Houses are going up all around her, in roads newly dubbed Paso Nuevo drive and Hidden Mountain Lane. For residents such as Mrs Jauch the attractions are palpable: more space for their bucks, eternal summers and fulfilment of what remains of the west's original frontier spirit.

But the costs are high too. Arizona's water table is being depleted as a result of homes being built on pristine desert rather than on agricultural land already used to grow thirsty crops such as cotton and alfalfa. Air quality is suffering from dust thrown up from developments and car exhausts, and highways are clogged with commuters travelling to and from the desert communities. In Phoenix, widening one of the main intersections to 24 lanes, 12 in each direction, has been mooted.

"At what point do we just stand up and start screaming?" said Wellington Reiter, dean of Arizona State University's design college, who has been involved in creating a light rail system - a first step towards public transport.

But he fears the pace of growth in the opposite direction, comparing the carbon footprint (the amount of greenhouse gas emissions caused by each person every year) of Phoenicians: 1,400kg, with that of the people of Hong Kong: 50kg.

Environmentalists tried unsuccessfully to introduce tougher restrictions on development in Arizona in 2000. Sandy Bahr of the state's Sierra Club said the unlikely alliance between developers and farmers hoping to make a fortune by selling their land for housing complexes proved too powerful.

"The policy-makers are burying their heads in the sand. Our whole economy is based not just on growth, but on rapid growth," she said.

With the American population passing 300 million, and projected to reach 400 million by mid-century, Arizona is the most extreme example of stresses being played out across the States. America is being paved over. Some estimates suggest that more than half of the built environment that will exist in the US in 2025 will have been constructed since 2000.

That translates in Arizona as an army of Tartesso Homes marching across the desert. The White Tank mountains will before long rise up from a carpet not of wild flowers but of haciendas. You could call it the final taming of the wild west, though it might not have been what Jack Swilling intended.