WASHINGTON (AP) - America's population is on track to hit 300 million on Tuesday morning, and it's causing a stir among environmentalists.

People in the United States are consuming more than ever - more food, more energy, more natural resources. Open spaces are shrinking and traffic in many areas is dreadful.

But some experts argue that population growth only partly explains America's growing consumption. Just as important, they say, is where people live, what they drive and how far they travel to work.

“The pattern of population growth is really the most crucial thing,” said Michael Replogle, transportation director for Environmental Defense, a New York-based advocacy group.

“If the population grows in thriving existing communities, restoring the historic density of older communities, we can easily sustain that growth and create a more efficient economy without sacrificing the environment,” Replogle said.

That has not been the American way. Instead, the country has fed its appetite for big houses, big yards, cul-de-sacs and strip malls. In a word: sprawl.

“Because the U.S. has become a suburban nation, sprawl has become the most predominant form of land use,” said Vicky Markham, director of the Center for Environment and Population, an advocacy group. “Sprawl is, by definition, more spread out. That of course requires more vehicles and more vehicle miles traveled.”

America still has a lot of wide-open spaces, with about 84 people per square mile, compared with about 300 people per square mile in the European Union and almost 900 people per square mile in Japan.

But a little more than half the U.S. population is clustered in counties along the coasts, including those along the Gulf of Mexico and the Great Lakes. Also, much of the population is moving away from large cities to the suburbs and beyond.

The fastest growing county is Flagler County, Fla., north of Daytona Beach; the fastest growing city is Elk Grove, Calif., a suburb of Sacramento; and the fastest growing metropolitan area is Riverside, Calif., about 50 miles east of Los Angeles.

The Census Bureau projects that America's population will hit 300 million at 7:46 a.m. EDT Tuesday. The projection is based on estimates for births, deaths and net immigration that add up to one new American every 11 seconds.

The estimated 11 million to 12 million illegal immigrants in the U.S. are included in official population estimates, though many demographers believe they are undercounted.

The population reached its last milestone, 200 million, in 1967. That translates into a 50 percent increase in 39 years.

During the same period, the number of households nearly doubled, the number motor vehicles more than doubled and the miles driven in those vehicles nearly tripled.

The average household size has shrunk from 3.3 people to 2.6 people, and the share of households with only one person has jumped from less than 16 percent to about 27 percent.

“The natural resource base that is required to support each person keeps rising,” Replogle said. “We're heating and cooling more space, and the housing units are more spread out than ever before.”

The U.S. is the third-largest country in the world, behind China and India. The U.S. is the fastest growing of the industrialized nations, adding about 2.8 million people a year, or just under 1 percent. India is growing faster but the United Nations considers it to be a less developed country.

About 40 percent of U.S. population growth comes from immigration, both legal and illegal, according to the Census Bureau. The rest comes from births outnumbering deaths.

“It's not the population, it's the consumption that can do us in,” said William Frey, a demographer at the Brookings Institution, a Washington think tank. “These are the luxuries we have been able to support until now. But we're not going to be able to do it forever.”

On The Net:

Census Bureau population clock: http://www.census.gov/population/www/popclockus.html