If the USA seems too crowded and its roads too congested now, imagine future generations: The nation's population could more than triple to 1 billion as early as 2100. That's the eye-popping projection that urban and rural planners, gathered today for their annual meeting in Las Vegas, are hearing from a land-use expert. "What do we do now to start preparing for that?" asks Arthur Nelson, co-director of the Metropolitan Institute at Virginia Tech, whose analysis projects that the USA will hit the 1 billion mark sometime between 2100 and 2120. "It's a realistic long-term challenge." The nation currently has almost 304 million people and is the world's third most populous, behind China (1.3 billion) and India (1.1 billion). China passed the 1 billion mark in the early 1980s. Nelson's projection assumes that current fertility rates remain constant but that longevity and immigration will continue to rise. Jeff Soule, director of outreach for the American Planning Association, hopes it will be provocative enough to inspire planners who anticipate development patterns and infrastructure needs to look beyond their lifetimes and localities. "We have to be more aggressive about looking out at the long term," Soule says. "It may get people thinking beyond their jurisdictions. … It's clear we have to think about such issues as food, water and basic transportation infrastructure." Nelson says China and India are accommodating billion-plus populations on less land area than the USA occupies. "We have a surprising amount of space in existing urban areas," he says. "We can easily triple the population in our urbanized areas with much of that growth occurring on, of all things, parking lots." Nelson advocates converting parking lots into commercial and residential buildings and extending light-rail lines and rapid transit to reduce dependence on cars. "We could accommodate half or more of the new population (on parking lots)," he says. "For the other half, we need to figure out which parts of urban areas need to be redeveloped. We should start asking these larger questions now." The population projection is provoking some skepticism. Robert Lang, Nelson's co-director at the Virginia Tech institute, says he expects immigration to decline, largely because birth rates in other countries are declining. "People are not going to have as many children, and their children won't have as many children, and there'll be (fewer) people to immigrate to the U.S.," Lang says. "I would rather focus on the near certainty that we will gain 100 million people by 2043. … No one plans for 100 years from now except to preserve a national park." Population projections for most countries do not extend much beyond 2050. Carl Haub, senior demographer at the non-profit Population Reference Bureau, has estimated that India's population could reach 2 billion around 2075. That won't happen, however, if India's fertility rates decline at a faster rate than they have been, he says. Nelson, who will become the founding director of the Center for the New Metropolis at the University of Utah this fall, says many events from disease to famine could throw his projections off course. "We could certainly have a comet hit the planet and pulverize the atmosphere," he says. "But what if none of these things happen? … Do we plan on a calamity, do we assume that half the population's planet might be wiped out? I don't think that's very responsible." What would a USA of 1 billion people be like? Would it be better or worse than the one we live in today? Enlarge AP file photo New York, seen here, has the second-highest population density of U.S. cities with at least 50,000 people, according to the 2000 Census. Only Union City, N.J. has more people per square mile, the Census found. Guidelines: You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. Read more