Annette Mills noticed the cryptic advertisement on the side of a city bus. Although the slogan 'Y6B: Coming October 12' had the ring of a millennium horror movie promotion, Mills, who runs the recycling program in Falls Church, Va., instantly grasped the sign's message.

On Oct. 12, the world's population will reach 6 billion.

Computer glitches that may occur at the turn of the century have received plenty of attention in recent months, but groups like Zero Population Growth, which came up with the 'Why 6 Billion?' campaign, and the Population Coalition, a group for which Mills volunteers as a discussion organizer, say overpopulation is a far greater challenge.

Suburban sprawl, species extinction, overcrowded schools and traffic congestion are directly related to runaway population growth, these groups say. By calling attention to the Oct. 12 global population watershed, they hope to broaden public awareness about the need for increased family planning, contraceptive research, the importance of population education in the schools and the need for a national population policy.

The United States, with a population of 273 million, is the third most populous country in the world, after China and India. Immigration and a low mortality rate contribute to an annual growth rate of 2.9 percent, putting it sixth in population growth behind India, China, Pakistan, Indonesia and Nigeria. By 2050, its population will be 349 million, the United Nations estimates.

Surging population also threatens to increase unemployment, according to the U.N.'s International Labor Organization, which estimates that by 2050 the global work force will increase by 235 percent in the world's 50 poorest countries, if population growth continues at its current rate.

While many people relegate population problems to the developing world, the issues are palpable in the United States, too, analysts point out. But it's hard to get people to make the connection between population growth and clogged highways, overflowing classrooms and disappearing green space, said Marilyn Hempel, executive director of the Population Coalition in Redlands, Calif.

'An awful lot of people are talking about those problems without talking about population,' she said.

The coalition, working primarily through local League of Women Voters chapters, hopes to change that with two projects. The first, Building Sustainable Communities, gathers citizens together to talk about the social, economic and environmental prerequisites of a healthy community. Mills is planning one in Falls Church in October to stimulate discussion about the effects on neighborhoods of lots of people consuming lots of natural resources.