If you feel like you're losing elbow room in the Lehigh Valley — whether in restaurants, in school, at the mall or on the road — you are.

In the last four years, the region has gained the equivalent of the city of Easton's population — 30,000 more people who now call the Valley home.

Experts did not predict this. In fact, the Lehigh Valley is likely to hit its population forecast for 2010 this year, a full five years early.

The growth is happening at a time when other major metropolitan areas in Pennsylvania are losing population or growing anemically. In fact, the Lehigh Valley is the fastest-growing metro region in Pennsylvania and among the top in the Northeast.

The surge is historic. Not since the 1920s has population increased this rapidly, not even during the baby-boom 1950s.

What's driving this population rise in a place viewed as a slow-growth region still fighting its way out of Rust-Belt depression?

Real estate.

Almost all of the increase comes from people moving here and fleeing high-cost metro areas to enjoy the benefits of the region's relatively low housing prices, which are the heart of the Lehigh Valley's modest cost of living. And unlike previous short-term spikes in population, this dramatic upturn may endure.

The population of the Lehigh Valley metro region, defined by the Census Bureau as Lehigh, Northampton, Carbon and Warren counties, soared by 4.14 percent from 2000 to 2004 to a total of 771,039. The bulk of that growth was in Lehigh and Northampton counties, whose combined population in 2004 was 609,000, the most current Census Bureau estimate.

From 2000 to 2004, the number of people living in Lehigh and Northampton counties grew:

An average of 1.25 percent a year. That may not sound like much until you realize that in each of the past two years, the region annually grew by about 8,800 people.

Five times faster than Pennsylvania and almost three times faster than the Mid-

Atlantic region of New York, New Jersey and Pennsylvania.

At a faster rate than the United States as a whole.

Migration mecca

The Lehigh Valley's last population explosion occurred five decades ago, mirroring the national baby boom after World War II. But the current population increase is altogether different.

Instead of a rise in births, it stems mostly from people moving into the area. Bedroom communities for the New York and Philadelphia metro areas are stretching their boundaries and converging on the Lehigh Valley.

This isn't the first time the Valley has been a migration mecca. The so-called ''New Jersey Invasion'' of 1989 saw hordes of Garden State residents fleeing high taxes and high housing costs, as Interstate 78 was completed through the Lehigh Valley.

This time, New Jersey Invasion II is bigger, augmented by a northward push from Philadelphia.