If you've been wondering about the line of cars that backs up every weekend at the Dale City exit off Interstate 95, wonder no more.

The monster of factory outlet malls, Potomac Mills Mall, has finished its second phase of development. The last of the stores in the second-phase construction will open by the beginning of September.

The mall is virtually 100 percent leased, with 170 stores and more than 1.25 million square feet of gross leasable space.

But the mall, built by Western Development Crop, is not complete.

The second phase had been scheduled to be the last, but Stacy Hornstein, Western's development director for Potomac Mills, said a third phase is in the planning stages.

The mall is drawing 125,000 to 150,000 customers a day on weekends, he said.

Debbie Stotz, Western's spokeswoman, said the new phase, like the completed portions, will feature specialty and outlet stores.

The new phase, however, will focus on European shops. Plans for the third phase have not been completed.

Western also developed Georgetown Park in Georgetown.

Potomac Mills is one of several developments scheduled in Prince William County, which is benefiting from the growth of the Washington metropolitan area.

The Metropolitan Washington Council of Governments predicts an 82.4 percent population increase during the next 15 years. The average Prince William household last year had a median income of $34,205 and a median size of 3.26 people. Forty-five percent of the county's population was between the ages of 20 and 44.

Mall stores include many that are new to the area, including IKEA, the Swedish furniture and houseware store; Cohoes, a discount women's clothing store that has its headquarters in New York; and Wacamaw Pottery of North Carolina.

The mall also has 10 movie theaters and a discount-drug store owned by Peoples Drug Stores Inc.

In other developments in commercial realty:

Stanford Telecommunications Inc. leased 25,000 square feet from the Mason Hirst Cos. at Lake Fairfax Business Center in Reston.

Carroon & Black Inc., an insurance agency, leased 21,000 square feet from the Alan I. Kay Cos. at Three Bethesda Metro Center, Bethesda.