Loudoun County officials have approved the jurisdiction's first major regional shopping mall, at Rtes. 7 and 28, and say they believe it will provide the area with long-awaited consumer convenience. But some officials also fear that it will worsen already nightmarish traffic.

After nearly 13 years of discussions and a year of intense bargaining, county officials said they have worked out a detailed plan with Lerner Corp., the developer of the Windmill regional shopping center, which includes more than $8 million worth of road improvements and equipment for the county's volunteer fire and rescue units. The officials said they hope the road changes will reduce some of the frequent traffic jams many shoppers experience at the Fair Oaks and Tysons Corner regional malls in Fairfax County, which for a long time have been the two major shopping malls for most Loudoun residents.

"There wasn't ever a question of whether there would be a regional mall in Loudoun one day, but it was a question of where and when," said county Administrator Philip A. Bolen. "Fourteen years ago, the time wasn't right."

But now, with the county's population of 70,000 expected to reach 117,000 by the year 2000 and major local and national companies developing huge residential, office and industrial parks along Rte. 7, county officials said they believe the county needs its own major shopping mall.

"We bargained hard with the applicant {Lerner} to make sure they gave us as much as they could," said Steve Stockman, a county supervisor. "But you never have enough proffers. We're hoping this project will set the standard for other developments in the county."

Last month, the county's Board of Supervisors voted 6 to 2 in favor of Lerner's Dulles Town Center project, which encompasses the Windmill shopping center and an adjoining office and industrial park. Stockman and the other supervisors voting for the development said the key factor in approving the project was making sure Lerner committed itself to improving Rtes. 7 and 28 and nearby roads to make sure the shopping mall and office park are easily accessible. They also wanted to ensure that the additional traffic would not cause further problems at the heavily used intersection.

Supervisors Ann B. Kavanagh and Thomas S. Dodson, who opposed the project, sought several road improvements beyond what Lerner and the majority of supervisors agreed to.

Dodson said he is concerned that the road improvements won't do enough to maintain a free flow of traffic from Rte. 7 in and out of the mall and that a better location for the mall might have been off Rte. 7 along the proposed extension of the Dulles Toll Road.

"I was looking for more, not only in terms of dollars, but a more practical solution to the transportation situation," he said.

Kavanagh said the county does not fully understand the impact of the future traffic, particularly at the interchange of Rte. 7 and Countryside Boulevard.

"It's clearly detrimental to the traffic flow on Rte. 7 and very detrimental for the community of Countryside," she said.

The 1.2-million-square-foot mall is not expected to open until 1993, but much of the road work is scheduled to begin within the next year. The road construction includes a major interchange at Rte. 7, Rte. 28 and Algonkian Parkway, the widening of Rte. 7 and the addition of several roads within the 554-acre tract.

Dulles Town Center will be slightly smaller than both the 19-year-old Tysons Corner Center at Rtes. 7 and 123 and the seven-year-old Fair Oaks Shopping Center at Rte. 50 near I-66.

Tysons and Fair Oaks are each about 15 miles from the Dulles Town Center site and each has more than 200 shops, along with several major department stores. Tysons and Fair Oaks each have Hecht's, Woodward and Lothrop and Garfinckel's stores. Tysons also has Bloomingdale's; Fair Oaks has Lord & Taylor, Sears and J.C. Penney's.

Arthur N. Fuccillo, general counsel for Lerner Corp., said the company expects Windmill will have three to five department stores and at least 150 smaller clothing stores, restaurants, movie theaters and other shops. When completed by 1995, the office and industrial park is expected to have 5 million square feet of space and an additional 1 million square feet of retail space.

The mall alone is expected to bring 4,300 new jobs to the county and generate more than $126 million in tax revenue in the first 20 years of operation. Fuccillo said Lerner expects to draw shoppers from western Fairfax, Loudoun and as far away as West Virginia.

But some local business owners said they are concerned that the mall will draw sales from the county's existing shops, especially in downtown Leesburg, which has numerous small specialty shops and restaurants. New shopping malls often force stores in small downtown locations to close, in part because shoppers have a greater selection of stores under one roof at a mall.

Norman Myers, owner of White's of Leesburg, a women's clothing store, and Clozone, a children's apparel shop, said he believes the mall will affect some shop owners in Leesburg, but not enough to cause severe problems. Downtown Leesburg has "charm and personality," unlike most shopping malls, and will continue to maintain a strong selection of high-quality shops, he said.

Betty Tatum, chairman of the county's Board of Supervisors, said she does not want another strip shopping center in Loudoun.

"We want to offer residents an alternative to Tysons Corner or Fair Oaks," she said. "We don't feel the mall is going to add more to our existing transportation problems because none of us wanted a situation at Rte. 7 and Rte. 28 where you were sitting in traffic like you do on Rte. 50 when you're trying to get out of Fair Oaks."

Pamela Wev, the county's executive director of economic development, said that the mall is clearly needed in the county and that the road improvements will help relieve some of the traffic. She said there is clearly a market for a mall in Loudoun, based on the county's rapid population growth and affluence. By the time the mall is opened, there will be at least 16,000 more housing units in the county, she said.

The county's population rose from 57,427 in 1980 to 70,305 last year. It's expected to hit 81,000 by 1990 and 117,000 by 2000. Median family income is $43,393, ranked in between Fairfax County ($45,976) and Montgomery County ($41,385) in the Washington area.