The sun has set, and at the intersection of Rte. 7 and 10th Street, things are going slow. A steady stream of headlights filing into the business district stretches several hundred yards from the stoplight: past the Safeway, past the tractor dealership, almost to The Seashell restaurant.

It is 6 p.m. in Purcellville, population 1,600, and it is rush hour. Seriously.

This may not be Tysons Corner, but townspeople will tell you if you have a minute or 30 that Purcellville, in western Loudoun County about 60 miles from Washington, does not have small-town, oh-just-something-to-complain-about rush hours, it has authentic, blast-the-horn, "%$&!-I'm-late" rush hours. And it has them twice a day, five days a week.

"The roads here are terrible," said Purcellville resident Bonnie Hutchison. "I go out after 9 in the morning and make sure I'm back by 3, because I don't want to get stuck in traffic."

Crossing the street can be a test of will.

"You try it at rush hour," dared Diane E. Gavura, owner of The Quilting Niche. "We play a game of chicken every day, just to get from one side to another."

The reason: Rte. 7, a major highway into the Northern Virginia suburbs from Winchester and points west, runs through Main Street.

To the surprise of many and the deep disapproval of some, steady residential growth has come to western Loudoun, Clarke County and even to some sections of West Virginia. The result is a heavy volume of commuter and commercial traffic through Purcellville. It starts building as early as 5:30 a.m.

The problem is so bad that contractors hired by the state are to begin work this week on a four-mile bypass carrying Rte. 7 around Purcellville. The bypass, similar to the one that carries Rte. 7 around Leesburg, will cost between $6 million and $10 million before its completion, expected in the fall of 1987, according to state traffic engineer Thomas Butler Jr.

The need for the bypass underscores the seemingly boundless nature of growth around the Washington area. Increasingly, employers, such as the Xerox Corp., which is building a multibillion-dollar facility near Leesburg, are moving into rural areas that once were primarily farm land or residential.

And, as it happens, employes are not abandoning their commutes; they simply are moving further out.

"It used to be that people lived in Leesburg and Purcellville and worked in Washington. Now they work in Leesburg and live in West Virginia," said James Bowman, president of the Purcellville Businessmen's Association. The reason, Bowman said, is that the cost of living in outlying areas is significantly lower.

Although Purcellville residents say that they will not miss the heavy traffic going through the middle of town, many are skeptical that the bypass ever will be completed.

"I'll believe it when I see it," said Hutchison, who lives with her husband Jack on a seven-acre plot outside Purcellville. The plot has been in his family since the turn of the century, Bonnie Hutchison said, and part of it has been sold to the state to carry the bypass.

" The state bought the land, sold it back, and bought it again," she said. "I'm getting tired of dealing with it, to tell you the truth."

The bypass, planned since the early 1970s, was delayed because of funding shortages and regulatory hurdles, Butler said.

Many residents, especially merchants, are ambivalent about how the bypass will affect Purcellville. Gavura, for example, said she despises Purcellville's traffic as strongly as anyone. But she wonders, in the next breath, whether the decline in traffic will mean a slump in her store's business.

"It kind of leaves me, 'Is this going to help me or is this going to hurt me?' " she said.

Bowman said he is confident that the bypass will help Purcellville business, because customers no longer will need to fear traffic jams when they go to shop in town.

Although Purcellville's traffic problems are real, some Northern Virginia officials are prone to ask how bad they actually are.

Western Loudoun, despite recent growth, still is primarily rural. And the fact that such an area gets millions of dollars for the Purcellville bypass on a primary road like Rte. 7 rankles suburban officials, who contend that they have more serious problems on secondary roads that the state is neglecting.

"It's not so much that Purcellville is getting money, it's a question of how the state assigns priorities," said Fairfax County transportation chief Shiva Pant. "About $10 million is being spent to carry 8,000 cars a day around Purcellville. We've got residential streets that carry more than that."

But, retorted Bonnie Hutchison: "We've been waiting 20 years for this road to be built. Those roads in Fairfax County weren't even there 20 years ago."

Butler noted that, if traffic volume were the only criterion the state used in selecting road projects, almost no problems outside of Fairfax County would be addressed.

For her part, Gavura yearns for the day when the need for a bypass would not have been debatable, it would have been absurd. The growth that the bypass symbolizes and is likely to bring more of is something that she could do without.

"You're talking to an old-schooler," she said. "I was born and raised here, and I don't like to see the growth. I like to see the farms and the open fields."