Northern Virginia's first major coliseum, the 10,000-seat arena of George Mason University, is nearing completion beside the young state university's main gate on Braddock Road south of Fairfax City.

The $16.7 million arena, to house school and public events, is expected to become a landmark for the fast-growing commuter university, which has been adding 1,000 students a year and has an enrollment of 15,500.

But the new arena is worrying area officials and residents who are already upset about traffic on Braddock Road, one of the region's most congested roads.

The Harlem Globetrotters and the new Walt Disney Ice Show have inquired about coming to the arena after it opens next spring, bringing visions of massive traffic jams to some residents.

"I think traffic's going to be a mess . . . on Braddock, Rte. 123 and in Fairfax City , especially the first few times they have events at the arena," said Fairfax City Assistant City Manager Jon Ellestad. "We're working with the university to minimize the traffic, but nothing will eliminate it."

Paula Patterson, who moved five months ago to the Kings Park West town house subdivision on Braddock Road opposite the university, said she thinks the traffic "will be horrendous."

But Fran Smith, also of Kings Park West, said, "I'm 100 percent for the university . . . and I don't think the arena can make the traffic here any worse."

In an effort to avoid major traffic tie-ups, the university already has announced it will schedule arena events only after evening rush hour and normally not on weeknights when university classes are in full session. The arena will have parking for 3,200 cars.

GMU also convened an area transportation task force, which concluded this past summer that traffic will be eased significantly in the future -- but not for several years -- by the coming of the Metro to Vienna, construction of the Springfield Bypass and the widening of Braddock Road.

Fairfax County's top transportation planner, Robert L. Moore, does not think the arena will greatly affect area traffic, "depending on the time of day events are scheduled."

If any event takes place during rush hour, it would have "a significantly adverse effect," he said. But regardless of the arena, Moore said, the two-lane sections of Braddock Road and Rte. 123 will have to be widened.

Virginia highway officials said widening of those sections, between Rte. 123 and Guinea Road, is scheduled to begin in 1987, although $1 million is to be spent next year to buy the right-of-way.

Fairfax City, which in the mid-1970s vetoed state plans to widen Rte. 123 in the city, is doing a comprehensive traffic study that includes looking at major changes to Rte. 123.

"We're willing to look at and discuss anything and everything" that may help relieve traffic problems in the city, Ellestad said.

George Mason was founded in 1957 as a two-year college of the University of Virginia. It moved to its present campus in 1964, became a four-year university in 1972, and has 8,000 full-time and 7,500 part-time students. The university is on state-owned land and does not need county approval to construct its buildings.

The new arena, with 9,200 theater-style permanent seats, and another 800 bleacher seats, will provide GMU's basketball team with a handsome home -- larger than the 9,000-seat gym of its sister University of Virginia in Charlottesville and comparable to the University of Richmond's Robins Center.

It also will bring school graduations indoors for the first time. Two recent graduations took place in the rain, said Donald J. Mash, university vice president for student affairs, who is overseeing the arena construction.

But the arena also is expected to host concerts and public events to help pay off its revenue bonds. The 1985-86 Eastern College Athletic Conference Southern Division basketball playoffs already are booked into the arena. No state funds have been used for the arena or for other university athletic facilities, the student union or dormitory buildings, Mash said.

The arena will be large enough to hold ice hockey, soccer and even a small circus, Mash said. It will be one of the largest arenas in Virginia and will have just more than half the capacity of the Capital Centre's 19,000 seats for basketball games and about the same capacity as the D.C. Armory or the D.C. Convention Center. The University of Maryland's Cole Field House in College Park holds 14,300.

The GMU arena will be open to rock concerts too, despite a controversy last winter when university President George Johnson told students he did not want rock groups such as "Meatloaf," which he said he'd seen spit on audiences.

Mash said yesterday, "We don't want groups that bite the heads off chickens and weird stuff, but the arena will be open for rock concerts."

A university policy board, with student representation, will be formed to advise the stadium organization on what groups are acceptable.

A professional stadium organization is expected to be selected to manage the new facility. Bids of three firms are under review by university officials: the Capital Centre in Prince George's County, the firm that operates Philadelphia's Spectrum and Pittsburgh's Three Rivers Stadium, and the firm that operates both the New Orleans Superdome and the Long Island Coliseum where the New York Islanders play.