The new 10-mile section of I-66 inside the Beltway will, by and large, provide a fast, truckless, stoplight-free route in and out of Washington as well as an almost exclusively commuter road during rush hour for car poolers.

But expect to find some traffic backups at both the Washington end, where car poolers will be merging with other inbound traffic as I-66 approaches the Theodore Roosevelt Bridge, and at the Beltway, where inbound autos not carrying car pools must line up to exit, some highway officials warn. The bridge is already clogged with traffic from Rte. 50 and the George Washington Memorial Parkway, say District highway officials, who predict "a big mess" there when all that traffic begins jostling for space on the three-lane bridge.

Motorists may also be confused by the unusual pattern of I-66 entrance and exit ramps in Arlington, say state and county highway officials and the American Automobile Association. That's because some Arlington streets have entrances to I-66 but no exits, or have entrances and exits on only one side of a street, inaccessible from the other.

In fact, there is only one full cloverleaf interchange (at Rte. 7 in Fairfax County) on the new section of I-66. A cloverleaf, with traffic able to enter and leave from all directions, was proposed on I-66 at Patrick Henry Drive, but Arlington officials vetoed it, "because it would have destroyed too much park land" and required demolishing many houses, said John Hummel, chief Arlington planner.

Arlington similarly eliminated or restricted proposed I-66 ramps to other county streets, and made some streets one way to protect neighborhoods and funnel I-66 traffic onto major cross-county streets, Hummel said.

Unfortunately, no existing maps clearly show I-66 and the new ramps or indicate where motorists can and can't get on I-66. The free Arlington County map of Public Open Space, published last year, shows I-66 and the ramps but not in detail.

From near the Ballston Metro station, for example, I-66 traffic to and from the west must use Fairfax Drive, while I-66 traffic to or from Washington must use nearby Sycamore Boulevard. But Hummel sees it as a short-term problem.

"Motorists are smart, particularly commuters," he said. "They'll figure it out."

Out-of-state motorists, however, may continue to be confused by the odd assortment of exit and entrance ramps, Hummel said, and some may even think Virginia has overdone it when they encounter three interchanges all named Lee Highway, a route named after Confederate Gen. Robert E. Lee. The three Lee interchanges in less than seven miles are there only because Lee Highway--Rte. 29/211--meanders on its westward course, crisscrossing I-66 three times.

There are two Fairfax Drive interchanges as well.

Some of the ramp confusion may be eliminated when interchanges are numbered, although the number signs, such as the electronic system that eventually will control traffic flow, will not be ready until spring.

The I-66 interchanges will be numbered 18 through 26, with 18 being the Beltway and 26 Jefferson Davis Highway, just beyond Key Bridge (the numbering starts at the I-66 terminus at I-81, west of Front Royal). Inside the Beltway, there are four eastbound entrance ramps and six eastbound exits, five westbound entrance ramps and four westbound exits.

Even so, in at least one case the interchange numbers themselves may end up confusing rather than aiding motorists. One of the potential trouble spots involves the I-66 interchange ramps with Glebe Road and nearby Fairfax Drive. Those two streets, though blocks apart, will have the same interchange number (23). That's because, when the two ramps linking each road with I-66 are considered as a whole, they complement each other, becoming, in effect, a sprawling, largely complete, interchange that permits vehicles to enter and leave I-66 from most directions.

Interchange 22 also includes ramps half a mile apart on Sycamore Street and Lee Highway.

I-66 was not designed to permit Arlington traffic to have access to all directions at each interchange. In general, traffic heading north on Arlington streets that connect with I-66 can get on the highway only to go into Washington, not west. And traffic heading south on connecting streets can go west on I-66 but not into Washington.

The reason is simple, according to Donald E. Keith, division administrator for the Virginia Department of Highways: "I-66 is not a local commuter route. It's an interstate highway."

On Glebe Road, for instance, southbound motorists cannot use the I-66 ramp into Washington. "They're not supposed to," said Keith. "They can use Lee Highway if they want to go in town."

"There are lots of places you can get on but not off, or off but not on," said AAA spokesman Thomas Crosby. "It's an abnormal interstate, a hybrid, and many of the exits and entrances they do have are too close together. We expect there will be unforeseen impacts on Arlington neighborhoods" as commuters try to find the lines of least resistance, Crosby said.

Hummel insists that, if commuters attempt to take shortcuts along residential streets, the county is prepared to cut them off at the pass by restricting turns during rush hour.

"There is a maze of one-way streets and ramps for I-66 in Arlington, but that's at their request," said Donald Harris, assistant design engineer for the state highway department in Richmond. Harris believes that Arlington officials and residents, many of whom unsuccessfully fought I-66, "kind of cut off their nose to spite their face" by restricting access to it.

Hummel foresees few major traffic problems except around Rosslyn, however, where thousands of commuters use the existing Lynn Street ramp to avoid Key Bridge and go over Roosevelt Bridge, which empties directly onto Constitution Avenue. They now will have to use Key Bridge or other Potomac River bridges or form car pools to get on the Lynn Street ramp.

But few probably will want to use it anyway, since District traffic officials have predicted Roosevelt Bridge will be a mess when six to seven lanes of traffic--from I-66, the parkway and Rte. 50--converge into three, especially because there are no car pool restrictions on parkway and Rte. 50 traffic using Roosevelt Bridge.

The fancy computer-controlled ramp meters, road sensors, TV monitors and flashing lights of the "variable message signs," designed to make driving easier and safer on I-66, are expected to have limited impact on I-66 traffic because the interstate is not expected to have heavy traffic. Fewer than 1,000 car pools and buses an hour are expected, say Virginia officials, who anticipate no traffic jams except at the bridge.

Finally, there may be some slight confusion over restrictions on trucks, because the Virginia Department of Highways and Transportation and the National Park Service, which also prohibits trucks on the parkway, define a truck differently.

For I-66 purposes inside the Beltway to Washington, a truck is defined as any hauling vehilce with six or more wheels. On the Park Service's parkway, small six-wheel trucks are permitted if they are not in use for commercial purposes, according to National Park Service officials.