Rush-hour traffic clogs Interstate 66 in Vienna. (Marvin Joseph/The Washington Post)

The Interstate 66 toll lanes will change the way people travel in one of the region's most congested corridors. Virginia is betting the shift to the new system, set to open in December, will be so dramatic that it will push more commuters to try transit.

The rush-hour, peak-direction, high-occupancy toll (HOT) lanes will kick off a makeover of nearly 10 miles of highway from Route 29 in Rosslyn to Interstate 495. The goal of the multibillion-dollar transformation is to move more people in fewer cars.

A roadway expansion and toll lanes on another 22.5-mile stretch from the Beltway to Gainesville will follow.

When tolling begins, every vehicle, except motorcycles, will need an E-ZPass transponder to use the new HOT lanes. Solo drivers who are willing to pay will be able to use the lanes during the morning and afternoon rush hours, a change in a decades-long restriction to keep the peak-direction lanes open only to cars with two or more occupants. Tolls will fluctuate based on congestion. The lanes will continue to be free for carpoolers who have a toll transponder set to HOV.

"This tolling operation is going to be set up. It is not a question of 'if' anymore, it will happen," said Fairfax County Supervisor Jeffrey C. McKay (D-Lee).

"It will change people's commuting patterns, said McKay, who is chair of the Northern Virginia Transportation Commission. "And now is the time to consider an alternative, to test it, to time it and to make sure you have the route that best works for you."

The state's goal is to encourage more carpooling, and even better, move them to transit. About 41 percent of morning rush-hour commuters in the corridor — not just on I-66 — are on transit, according to the commission. A fraction are on local and commuter buses and on Virginia Railway Express commuter trains, whose Manassas line service started in 1992.

The biggest share of transit commuters use Metro. The Orange Line became a vital mode of transportation for residents in the corridor when it was extended just over nine miles to Vienna, opening four stations along I-66 in 1986.

Three decades later, the tolling system yields another defining moment for commuters.

"For the first time, really since Metro came to I-66, people should probably take the time to consider transit and whether it can improve their commutes and bottom-line financially," McKay said.

The state has spent $10 million to improve public transportation in and around the I-66 corridor, including expanding transit capacity. The effort includes new bus routes that will take commuters from as far as Gainesville to the Pentagon via I-66, and from Fairfax to Foggy Bottom.

These investments, officials say, will boost capacity to carry an additional 5,000 people on transit through the corridor when tolling begins. The projects range from additional rush-hour trips on bus routes that parallel I-66, to bus stop improvements in Arlington, to a new 250-space park-and-ride in Loudoun County.

But will the changes be enough to persuade drivers to give up their cars?

Some motorists say the available options, chiefly Metro, aren't reliable enough for the state to make a strong case for transit. Bus service is limited in neighborhoods — particularly in the outer suburbs where residents often have to drive to a park-and-ride facility to catch a bus. Commuter buses are mostly intended to serve the rush-hour, peak-direction crowd, making them inconvenient for those with unpredictable work schedules or irregular hours.

Still, transportation officials are hoping the variable tolling — which based on experience with the I-495 express lanes means tolls could top $30 — will discourage solo driving and encourage commuters to try transit or carpooling.

"Congestion is so bad and the potential prices will be such that people will seek out these options," said Stewart Schwartz, executive director of the Coalition for Smarter Growth. "But people need to know these options are available."

A portion of the I-66 toll revenue will go to supporting more carpooling and more commuter bus service.

In Fairfax, a new express bus line will provide a one-seat ride for 400 people to commute from the county Government Center park-and-ride to the State Department in Foggy Bottom each morning and the same number back to Fairfax in the evenings. The county is proposing to launch another bus line along I-66 after tolling begins.

In Prince William, a new bus route from Gainesville to the Pentagon launched in December in preparation for the changes in the corridor. Ridership has not picked up, with only about 200 passenger trips daily, with many of them riders who switched from other routes. But officials say they expect ridership to grow as the tolling begins.

The transition also could boost the corridor's slugging culture. Slug lines, or instant carpooling, is a Washington area commuting tradition in which drivers who want to use HOV lanes stop to pick up passengers along the way. Thousands of people slug in the I-95 corridor, but the practice is not as well developed in the I-66 corridor.

"This is an opportunity for people to reassess how they are commuting," said Tom Biesiadny, director of Fairfax County Department of Transportation.

A second round of projects is in the works for the area, according to the Northern Virginia Transportation Commission, which is managing the transit investments.

Under a 40-year agreement with the state, the commission may provide funds to operate new or enhanced service for up to eight years. The goal is to increase capacity of transit in the corridor to get cars off the interstate.

The key now, McKay said, is to get commuters to think about whether they want to adjust their routine before the tolling actually starts. Past experiences has shown that changing people's driving patterns isn't easy.

"Most people aren't going to even give the research that is necessary to even weigh the pros and cons of that until tolling starts and hits their pocket," McKay said. When they do, he said, they have to find robust, convenient and affordable transit options.

"Some people will look at this only once and if the transit service doesn't meet their needs, they will probably just suck it up and keep driving."