Traffic seen in a recent view at rush-hour along Interstate 66 in Vienna, Va. (Marvin Joseph/The Washington Post)

The commute of thousands of Northern Virginians will change dramatically at the end of the year when tolling begins on a significant stretch of Interstate 66, one of the region’s most congested highways.

Rush-hour, high-occupancy toll lanes are scheduled to open in December, kicking off a major makeover of the nearly 10 miles of highway from Route 29 in Rosslyn to Interstate 495. Additional transit options also are planned as part of the state’s multimodal strategy to improve traffic flow along the corridor.

The addition of the I-66 lanes will mean about 64 miles of Northern Virginia highway operating under a toll system all or part of the time.

“We are really at our limit of adding any more pavement to 66 inside the Beltway,” said Amanda Baxter, the Virginia Department of Transportation’s manager for the project.

“The roadway is what the roadway is, and it only can handle so many cars at one time,” she said. “We are giving people options.”

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, an important change in a decades-long restriction to keep the peak direction lanes open only to cars with two or more occupants. When tolling begins, those high-occupancy vehicles can continue to use the highway free but will need a toll transponder that can be set to carpool mode.

Hybrid drivers will lose a privilege to drive solo in HOV lanes free and will be tolled if riding alone.

“The biggest push right now is getting people to understand that if you do not have an E-ZPass and you need to be on these lanes only during rush hour, you need an E-ZPass,” Baxter said.

About half of the vehicles that now travel on that portion of I-66 are equipped with E-ZPass transponders, officials said.

The lanes will operate similarly to those on the Capital Beltway, where tolls fluctuate based on the level of traffic congestion. The 495 express lanes, however, operate 24 hours a day and have parallel regular lanes. I-66 inside the Beltway will be all toll lanes during peak hours in the peak direction, but weekdays only.

The express lanes last month hit a billion miles traveled, a milestone for the system of toll lanes that opened five years ago on I-495, and two years later on Interstate 95. Officials and experts say the lanes have performed as expected, providing a quicker and more reliable trip for toll payers and carpoolers than do the regular lanes. Many users choose the lanes to commute to work, but a sizable number of cars, particularly on I-95, use the lanes for vacation travel.

A May survey by the express lanes operator, Transurban, found that nearly 8 in 10 drivers in the D.C. region say the express lanes benefit the region. Still, users have generally been suspicious about the congestion-based toll system and complained about price gouging.

As part of the transition for the I-66 lanes, rush-hour periods will be extended 90 minutes in the morning and the evening, which means solo commuters who currently travel on I-66 just before or after the HOV restrictions go into effect will have to adjust their trips around the expanded peak hours if they want to avoid paying the toll. The new peak hours will be 5:30 to 9:30 a.m. eastbound and 3 to 7 p.m. westbound.

For commuters, adjusting to the new requirements and rules may be the biggest challenge in coming months. This will be the first rush-hour-only, peak-period toll system of its kind in the United States.

"Driving in HOT lanes is about the most complicated thing we ask local commuters to do," said Robert Thomson, who wrote extensively about the I-66 toll plans in his former role as The Washington Post's Dr. Gridlock. "You need to have an E-ZPass — the right kind of E-ZPass — and you need to know the hours the HOT rules are in effect and you need to be aware of how 'dynamic tolling' works, meaning the toll can vary from time to time and day-to-day with no upper limit."

If the toll plan works as intended, he said, the end result should be better traffic flow even when the lanes open up to solo drivers. Success depends heavily on multiple factors, including on persuading today’s drivers to move to carpooling and bus alternatives.

State and local officials last month launched $10 million in transit projects they say will lead to moving as many as 5,000 additional people through the corridor after the tolling begins. Three new bus routes will be added, and service will increase on two existing routes. A new park-and-ride lot in Loudoun County will support current and future bus service. Some toll revenue will be used for more transit improvements.

“These and future transit improvements working in tandem with the I-66 Express Lanes will help us relieve the congestion that impacts our quality of life and the economy, while achieving our ultimate goal of moving more people through the I-66 corridor,” Virginia Secretary of Transportation Aubrey Layne said.

Testing of the tolling system began this summer, soon after workers wrapped up installation of nine toll gantries and dozens of signs over and around the interstate.

Drivers are passing under the gantries, which house cameras and electronic equipment to communicate with E-ZPass transponders. The system has been tracking how efficiently the gantries are reading toll transponders. The cameras that will take photos of the rear license plates of cars without an E-ZPass have been flashing. In coming weeks, drivers should notice more illuminated signs with alerts about the tolling system and crews on the side of the road configuring computer equipment.

As part of the effort to expand capacity, an eastbound lane will be added next year from the Dulles Toll Road to Ballston. That expansion is expected to alleviate the biggest chokepoint for Washington-bound traffic, at the toll road where about 70 percent of traffic merges moving into the District and Arlington.

On I-66 outside the Beltway, a $2.3 billion expansion will add toll lanes by 2022. That project spans 22.5 miles from the Beltway to University Boulevard in Gainesville in Prince William County.

Peter Hesse uses I-66 to commute from his home in Chantilly, in Loudoun County, to downtown Washington. With his electric Ford Fusion, he’s used to being able to drive in the HOV lane. Come December, he’ll have to pay for the privilege.

“How much money out of my paycheck goes toward tolls in traffic becomes the challenge,” said Hesse, an information security professional. “What percentage of my income and my time and my family is it worth?”

He said he worries that the tolls will be unaffordable. On the 495 express lanes, there have been reports of tolls upward of $30.

“You have to make more money than I do to not have that really bother you,” he said.

Hesse said he may explore other options, including Route 50. But none of the other options are very appealing. Interstate 66 is the most direct east-west route between his home and downtown.

State transportation officials say they don’t anticipate many people will shift their commute to already congested nearby roads such as Routes 7, 29 and 50, according to data they’ve collected during the testing period.

Thomson said the state’s calculation is that “more commuters will be drawn to the HOT lanes than will be repelled by them, resulting in a net gain in overall traffic flow along alternative routes.”

The key question is whether such a system will lead to moving more people through the corridor, or just more vehicles. Virginia is betting the system will provide more support for people who choose to carpool or take commuter buses.

“That strategy is a key to success — if it is a success,” Thomson said.