The Interstate 395 high-occupancy/toll (HOT) lanes are seen near the Duke Street interchange in Alexandria. This is part of the 95 Express Lanes project using a 29-mile portion of the existing reversible HOV-3 lanes on I-95 and I-395. (Linda Davidson/The Washington Post)

The high-occupancy/toll (HOT) lanes in the Interstate 95 corridor in Northern Virginia will add an additional two miles next week.

The expansion in Stafford County is a small step toward bringing the Express Lanes farther south to Fredericksburg. Plans are underway to add an additional 10 miles of HOT lanes by 2022.

The new northbound and southbound ramps will open to traffic ahead of schedule on Oct. 31, Virginia transportation officials said. The $50 million project adds entrance and exit ramps south of Exit 143 at Garrisonville Road. About 146,000 vehicles travel on I-95 near Garrisonville Road each day.

The 95 Express Lanes opened three years ago, spanning 29 miles from just north of Garrisonville Road in Stafford to the vicinity of Edsall Road on Interstate 395 in Fairfax County.

They are part of a network of HOT lanes in Northern Virginia that includes 14 miles on the Capital Beltway, from the Springfield interchange to just north of the Dulles Toll Road.

And HOT lanes are set to open on Interstate 66 later this year. In that project, the state is adding 10 miles of toll lanes inside the Beltway; the lanes will be tolled only on weekdays during rush hours.

Officials say that in the five years since HOT lanes were introduced in Northern Virginia, residents have become familiar with the system and that many highway users already have the E-ZPass transponder required to use the lanes in their vehicles.

The complicated part, however, remains understanding how the toll system works.  In the I-95 and I-495 (Beltway) lanes, the tolls can change every 10 minutes in an attempt to keep travel speeds constant, varying with traffic volumes.

“Ideally what we want to do and strive for is to maintain the toll prices and change them so often that the lanes are, on average, free-flowing at about 60 miles an hour regardless of what happens on the regular lanes,” said Michael McGurk, a spokesman for Transurban, the company that operates the I-95 and I-495 lanes.

That means if the regular lanes are at a standstill, traffic in the express lanes is still moving — which means tolls are higher when demand is highest.

State transportation officials say the goal of the I-66 HOT lanes will be to maintain a minimum average speed of 45 mph, and the price will vary to control congestion to achieve that goal. There is no cap on how high tolls can go.

There have been reports of tolls topping $30 for the I-495 lanes.

“What the rates are doing at that point is they are trying to discourage anyone else from getting on the lanes,” McGurk said.  “So that will have the effect of clearing whatever is going on in the express lanes and not exacerbating any of the delays.”

An important distinction between the I-495 and I-95 HOT lanes and the new I-66 lanes will be the HOV requirements for a free ride. The minimum for the I-495 and I-95 HOT lanes is three riders; the I-66 lanes will be HOV-2, at least until 2022, when the lanes are to be extended into Prince William County.

This difference could create confusion come December.

“You are going to have people who accidentally leave the transponder on HOV mode coming from 66,” McGurk said. “That is definitely going to be a new dynamic when 66 opens.”

The express lanes last month hit 1 billion miles traveled, a major milestone for the system, which will mark the fifth anniversary of its opening next month. 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.

The extra two miles of express lanes on I-95 will open after more than a year under construction. The southbound lanes are to open to traffic on Oct. 31. The new northbound ramp is to open early on Nov. 1.

Officials say the extension will operate under the same rules and reversal schedule as the 95 Express Lanes. All drivers need an E-ZPass to use the toll lanes. Drivers riding with two passengers need an E-ZPass Flex set to carpool mode to ride free.