They are like quiet country roads, rising and banking, then dipping out of view, the serenity broken by nothing more than the occasional vehicle cruising through the soft turns. Traffic is so sparse that motorists - the few that there are - usually can't see the car ahead.
Yet these are anything but rural byways. Rather, they are the little-known and seldom used high-occupancy vehicle lanes of the Big Dig tunnel system, curving in and out of the city not far from the skyscrapers of South Station. When they were opened two years ago, with their very own tunnel under the Fort Point Channel, state officials predicted they would change the way Boston area drivers commute to work.
They've done nothing of the sort.
The roads sit largely unheralded and unused, given that the adjacent Interstate 93, the highway they are supposed to relieve, is rarely clogged by traffic in that area. The cost for these three miles of open pavement: an estimated $250 million.
Even during rush hour, traffic is sparse. A Globe reporter, watching the commute for an hour one morning, counted 181 cars and buses, or about three cars a minute, in the northbound I-93 lane, which takes vehicles either to South Station or toward Logan Airport. The southbound lane, which drivers can access near the Massachusetts Turnpike interchange, carried 122 cars and buses - about two per minute - during the same period.
The Massachusetts Turnpike Authority, which oversees the Big Dig, conceded that far fewer commuters are using the lanes than the 1,600 cars an hour they were designed to accommodate.
"That system was built looking towards the future, not the present," said Mac Daniel, spokesman for the Turnpike. "We acknowledge the lanes aren't being used as much as they could."
But for now it's a quarter-billion-dollar engineering curiosity - a lonely carpool lane used largely by buses, taxicabs, and limousines.
"You don't build something that's expensive and that isn't working and then say it's for the future. It doesn't make sense," said Senator Mark Montigny, Democrat of New Bedford, who is a member of the Transportation Committee. "It's like building a bridge to nowhere."
The high-occupancy vehicle lanes were conceived nearly two decades ago to allow the Big Dig to meet state and federal clean-air rules by encouraging shared commutes. At the time, traffic planners believed Americans would carpool in droves if given an incentive, such as special-access lanes, and fewer cars would mean less pollution. But by the time the Boston lanes opened, many of those same engineers had concluded that HOV lanes did little to ease traffic gridlock.
Since then, the Turnpike has paid scant attention to the lanes. A 1991 state law requires state highway officials to file regular reports on how many drivers are using carpool lanes on other parts of Interstate 93, but not within the Big Dig, said Joe Ferson, a spokesman for the Massachusetts Department of Environmental Protection. The Federal Highway Administration does not require states to keep track of HOV lane use unless local planners want to open them to solo drivers.
Turnpike statistics gathered at the Globe's request by using road sensors show the lanes carried an average of 59 to 167 vehicles per hour during the month of September, with heaviest use on the southbound portion of I-93.
General lanes on I-93 within the Big Dig carry 10 to 20 times as many vehicles, on average, according to Turnpike statistics. In addition, the Turnpike cannot say how much time, if any, a driver who uses the lanes will save during a rush-hour commute. But during a typical rush hour, traffic rarely backs up on the stretches of road that the HOV lanes are meant to relieve.
Because builders of Boston's HOV roads had to dig tunnels for some lanes and buttress others on overpasses, their estimated cost of more than $80 million per mile dwarfs the standard in other states, which ranges from $10 million to $30 million per mile. And because the lanes were designed to help cars and buses with a few specific shortcuts in mind - to Logan Airport and South Station - they do not offer much of an alternative to carpoolers who are trying to drive into the middle of the city.
In fact, their complexity is part of the reason many drivers find them daunting or confusing to use.
To access the lane heading northbound on I-93, drivers enter on the left side of the highway north of the South Bay shopping center. After less than a mile, the lane splits, with one exit toward South Station and another veering east toward Logan Airport. The latter road descends into a one-lane tunnel beneath the Fort Point Channel before merging with regular traffic near the entrance to the Ted Williams Tunnel.
Southbound drivers can enter a short stretch of the lane on I-93 either at South Station or just south of the Turnpike, but cannot access the Massachusetts Avenue exit if they do. Turnpike officials had lofty goals when the lanes opened two years ago. Matthew Amorello, then Massachusetts Turnpike Authority chairman, forecast dramatic improvements in the lives of commuters.
"We're confident that this change will get more people carpooling or taking the bus into Boston," he said at the time.
Now, Turnpike officials say the lanes were put in for the future, when congestion will presumably grow in the Big Dig. If engineers waited until things got bad to build carpool lanes, planners and engineers would need to carve out and secure land. Designers would have to reconfigure tunnels and overpasses. In other words, it would cost even more, Daniel said.
And one way or another, traffic planners believe the lanes will eventually attract drivers, either for carpooling or some other dedicated use.
"As time goes on and things get tougher and things get more crowded, people will find creative ways to use unused space," said Tony DiSarcina, a private civil engineer who worked on the initial traffic studies used to justify building the lanes.
DiSarcina said it was hard to determine how busy the lanes would be when they were designed in the 1980s, because engineers had no history of HOV lanes in the downtown area. Because of that, he said, the estimates were off, though he could not say how much.
Daniel said the Big Dig carpool lanes might someday connect with better-used carpool lanes north and south of Boston. But the state has no plans to build a connecting road and no money set aside that would pay for it.
In Boston, the biggest beneficiaries are bus passengers traveling in and out of South Station. The special ramps allow drivers to bypass downtown traffic by dropping passengers directly into the bus terminal or the airport.
"The time savings is absolutely incredible," said Bob Schwarz, executive vice president of Peter Pan Bus Lines, the private company that takes about 50 trips in and out of the station every day.
Taxicab and limousine drivers are also frequent travelers on the lanes, especially those portions that merge into the Ted Williams Tunnel toward Logan Airport.
Local officials did not decide to build the lanes on their own. Like most communities, Boston built them to comply with state and federal air quality standards. They are the most popular among several ways to meet those requirements because they can be, in some cases, cheaper to build than public transit, and, with enough carpooling, can reduce smog.
The lanes also helped the Big Dig secure federal antigridlock money - part of the package of federal aid that amounted to $8.549 billion for the road system. A Federal Highway Administration spokeswoman said she could not provide an exact amount that the Big Dig received for the HOV lanes.
Carrie Russell, a lawyer for the Conservation Law Foundation, said the Turnpike must do more to promote the lanes and pay attention to who is using them.
"They need to be monitoring them. If there's a problem with them, they need to be taking steps to make them work," Russell said. "This is part of the state's plan to come into clean air compliance."
But the effectiveness of even well-charted HOV lanes has been questioned by specialists. Many qualifying vehicles are made up of family members who would have traveled together regardless of whether they had a dedicated lane, according to Pravin Varaiya, an engineering and computer science professor at University of California, Berkeley who has spent four years studying their effectiveness.
"HOV, as congestion mitigation or encouraging carpooling, has just proved not to do that except in a few places," Varaiya said. "The empty lane syndrome is common."
After the Globe began asking about the lanes, Daniel said the Turnpike Authority would begin promoting the lanes more aggressively, with digital signs and a possible advertising campaign.
At this point, the lanes remain off the map to many drivers, even avid transportation promoters. DiSarcina, the engineer who helped conceive them in the 1980s, did not know they were open. And Michael S. Dukakis, the former governor who helped plan much of the Big Dig, did not know they were part of the project.
"I've never seen 'em," he said. "Where are they?"
Noah Bierman can be reached at email@example.com.