BOSTON — State transportation officials are considering whether to keep the carpool lane along Interstate 93 open to general traffic for the next two years to help alleviate traffic congestion from the Tobin Bridge project.

The Tobin Bridge, a primary access point to Boston for those driving on Route 1, is seeing heavier-than-usual traffic backups as crews begin a two-year renovation of the aging landmark. Commuters have been finding alternative routes in and out of the city, which has increased traffic on I-93 and other highways.

To help relieve the congestion, the state Department of Transportation opened the high-occupancy lane between Medford and the Zakim Bridge in Boston to all motorists, regardless of how many people are riding in a vehicle.

The switch is meant to be temporary, but MassDOT officials said Monday they are considering keeping it open to traffic for the duration of the bridge project.

"This is strictly a temporary measure and there is no plan or intention on our part to leave it in place long term," Highway Administrator Jonathan Gulliver told a meeting of MassDOT's board of directors on Monday. "It could be as long as two years because the intention is to alleviate traffic predominately associated with that Tobin Bridge/Chelsea Curves project."

But the HOV lane closure is drawing criticism from environmental groups and transportation advocates who say the move is wrongheaded.

Chris Dempsey, director of the advocacy group Transportation for Massachusetts, said the state needs to analyze traffic data to figure out if the lane closure is adding to commuter woes.

Dempsey said the data will help determine whether the I-93 lane closure "has slowed down buses and other HOVs that once had this lane reserved for them."

Gulliver said the state will conduct vehicle counts along I-93 beginning this week to gauge what the impact on traffic congestion has been, and analyze if it is working.

"As it stands, we don't have the data to make a definitive decision," he told the board. "It's strictly anecdotal at this point."

Transportation Secretary Stephanie Pollack said the decision to close the lane was based on MassDOT data suggesting that the move would alleviate congestion during the project.

"The modeling we did showed that there was capacity in the lane, and that we could add traffic without slowing the lane down significantly," she said Monday. "But modeling is not always right, so if the real data indicates that we are slowing down HOV users, we need to know that."

The 2.6-mile HOV lane north of the city is one of two sections of the highway designed to encourage carpooling during peak hours. The other is along a five-mile stretch of the I-93/Southeast Expressway between Furnace Brook Parkway in Quincy and Morrissey Boulevard in Boston.

Travel in the HOV lane north of the city is usually restricted between 6 and 10 a.m. on weekdays. During those times, vehicles must have at least two occupants to use the lane.

On the Southeast Expressway, so-called "zipper lanes" shift for the morning and afternoon commutes, allowing HOV travel northbound from 5 to 10 a.m. and southbound between 3 and 8 p.m. on weekdays.

The Tobin Bridge's two-year rehab will involve lane closures on the northbound and southbound sides of Route 1.

Two of the three northbound and southbound travel lanes will be open during the day, and one of the three in each direction will be open overnight.

The Massachusetts Bay Transportation Authority is encouraging people to use the commuter rail and offering free fares for inbound trips on Silver Line 3, between Chelsea and South Station, as well as additional Blue Line capacity. Officials are also encouraging commuters to use the Haverhill or Newburyport/Rockport rail.

MassDOT said crews are using accelerated measures to speed up the maintenance project, which is expected to wrap up by 2021.

Monica Tibbits-Nutt, a member of the MassDOT board and the T’s Fiscal and Management Control Board, said the HOV lane closure defeats the purpose of the state's carpooling program.

"It seems to fly in the face of what we’re trying to do — ease congestion," she said at the meeting Monday. "This is causing significant delays for our bus services."

Christian M. Wade covers the Massachusetts Statehouse for North of Boston Media Group’s newspapers and websites.