BOSTON - The bad news for North Shore commuters is about to become official.

The Massachusetts Port Authority board of directors is set to formally hike the toll on the Boston-bound side of the Tobin Bridge toll from $2 to $3 - the second $1 increase in as many years.

But the board will also likely approve a 50-cent discount to the new $3 toll for frequent drivers who use the FASTLANE program. The discount is the result of a compromise between the authority and North of Boston lawmakers who have been fighting the proposed toll hike since word of it came out in November.

The toll increase will begin April 4 under a proposal the MassPort board will vote on Thursday, MassPort spokesman Jose Juves said.

The hike is projected to generate another $10 million to $11 million per year in toll revenue, Juves said. The money will be split "about 50-50" between helping to pay for the $14.6 billion Big Dig transportation project and long-term improvements and maintenance to the Tobin Bridge, he said.

"I'm happy with the compromise but it's not a total victory by any means," said Senate Democratic Leader Fred Berry of Peabody, who led opposition to the toll hike by organizing a boycott of recent MassPort hearings across the North Shore and by purchasing advertisements in local newspapers promoting the boycott.

The boycott was one of the first endeavors of the newly formed Northeast Legislative Caucus, which includes approximately 30 lawmakers from the North Shore and the Merrimack Valley.

"I don't deny MassPort needs the additional money to pay for the Big Dig," added Berry, co-chairman of the caucus along with state Sen. Thomas McGee, D-Lynn. "But I just don't see why the North Shore commuters have to foot the whole bill. MassPort resisted the discount all along and we had to really fight for it. But in the end they realized they had 30-plus legislators who weren't going to back down. I give them credit for throwing us a bone." To get the toll discount, drivers must use the FASTLANE program by purchasing a transponder device that is attached to their motor vehicle window. The program is already in use on the Tobin Bridge, but only to speed vehicles through an automated toll lane. Currently there is no discount. The transponder costs $27.50, but lawmakers are trying to get MassPort to make them available for free for a limited time to drivers who can prove they live on the North Shore.

State Sen. Steven Baddour, D-Methuen, the co-chairman of the Legislature's Transportation Committee, said the FASTLANE discount is currently available only on the Massachusetts Turnpike.

"MassPort is an independent authority that doesn't have to listen to us and generally doesn't," he said. "They could have ignored us. But they were bombarded every week by 30 legislators. This is a perfect example of why we need this caucus. This would not have happened without a united front." State Rep. Michael Costello, D-Newburyport, called the discount "extremely good news" but also said it is not the end of the issue.

"We were clear the discount is only a stop-gap measure," he said. "Commuters from the north and west pay for the rest of the state's ride into Boston. The South Shore and (Interstate) 93 drivers get a free ride. We need to find a way to make paying for the Big Dig fair to everyone."

While Baddour agreed the toll hike is unfair, he said its roots go back to legislation approved in the mid-1990s that dictate how the Big Dig is to be paid for.

"Tolls are the only means MassPort has for raising revenue," he said. Juves said the additional revenue would be used to cover the nearly $15 million the agency must contribute next year to help pay for the massive Boston transportation project. The toll revenue will also be used to pay for $68 million in bridge repairs and improvements over the next 10 years, he said.

MassPort first announced in November that it intended to increase the toll on the bridge to make it the same as the Sumner/Callahan and Ted Williams tunnels.

The Tobin Bridge spans the Mystic River and is the primary connection between the North Shore and Boston proper. Between 10 million and 11 million motor vehicles traverse the bridge annually, Juves said. Earlier this month, a committee consisting of state transportation officials, legislators and appointees of Gov. Mitt Romney concluded that the current formula for paying for the Big Dig is unfair to those who live in the North Shore or MetroWest regions of the state. Nevertheless, the panel did not endorse ideas for remedying those inequities.