Massport’s purchase of 18 acres in South Boston is part of a 3,100-foot freight corridor designed to siphon trucks traveling to and from the Conley Container Terminal off residential streets.

Massport paid $7 million last week to acquire three parcels off East 1st St. from the MBTA.

The land is needed to complete the two-lane Thomas J. Butler Freight Corridor and Memorial Park. The project will reroute truck traffic through the former Coastal Oil site and Exelon Corp. power plant parcel to Summer Street south of the Reserve Channel. Massport acquired the former Coastal Oil site in 2008. It has acquired one of two parcels needed on the Exelon property and is in the process of buying a second one, spokeswoman Miraj Berry said.

The bypass road is tied into the ongoing $35-million expansion of the Massport-owned Conley freight terminal to provide capacity for an additional 250,000 cargo shipping containers per year.

The project also includes construction of the 4.2-acre Butler Memorial Park, which is designed to buffer the terminal from residential neighborhoods.

The freight corridor is named after Massport’s late director of government and community affairs.

"This bypass road will create healthier living conditions and make the streets safer for residents while both providing an important link to the local and regional economies and honoring the legacy of a man who was so important to South Boston and Massport," Massport said in a statement released Thursday.

Construction is expected to be completed in 2016.

The three MBTA parcels were previously used for a power station that was dismantled in 2009.