Massachusetts will move toward what one congressman described as "a new chapter in Cape Cod's history" through an agreement with the federal government to replace the two bridges connecting the Cape to the mainland with wider, state-controlled successors.

Transportation Secretary Stephanie Pollack and U.S. Army Corps of Engineers Col. William Conde signed a formal memorandum of understanding Tuesday to embark on a yearslong joint project to replace the 85-year-old Bourne and Sagamore bridges, two economic and transportation linchpins that have been functionally obsolete for years despite connecting the roughly 215,000 Cape Cod residents to the rest of Massachusetts.

"These are more than bridges. These are lifelines," said Congressman Bill Keating, whose district includes both spans, in a video message played during the virtual signing event. "We need modern bridges that will deal with modern vehicles, modern commercial transport and open up passageways for bicycles and pedestrian traffic."

"This is indeed a milestone of significance," Keating added, standing outside with the Bourne Bridge towering in the background. "It's a new chapter in Cape Cod's history."

Under the agreement, the Corps will retain ownership and management of the bridges for the duration of the construction and demolition project. The federal government plans to pick up the tab, which the Corps previously estimated could range between $1.45 billion and $1.6 billion.

Once construction is complete and the new bridges enter service, MassDOT will take over their ownership and operation. State and federal officials described the switch as a more natural fit that allows the department to align its work on surrounding roadways with the highly traveled canal crossings themselves.

"All of us have driven over the Bourne and Sagamore bridges and noticed how narrow the lanes are, how close the oncoming traffic seems, that they're tough if you're on foot or on a bicycle," Pollack said during the event. "The bridges have served us well, but their days are numbered, and they need to be replaced."

"The path we've developed, to use a bad pun, is a terrific bridge to the future," Gov. Charlie Baker said at the event.

A 2019 MassDOT study found that, in 2014, average daily traffic was 56,500 on the Bourne Bridge and 65,900 on the Sagamore Bridge during the summer peak. Those figures are projected to continue to increase in the coming decades.

The agreement is a major milestone toward replacing two crossings that have been in place since before World War II, but several key details have not yet been settled.

Scott Bosworth, MassDOT's undersecretary, warned that the mutual journey will be "extraordinarily difficult."

Officials will need to settle on a final design. When it concluded in October that replacing the four-lane bridges is the best approach, the Corps suggested constructing bridges with four standard lanes of travel as well as two auxiliary lanes for entrance and exit and additional space for pedestrians and bicyclists.

Funding is also still uncertain. Baker administration officials publicly touted that both parties agree paying for construction and demolition will be a federal responsibility, but a MassDOT spokesperson clarified that the memorandum signed Tuesday does not formally bind any federal agencies to cover the costs.

State and federal partners will work on a finance plan, and how and when the roughly $1.5 billion would be appropriated is unclear.

The three members of the state's congressional delegation that joined Tuesday's event -- Keating, U.S. Sen. Ed Markey and U.S. Sen. Elizabeth Warren -- all pointed to a $1.5 trillion infrastructure bill that the House approved last week as a crucial step toward making more federal dollars available for much-needed projects.

Keating said the bill included amendments he filed that would allow the Corps to access a gas tax-funded pool of money for multi-year building projects such as bridges.

Markey and Warren called on the Senate's Republican majority to advance the bill, with Markey indicating it would direct at least $5.5 billion toward surface transportation projects in Massachusetts.

"Every Massachusetts resident knows that this project is long overdue, that we need these new bridges to improve safety, reduce traffic and boost the regional economy," Warren said. "But let's be clear: the Cape Cod bridges project is not a unique situation. We have significant infrastructure needs around the Commonwealth and around the country. The federal government must step up to fund new infrastructure."