Long-awaited study concludes replacement of aging spans more cost-effective than rehabilitation.

CONCORD — In a report released Thursday night, the Army Corps of Engineers recommended replacing the aging Bourne and Sagamore bridges, with construction possibly starting in 2025. The recommendation came in a draft version of the Corps’ long-awaited Major Rehabilitation Evaluation Study, which weighed rehabbing the two 84-year-old bridges against replacing them.

The Corps, which owns and maintains the bridges, determined that the estimated $1.5 billion to replace them would be more cost-effective than the similar amount it would take to rehabilitate them.

As recommended, each new bridge would have four travel lanes, two in each direction, and an auxiliary lane on both sides. There also are plans for a median, shoulders and a separate bike and pedestrian lane, which the current bridges do not have. The lanes also would be widened from 10 to 12 feet.

The existing bridges would remain open during construction and be demolished after the new bridges are finished. The new Sagamore Bridge would be built to the west of the existing bridge and the new Bourne Bridge to the east of that span.

Replacing the bridges would have minimal disturbances to traffic during construction, according to the report. But the existing bridges would still need maintenance during that time.

Photo Gallery: The Bourne and Sagamore Bridges

The Corps estimated that it would need to acquire 11 acres for the new Bourne Bridge and about 4.5 acres for the Sagamore Bridge. The cost of acquisition, improvements and relocation of businesses was estimated at $7.8 million per bridge.

The Cape Cod Canal was built in 1916 to cut down on travel times for ships heading south from Boston. The bridges were built about 20 years later, at a time when the Cape’s population was about 26,000 people and vehicles were much smaller than the SUVs and trucks that cross the bridges every day now.

Both bridges are structurally deficient and do not meet modern-day standards.

“The existing bridges were constructed 84 years ago and require increasingly more frequent repair and maintenance, which is costly and causes significant impact to traffic crossing the Cape Cod Canal,” the Army Corps wrote in a statement.

With the release of the draft report, the Corps is kicking off a public comment period to start later this month. Public meetings will be held in Bourne, Plymouth, Boston, Eastham and Hyannis.

Major Rehabilitation Evaluation Report and Environmental Assessment by on Scribd

The Corps will review those comments and then finalize its report and give its decision, possibly by spring. That timeline could provide for construction to start in 2025.

The final report will be submitted to Corps headquarters in Washington, D.C,. for a decision in February. Public comments on the report will be taken through Nov. 1.

“I am pleased to see that the Army Corps. of Engineers’ report concludes that the best and most cost-effective way to manage the vital transportation link over the Cape Cod Canal is to construct two modern bridges to replace the Bourne and the Sagamore,” U.S. Rep. William Keating, D-Mass, said in a statement. “Two new bridges built up to modern standards will improve traffic flow, allow better access to the Cape and Islands in the coming years, and provide vital evacuation routes … As a year-round resident of Cape Cod who depends on the Bourne Bridge, I strongly feel that this is the best outcome for our Cape and Islands community and the greater Southeastern region as a whole. Our federal delegation stands together in pursuing funding in Congress for this important project.”

The Massachusetts Department of Transportation came to a similar conclusion about the bridges in May. After a five-year study, the state proposed bridges that were 138 feet wide — 90 feet wider than the existing ones. The department envisioned each bridge with six 12-foot lanes, a 10-foot-wide median, a 12-foot-wide shared-use path and a 6-foot sidewalk on either side.

The state also recommended revamping several of the roads leading to the bridges; bonds for that work were proposed in Gov. Charlie Baker’s $18 billion bond bill earlier this year.

A hearing on that bill is scheduled for Oct. 8, Cape Cod Regional Transit Authority Administrator Thomas Cahir said.

Multiple bills before Congress could help pay for the bridges, including a $5 million amendment in an appropriations bill that would pay for their design and a $1 billion bill that would bolster the nation’s evacuation routes, of which the bridges are a component.

Rehabilitation of the bridges was estimated to take 3½ years and result in temporary and full closures.

Other scenarios looked at in the Corps report include replacing replacing both bridges with a single bridge; the addition of a third automobile bridge not overseen by the Corps; replacement of the bridges with tunnels; and even filling in the canal.

Follow Ethan Genter on Twitter: @EthanGenterCCT.