One bridge is more than a century old, and 1,100 feet long. The other is about half that age and a few hundred feet shorter. Both are on city streets that carry tens of thousands of cars a day in and out of Boston, one over a highway and rail tracks, the other over a river.
One took less than six weeks to rebuild. The other will take five years.
The Commonwealth Avenue bridge project was that rare thing in construction: a big job, done fast. State highway officials used a technique called accelerated bridge construction, in which many of the main components are built off-site and then dropped into place by crews working in 24/7 sprints. Though finishing work will continue in the coming months, the hard part is already over.