That’s where things stood until last year, when FDOT notified the county of worsening conditions after inspecting the West Bridge in January and the Bear Cut Bridge in June. Both were built in 1944 and expanded in the early 1980s; the newer portions are structurally sound, the county says. The inspections are exempt from public disclosure under state law, to protect national security.

By October, FDOT recommended shutting down the most deteriorated westbound lane on the Bear Cut Bridge. The next month, the county and state agency agreed to reopen the lane, with weight restrictions for heavy vehicles. But once the restrictions went into place in December, they didn’t work. Some trucks had trouble getting supplies to and from Key Biscayne. And other trucks flouted the new rules and rode on the weakest lane, Antonio Cotarelo, the interim county engineer, said.

On Jan. 3, both lanes were shut down — a sudden move that appeared to have come out of nowhere. Mayor Carlos Gimenez said in an interview last week that, as late as last fall, he thought the necessary repairs for the bridges were not urgent.

“It wasn’t communicated to me at that time that the bridge had deteriorated that much,” he said. “What I was told about this issue back in the fall was that my public works folks thought that this was [coming] in the next two or three years.” Last week, commissioners on the transportation committee reluctantly gave initial approval to waiving competitive bidding for the bridge work. To finance the $25 million in repairs, Miami-Dade plans to issue bonds backed by Rickenbacker Causeway tolls, which would have to be raised to $1.75 from $1.50 for cars. According to the 2012-13 county budget, a causeway fund, supported by tolls on the Rickenbacker and Venetian causeways, has only $4.6 million set aside for capital improvements.

Zapata, one of several commissioners to say he will not sign off on the hike unless the county makes a long-awaited move from C-Pass to SunPass tolls on the causeways, said Tallahassee was partly responsible for the emergency rebuilding because FDOT did not inspect the bridges more frequently. He suggested the state should pay for some of the repairs — even though Pego, the FDOT district secretary, said his agency does not support “earmarks” for particular projects.

“It’s not an earmark,” Zapata responded. “I think part of the emergency situation was done because FDOT did not, I would say, properly inspect those bridges.”

A previous version of this article omitted the first name and complete title of Interim County Engineer Antonio Cotarelo.