KEY WEST , Fla. — Officials in the Florida Keys announced what many coastal governments nationwide have long feared, but few have been willing to admit: As seas rise and flooding gets worse, not everyone can be saved.

And in some places, it doesn’t even make sense to try.

On Wednesday morning, Rhonda Haag, the county’s sustainability director, released the first results of the county’s yearslong effort to calculate how high its 300 miles of roads must be elevated to stay dry, and at what cost. Those costs were far higher than her team expected — and those numbers, she said, show that some places can’t be protected, at least at a price that taxpayers can be expected to pay.

“I never would have dreamed we would say ‘no,’” Ms. Haag said in an interview. “But now, with the real estimates coming in, it’s a different story. And it’s not all doable.”

The results released Wednesday focus on a single three-mile stretch of road at the southern tip of Sugarloaf Key, a small island 15 miles up Highway 1 from Key West. To keep those three miles of road dry year-round in 2025 would require raising it by 1.3 feet, at a cost of $75 million, or $25 million per mile. Keeping the road dry in 2045 would mean elevating it 2.2 feet, at a cost of $128 million. To protect against expected flooding levels in 2060, the cost would jump to $181 million.