“The investigators’ primary focus was on crash avoidance,” the N.T.S.B. statement said. “But in those instances when crashes do occur, they said the use of a helmet was the single most effective way for riders to reduce their chances of receiving a serious head injury.”

So what’s the problem?

For critics, the issue with the agency’s headgear recommendation was not necessarily that it promoted helmets but that it encouraged states to require them — perhaps by passing laws that would be enforced by tickets or fines.

The League of American Bicyclists, an advocacy group based in Washington, is opposed to that idea.

“We certainly promote helmets,” Ken McLeod, the league’s policy director, said. “Helmets do make individual bicyclists safer. We just think a mandatory helmet law is the wrong policy for federal or state governments to pursue.”

Mr. McLeod said that helmet laws might stop people from riding bicycles, which would make things like bike sharing programs unsustainable, discourage exercise and ultimately weaken the demand for other things that help cyclists, like protected bike lanes.

He said the laws could also be implemented in bad faith — for example, with the intent of keeping bicycles off the streets — and might be unevenly or unfairly enforced by local law enforcement agencies.

Jennifer Homendy, an N.T.S.B. board member, said the helmet recommendation was a small part of a very comprehensive report that devoted much of its focus to roads, drivers and cars.