The boxes have long been used by car companies to assess the performance of their vehicles. But data stored in the devices is increasingly being used to identify safety problems in cars and as evidence in traffic accidents and criminal cases. And the trove of data inside the boxes has raised privacy concerns, including questions about who owns the information, and what it can be used for, even as critics have raised questions about its reliability.

To federal regulators, law enforcement authorities and insurance companies, the data is an indispensable tool to investigate crashes.

The black boxes “provide critical safety information that might not otherwise be available to N.H.T.S.A. to evaluate what happened during a crash — and what future steps could be taken to save lives and prevent injuries,” David L. Strickland, the safety agency’s administrator, said in a statement.

But to consumer advocates, the data is only the latest example of governments and companies having too much access to private information. Once gathered, they say, the data can be used against car owners, to find fault in accidents or in criminal investigations.

“These cars are equipped with computers that collect massive amounts of data,” said Khaliah Barnes of the Electronic Privacy Information Center, a Washington-based consumer group. “Without protections, it can lead to all kinds of abuse.”