Plans to implement the technology are under way in cities from San Francisco to Hartford, Conn., and Eugene, Ore., to Columbus, Ohio.
Linked to video cameras already in wide use, the microphones will offer a formidable new tool for security and law enforcement. With the new systems, experts say, transit officials can effectively send an invisible police officer to transcribe the individual conversations of every passenger riding on a public bus.
But the deployment of the technology on buses raises urgent questions about the boundaries of legally protected privacy in public spaces, experts say, as transit officials — and perhaps law enforcement agencies given access to the systems — seem positioned to monitor audio communications without search warrants or court supervision.
“This is very shocking,” said Anita Allen, a privacy law expert at the University of Pennsylvania. “It’s a little beyond what we’re accustomed to.”