"It's one thing if a crime has been committed or if there's a legitimate demand from law enforcement with a warrant," he said. "But it's another thing if the airport has decided to simply share this information with law enforcement for their own purposes."

The data collection not only affects travelers but also people who pick up their relatives and friends at the airport. And it comes at a heightened time for free speech protests at the airport: In recent months hundreds of people flocked to SFO to protest President Trump's travel ban affecting people from Muslim-majority nations and refugees worldwide.

Cagle says the new policy allows the airport to hold onto the information for too long.

"If data is kept at all, it should only be kept for days or weeks, at most, not placed in a multiyear database," he said.

The airport's license plate policy was prompted by legislation authored by state Sen. Jerry Hill (D-San Mateo) and signed by Gov. Jerry Brown in 2015. It went into effect last year.

Senate Bill 34 requires security protections for data collected by the plate readers and "public disclosure" about the use of the technology by public agencies, which have to post their policies online.

SFO emphasizes that the main role of its license plate readers is to help administer FasTrak accounts at its garages and to track "commercial operators," like taxis and limos, on its roads.

"It's important to remember that the primary purpose for the system that we've established here is for revenue collection," airport spokesman Doug Yakel said.

"Is there a correlation to law enforcement efforts? Of course," Yakel said. "It's a benefit to thwarting vehicle theft and other types of crimes. But that's really not primarily how it's used."