The head of the top US road safety agency cautioned autonomous vehicle developers in a letter Wednesday about what he called an “unacceptable” pattern of driverless cars interfering with the work of law enforcement and other first responders.

“To state it bluntly: An AV that cannot safely interact with first responders is a danger to the general public,” Jonathan Morrison, the administrator of the National Highway Traffic Safety Administration (NHTSA), wrote in a letter that he described as a “call to action” for technology developers.

Morrison wrote that NHTSA has documented a “clear pattern” of interference over the last few months, including incidents in which the vehicles drove into active emergency scenes, blocked ambulances and firefighters, and didn’t respond in situations involving flashing lights, fire, and traffic cones.

He directed developers to "immediately focus their resources on fixing this issue,” and said that the NHTSA would schedule meetings with each company by the end of July to hear about solutions. Neither Waymo nor Zoox responded to WIRED’s request for comment.

Officials with several city law enforcement and emergency departments told NHTSA representatives in a closed-door meeting earlier this year that Waymo robotaxis were impeding some emergency responses, WIRED reported in April. One fire chief called the vehicles’ behavior in those situations a “safety issue for our crews as well as the victims."

A San Francisco official told the agency that the company’s tech was “backsliding,” a...