DETROIT (AP) -- America’s top auto safety regulator is stepping down.

David Strickland, who ran the National Highway Traffic Safety Administration for the past three years, announced his departure on Thursday.

Transportation Department spokesman Nathan Naylor says Deputy Administrator David Friedman will become acting administrator of the agency.

No departure date for Strickland was announced.

Friedman was appointed in May. Before joining the agency he worked for a dozen years as a fuel economy expert for the Union of Concerned Scientists.

Under Strickland, the safety agency fined automakers millions for being slow to report defects. He also pushed to develop guidelines for self-driving cars.

The agency monitors auto safety complaints and investigates safety problems. It also deals with government fuel economy requirements.