The leadership shuffle at the U.S. Department of Transportation continued Thursday with the resignation of its top safety official, David L. Strickland.

Transportation Secretary Anthony Foxx also accepted the resignation of his chief deputy, John Porcari, on Wednesday. Porcari is a former Maryland transportation secretary who spent more than four years in the federal role.

Strickland has headed the National Highway Traffic Safety Administration for four years and was a key player in former transportation secretary Ray LaHood’s safety-first initiatives against distracted driving and drunken driving and promoting roadway safety.

NHTSA is responsible for setting vehicle safety standards, investigating automotive safety defects and monitoring vehicle recalls; setting and enforcing regulations on fuel economy; and compiling state traffic fatality data.

Foxx moved quickly to replace Porcari, reassigning Federal Highway Administrator Victor M. Mendez to take over as acting deputy secretary.

Finding someone whose credentials rival Strickland’s will be a challenge. Strickland is a Harvard-educated lawyer who came to the job after eight years as counsel to the Senate committee that oversaw NHTSA.

“David Strickland has an impressive list of accomplishments during his time at NHTSA,” said Karl Brauer, senior director at Kelley Blue Book, a vehicle valuation company headquartered in Irvine, Calif. “He’s clearly a passionate safety advocate [who] brought that passion to the role. While several unresolved issues remain on David Strickland’s docket, including the Tesla investigation, he would likely never find a moment where all open cases are resolved. The agency has survived changes in leadership before, and it should survive this one without letting anything fall between the cracks.”

David J. Friedman, NHTSA’s deputy administrator, will serve as acting NHTSA administrator.

During Strickland’s tenure, the federal agency finalized guidance to states on “driverless” cars, provided automakers contemplating new electronic features with guidelines on distracted driving, and handled investigations into issues with Jeep SUVs, the Chevrolet Volt and the Tesla Motors Model S.

The Department of Transportation confirmed his resignation. There was no immediate comment from Strickland on his departure or future plans.

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