Investigators believe Deputy Michael Haak, who was off-duty and in his personal vehicle, had stopped between South Street and Artesia Boulevard because of a mechanical problem.
He walked on the traffic side of his stopped vehicle and was struck by a driver in the right lane, the sheriff's department said.
Investigators say the driver, a 54-year-old Buena Park man, stopped at the scene and cooperated with investigators. There was no indication of intoxication, officials said.
Haak, 52, had worked for the department for 29 years, most recently assigned to the Lakewood station.
He is survived by a wife, three children and a sister.
Capt. James Wolak told deputies at the Lakewood station: "I know many of you worked with Mike and words alone will never be able to ease the overwhelming grief you must be feeling. Please know that Mike loved being a deputy sheriff and lived a very full life."
The California Highway Patrol is investigating the incident.