A motorist with a car telephone called authorities after he said a man waved a pistol at him from another car on Interstate 10 near Yucaipa on Thursday.
California Highway Patrol officers later pulled over the alleged gunman's car, arrested the driver, William Worrell, 53, and confiscated a loaded .38-caliber revolver.
"It's that whole thing of guns and traffic frustration starting all over again," San Bernardino County Sheriff's Deputy Ron Miller said. "The road narrowed from four to three lanes and this suspect thought he had been cut off and started waving his gun."
The CHP said Worrell was booked on suspicion of brandishing a weapon and carrying a loaded weapon in a car.