Fire Department spokesman Cecil Manresa said the shooting was reported on Interstate 110 shortly before 5 p.m. Saturday.
KNX-AM reports the man was shot twice and tossed out of the car in the fast lane.
Police Sgt. Herb Chang said the victim remained hospitalized late Saturday in critical condition.
The shooting forced officers to shut down all southbound lanes by the 9th street exit for two hours to investigate the crime scene, causing traffic to back up for miles.