The incident occurred in the area of San Gabriel Boulevard and Duarte Road at about 8:38 a.m.
Police said in a news release the driver of a Mercedes pulled alongside a Subaru that was driving "erratically."
The driver of the Mercedes "attempted to contact the driver," and at some point during the confrontation the driver of the Subaru shot himself in the chest.
He was taken to the hospital, but is expected to survive.
The incident is under investigation.