A short version of the story
Richard Ankrom is an artist and sign painter. One day he was driving north on the 110 freeway in LA. He missed the exit to the Interstate 5 North he wanted to take and got lost. He later understood that the I-5 exit was not indicated on the green overhead sign. In 2001 he crafted the I-5 North sign himself. With a group of friends he assembled the sign at the place where it supposed to be.
The nice part of the story
The company running the freeway found out about the sign from an article in a local newspaper. Ankrom had hoped he could get his sign back after they took it down; he wanted to hang it in an art gallery. However, the company didn’t take the sign down. The guerrilla sign had passed the inspection.