underpass on the 5 North

Driving along the 5 North one night through the short tunnel just north of the 118, I drive past an 18-wheeler, and as I do, he taps his horn. I was a bit taken aback, since I was in my lane, and he was in his. But then I was told that honking in that underpass is a strange cultural thing. During rush hour, if one person honks, other people would join in and honk, making a veritable sonic cacophony in celebration of, uh, traffic.