Historical Hollywood Freeway Query: "Night Work Ahead When Flashing"

[Occidental Tourist]

I was recently watching an old TV show whose opening included a shot of the predecessor of this BGS on the southbound Hollywood Freeway from the early 1980's.  Across the top of the mainline pull-through there was a row of black text on yellow background that read "NIGHT WORK AHEAD WHEN FLASHING".  Above it had been installed two yellow flashers.



Does anyone remember this sign and know what the night work was on the freeway that prompted a semi-permanent sign installation on an existing BGS?