Los Angeles closes streets for a few large events each year. If you are not involved, you may not hear about it until you are stationary.
The marathon
The course runs from Dodger Stadium to the coast, crossing a large part of the city on a Sunday morning. Roads along it close from early and reopen in stages through the afternoon. Hollywood, West Hollywood, Beverly Hills and Brentwood are all affected in sequence.
The other regulars
CicLAvia closes long stretches to cars several times a year. Pride in West Hollywood, the Rose Parade in Pasadena on New Year's Day, and various film shoots that close individual blocks with a day's notice.
If you have a flight
This is where it actually bites. A Sunday morning airport run that crosses a closed course is not a longer drive — it is a different drive, often much longer. Leave earlier than you would on any other Sunday.
Dates shift each year. If you are travelling on a Sunday morning in late winter, it is worth ten seconds of checking.