And I simply do not understand how 1000s of people in cars get off work at three.
Perhaps you don't appreciate the workingmen's schedule.
Hundreds of establishments follow a two or three shift work day that runs 7AM-3PM, 3PM-11PM and 11PM to 7AM - ask any nurse! To avoid these travel times, other business work shifts like 8-4, 9-5, or 10-6. So, in the afternoon hours we generally have four surges of entering traffic at 3,4,5, and 6. The eastbound traffic starts to thin out just about the time the Inland Empire folks start their westerly trek to the dining/entertainment venues of LA.
And come the weekend, there are only 7 basic ways out of the basin: 4 headed east(the 210, 10, 60 and 91;two headed North (the 5 and the 101); and one headed south (the 5). All the others, like the 405 and 605 are just bypass spurs to the main arteries.
der Brucer