The primary reason for traffic slowdowns on these highways was rear-end collisions blocking traffic.
You've apparently never driven on U.S. 101 in the SF Bay Area; the primary reasons for traffic slowdowns are:
(1) Auuuuuuuugh! There's a huge ball of light up in the sky! We fears it, my precious!
(2) Look! An accident! Is there blood? Hey, Bill, can you see any blood?!?!
(3) I must get in the fast lane because it is the "fast" lane, even though I'm coming up on my exit!
(4) I must get from the fast lane all the way over to the exit lane, but it's OK if this takes forever, I was in the fast lane for 50 feet, dammit!
(5) Yes, I know it's after 3 PM and before 7PM! What do you mean, the lane to the left of mine is "The Car Pool Lane"? I'm driving slow in the middle lane; if you want to pass, you should get into the car pool lane and pass, then get back into this lane; you probably won't get a ticket anyway...
(6) Let me race up in this lane that I need to be out of before too long, rather than getting over now, even though I see barricades ahead, because I know some dumbass will let me in, right? Right? Hey, dumbass, I'm talking to you!
(7) I want to get on one of the bridges, but I don't want to wait behind all the people who also want to get on one of these bridges, so I'm going to block the next lane over until someone lets me in just to punish everyone else... if I have to wait, then everybody else damn well has to wait, too.
That probably should have been a countdown; fast lane discipline while car pool hours are in effect is probably the number one cause of traffic slowdowns, followed by "I'm too stupid to get over ahead of time", with "Auuuugh! Ball of fire!" in third place...