Traffic is confusing

I was driving home late last night from my friend's house after spending the afternoon with him and his family. It was darn near 9 when I actually got on the road. The drive from LA to San Diego is actually pretty nice proivided there is no traffic, sadly there was tons of traffic which doesn't make any sense to me. Even more perplexing was the solid wall of cars coming the other way, barely any gaps in the army of headlights, I wish I could have taken a picture there was so much traffic. I don't understand why on a sunday night at 9+PM there is so much traffic going between the two cities, it was bad enough that I didn't once even consider using the cruise control. What are you people doing?