Have you been to Los Angeles? On what space do you expect them to do this on, without interrupting the already bad traffic and with what money? I'm genuinely curious as to your idea and how it would work.
I've been there several times, and I'm aware of how difficult it would be. My comment wasn't predicated on the idea that improving traffic in LA would be painless—quite the opposite. In order to commit to proper urban planning, there are tradeoffs and the construction is disruptive and inconvenient for a time. But it must be done. And LA can afford to do it.
Inconvenient for a while is an understatement. It took them nearly 10 years to expand a portion of freeway from Temecula -> Riverside and it's honestly not helped at all, made it worse if anything. California may be good at some things but completing road work is not one of them. A recently repaved and widened portion of the 52 near me in San Diego feels like riding a roller coaster with all the dips it has.
And even if they somehow do manage to do it, I'm not sure at all that any lasting improvement will be made. An urban planner friend tells me that people decide where to live based on commute time, not miles driven. So if you put in more roadway, people move to cheaper places farther out, using more road-miles than before, at least until the speeds drop back down. 
I'm not sure what practical intervention you're suggesting there. We can rebuild existing buildings to be denser, but I'm not sure how you'd make that cheaper than a new suburban home, let alone as appealing to the kind of person who currently buys a new suburban home.