Question: I've placed some medium and high-density roads between the ultra-pollution industrial side of the city and the medium density residential/commercial side of the city. Now my city is at a size where all those roads are filled with traffic every day. And unfortunately I also built some low density R/C zones on these east-west connectors.

How should I handle this? Add a new larger connector between them? Bulldoze the existing road and make it bigger (removing the existing infrastructure along it)? Busses? I have lots of space available still.

Answer: 
If you have a tight grid with lots of intersections, the intersections themselves can slow traffic down. Removing every second road (sometimes needing to demolish buildings) can ease the business of the intersections and improve traffic congestion a bit.
Now, with the raise and lower street tool, you can raise an avenue high enough so it makes basically makes a freeway through your city. Then, make exits onto different parts of your city. Avenues are your best chance of getting sims around. I had a problem like this in one of my cities. The streets were so crowded 24/7, that fires would start and fire trucks wouldn't get to the street in time, burning down the city as a result.