What's a good strategy for dealing with traffic?

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.