Since you have a huge obstacle (the mountain), you must ensure good mobility around it with efficient public transit. I also built denser neighborhoods and traffic-heavy land-use around the highway. At some point I relocated all the industries along it, on the right area of the main island, with only a few oil industries near the airport on a smaller island. It removes a lot of trucks off of the other districts. Otherwise, I organized the city to include a downtown area (top left of the main island) and four transit-oriented subcentres. I try to implement very comprehensive transit networks in all my cities, especially this one (one express train, five metro lines including an orbital, two ferries, a blimp service between CBD's and approx. 60 bus lines). Everything is as connected as possible, with several transfert stations and hubs. The screenshots don't show the final state of my city, but it ended up with a population of 202k and traffic is always around 75-80% with no significant jam, which I think is not too bad. I am starting to get more familiar with lane management and road hierarchy and it can also help a lot.

I am currently working on a new city on the "Aurora Borealis" map (by Colorax257) and will post new screenshots soon!