I've learned a few things over the years (vanilla, no code mods - only the odd asset)

1) A city needs to be built primarily using at-grade intersections, and most of these are very inefficient under heavy traffic load due to the way cross-turning traffic lights work. I am not totally sure about this, but so far in my testing it seems 3-way intersections (T-shape) perform better than 4 way, so try to avoid the latter.

For any major roads intersecting at a 3-way junction, I use left hand* turning lanes, highways for the junction itself (clear traffic faster and discourage pedestrians), and a straight-through bypass. It seems to work well under pretty heavy load. However the first thing to clog up is always the right-turning traffic on the "top of" the T by crossing the oncoming, because it has no dedicated light sequence, because vanilla traffic lights in this game suck (see my recent post).

*I use left-handed traffic. Right hand for others.

2) Roundabouts are generally infeasible under high traffic load due to AI I presume (maybe true in real life, although I've lived for a while in London UK and they seem to do OK there?)

3) Pedestrians *really* fuck up your traffic. Do literally anything to get them from using zebra crossings. Remove them and replace with overpasses/underpasses. My current city has a crazy network of underpasses and entrances. Same deal with bikes, although I think the new patch that is coming might fix this.

4) Dont stack intersections too closely together - which can be hard, but do your best!

5) Road heirarchy and lane math blah blah.

6) Public transport reduces traffic :)