First step for sim city 4. NAM. Network addon mod, fixes all the pathfinding so the sims don't take that street that's parrallel to the avenue, causing it to be 1500% congested. Can be found on
this rather handy site, or more specifically for the file itself,
here.
What does this mod do? basically, it fixes the pathfinding, lets you tweak it, gives you the option of making your city LHD, and adds in a lot of "puzzle" pieces. These range from new highway intersections and roundabouts, to a modular system to make raised roads, rail, avenues etc. You can pick the traffic simulator you wish to choose when installing, i'd reccomend simulator Z unless your PC is very underpowered. Don't go for the park and ride one though, your sims will never drive to a place of work, (only to a parking garage).
Here's an example. In the example you can see street, and road roundabouts of two sizes, aswell as the modular road peices in action that's let me build two rail overpasses, and also the smoothed rail bends.
Other good mods are:
Hole digger mod (search for it on the site i linked above), this allows you do change the height of tiles very accuratly, which is helpful since the modular items in the NAM are 15M taller, meaning you can, say, have a ground level highway sunked with a road bridge going over it at a constant level. Also note that placing a street on a single tile makes that tile level. placing a single street tile next to the first one (without connecting them), makes both tiles the same level, etc. You can get area's of your city flat in this way.
If your into realism, a slopemod, and a zero bridges height mod. This limits the slope of roads etc. greatly, provies a challange if you want it, and adds realism. Not a necessaity, depends on your playstyle. The bridges mod allows bridges over water that don't have to have the large dirt buildup each side.
the RHW (rural highway mod) is another must, lets you build modular highways like this
There's plenty of other handy mods out there, there's a few fixes for different things (something about the opera house causing a demand slump), and, there is more custom buildings/content than you could ever hope to poke a stick at. There's anything from eastern europe themed stuff to Dubai themed stuff and anywhere in between out there if you look for it.