How to plan a good street layout

It really depends whether or not you have rush hour installed. I do, however I played without it for a long time before that. If you don't have it, make a design in which local roads or streets are in a design where it is either impossible or inconvenient for them to be used as thoroughfares, which can help reroute traffic onto thoroughfares.

In rush hour it is a lot easier. You can use one way roads to make a pattern where it easily enables traffic to get in and out of an area quickly and onto the main roads, but do not enable traffic to cross the area easily unless they are on a main road. Also in rush hour public transport helps a lot more than it does in the vanilla version, due to some changes to do with they way sims commute. If this doesn't work, just use streets, they are slow and so for the most part sims will avoid them. Note that this doesn't work in vanilla SimCity 4.

In both cases, the thoroughfares could be lined with commercial zoning, as traffic increases desirability for both commercial services and commercial office buildings.

When planning your your city, make sure to 'assign' certain parts of the city for industrial, residential, specialty etc. Then, make the style of road you want, usually adjusting it the terrain you are on.

If you do curved, follow the guidelines. If you do grid, follow the guideline. The guideline is always good as it gives way for expansion.

To make sure a grid layout doesn't have traffic jams, make sure that not all the roads connect to the avenues. Could you imagine having a single road in real life that short with 10 traffic lights? Get only one or two roads to connect that 'section' so to speak to the avenues.

Always make an extra avenue from the starting avenues to connect/divide your 'sections'.

When making a good city, make sure to keep one road specified to be your "traffic jammer". It will be less costly to upgrade that one road then every road. Keeping your residences low-level might help, but place them somewhere away from all major traffic.

Having skyscrapers and major buildings all close together will not only be easier to control with fire stations and police, only roads in that general area will need to be upgraded. DO NOT BE CRAZY AND UPGRADE EVRY ROAD!

When finished making the layout of your dream city make more roads connecting to that main road to help "lighten" the load on that road. Have many roads connected to each other and you will be fine.