In plain Simcity 4, Sims will take the shortest route by distance to their location. This leads to some rather illogical travel paths where sims will take the overburdened street and ignore your empty 6 lane highway right next to it. The only way around this is decentralisation. Don't just have an industrial third, commercial third and residential third to your city, have several zones dotted around. Mass transit also helps, but again there is a tendency for sims to ignore it. Spamming bus stations is the easiest way to get mass transit going.
There is a mod, called the Network Addon Mod that fixes this issue by making Sims take the shortest route by time to their location. It also gives you far more options for transport.
Decentralising your zones also helps with NAM installed, but you can also work by having some redundancy too - like having a subway and highway going to the same location. This works far better under NAM than under plain Simcity, where Sims will only ever avail of one option, irregardless of how overused it is.