SimCity 4: Rush Hour/New Roads

Rush Hour added a lot of new roads to add some variety.

Avenues
Avenues are a new type of road that have more capacity than regular roads. However they're two tiles wide which makes upgrading an existing road network very hard. Another disadvantage is that you cannot switch sides easily on an avenue (so build crossings if you intend to build a long stretch of avenue).

One very cool thing is that avenues can connect directly to highways, which makes them very useful. If you plan them well, they can reduce traffic congestion a lot.

One-way roads
One-way roads are another way to help with traffic congestion if there's no space for an avenue. The direction you drag the one-way road determines which way the traffic will go, which is indicated by large arrows. One important note is that people must not only be able to go to work but also return back home - a dead-end consisting of a one-way road will never work for commute purposes.