Roads are a vital thing in you city. Roads are free to build, but cannot overlap other buildings. They also only start as a 2 laned road. Sims will abandon their homes if the roads are to crowded.
2 lane road
4 lane road
6 land road
Avenue- adds palm trees in the middle of the lanes
Boulevard- changes the type of trees
Streetcar avenue- replaces the trees with streetcars. Upgrade many roads to have street cars flowing. They will not appear until you upgrade 4 roads that are connected to streetcar level
OMEGA Maglev train- this changes the street cars to Maglev trains, which are similar to monirails. Same requirments from streetcars to see the trains apply