Roads and transport system should be constantly upgraded. In general, roads should be improved as city grows. But when city becomes completely jammed, all that is left to do is complete rebuild. Three lane roads should be replaced with highways - it will greatly improve city communication.

If you decide to replace roads with highways, you will see that highways need bit more place. Additionally, exits take lots of place as you can't simply create a crossroad. Because of that, it is wise to leave some free space in the city center for additional structures.

When even highways start jamming because of too many trucks, railway becomes great help. Although connecting it to the center is expensive and forces you to demolish some buildings if you don't have free space, it delivers resources exactly where you need them, which is a great thing for your roads. Railway can't intersect with roads so you will have to build many flyovers. It is definitely worth the effort if you need it.

Final transport buildings - ports and airports work similarly to railway - if they will be build, they will greatly improve your city transport efficiency.