Because of their large and complex construction, they are also expensive to maintain. However, a well placed highway system between residential and commercial/industrial zones will provide useful service and due to its high road capacity, many traffic problems that occur on streets and roads will be reduced.
Highways, debuting in SimCity 2000, are elevated. Highways are connected to roads via onramps. Intersecting highways create an otherwisely simple interchange.
Elevated highways return in SimCity 3000. Highway interchanges are made by adding either onramps or highway pieces to intersecting highways.
A popular trend among players is to directly connect avenues (two parallel roads) with a highway. However the game treats this interconnection as broken: no traffic will use this particular connection.
SimCity 4: Rush Hour included two types of highways: elevated highways and the all-new ground highways. Avenues and highways can now interconnect, as well as the addition of cloverleaf interchanges and T-interchanges.
A popular mod for SimCity 4 is the Real Highway Mod, included as part of the Network Addon Mod, which allows the player to build a more realistic looking highway, as well as allowing the player to create a custom interchange.
Highways and railways are pre-built in SimCity (2013) for connecting cities. While railways can be extended for building stations, highways cannot be modified much more than connecting the main road to the T-interchange.