A city performs different but interdependent functions related to its connectivity. Although a city can have several forms of connectivity, there is usually a dominance of a particular form based upon the main economic functions and specialization. This involves a range of activities, each having its own connectivity:
- Production and distribution. Activities related to production and distribution rely on a specific array of modes and terminals. The connectivity provided by long-distance maritime transportation relies on the port district as the main nexus, particularly at major port cities. Heavy industries (e.g. steel, petrochemicals) are mainly linked with ports and rail yards. Manufacturing is more associated with highways and to some extent intermodal rail. Logistics zones (distribution clusters) are also relying on a mix of highway and rail connectivity. Some are directly adjacent to port, airport, or intermodal rail facilities.
- Mobility and accessibility. Mainly supports the mobility of residents (movements of passengers from/to residential areas) and provides for their consumption needs (movements of freight to shopping districts and home deliveries). Airport districts have also become important clusters of activities supporting connectivity to a regional and international system of cities. The growing importance of air freight is also reinforcing the importance of some airports as nodes for distribution.
- Transactions. Relates to the range of activities managing the allocation of resources (capital, labor, materials, etc.). Financial and management districts depend on the connectivity provided by information and telecommunication technologies.
Two basic forms of interdependent nodes are at the core of the urban spatial structure:
- Connectivity nodes. Relate to locations that transfer passengers and freight, thus offering accessibility to resources and markets within and/or outside the urban area. They include terminals such as ports, rail stations, airports, and distribution centers. Most cities owe their initial development to a location that grants connectivity to local, regional and/or international systems of circulation, commonly a port site. In a contemporary setting, airports are playing a greater role. Connectivity nodes are often dependent on the specific geographical requirements of each transport nodes, notably in terms of space consumption. Terminals such as ports, railyards, and airports can be large consumers of space for them to handle large volumes of passengers or freight.
- Economic nodes. Refer to locations that perform a secondary (manufacturing), tertiary (services) or quaternary (management, research, education) function of economic significance. These functions are extremely varied and can include transformation, management, education, retailing, and leisure. Economic nodes tend to cluster and are often dependent on accessing a connectivity node. Such clusters often take the form of central business districts, commercial strips, industrial districts, logistics zones, or airport districts.
The presence of nodes requires links, which can be serviced by different transport modes. Road and transit links are obviously local in scope often taking the form of a grid that characterizes the form of many cities. Rail, maritime, and air links integrate the city to a wider context of distribution and trade, often global in scope. The prevalence of forms of connectivity, the complex set of relationships between nodes, and their links imply an urban form that is unique in each case.