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Transportation systems are composed of a complex set of relationships between the demand, the locations they service and the networks that support movements. They are mainly dependent on the commercial environment from which are derived operational attributes such as transportation costs, capacity, efficiency, reliability and speed. Such conditions are closely related to the development of transportation networks, both in capacity and in spatial extent. Transportation systems are also evolving within a complex set of relationships between transport supply, mainly the operational capacity of the network, and transport demand, the mobility requirements of a territory. This chapter consequently investigates the relationships between transportation networks and their spatial structure.

Transportation systems are commonly represented using networks as an analogy for their structure and flows. The term networkrefers to the framework of routes within a system of locations, identified as nodes. A route is a single link between two nodes that are part of a larger network that can refer to tangible routes such as roads and rails, or less tangible routes such as air and sea corridors.

The territorial structure of any region corresponds to a network of all its economic interactions. The implementation of networks, however, is rarely premeditated but the consequence of continuous improvements as opportunities arise and as conditions change. They result from the influence of various strategies, such as providing access and mobility to a region, and technological developments. A transport network denotes either a permanent track (e.g. roads, rails and canals) or a scheduled service (e.g. airline, transit, train). It can be extended to cover various types of links between points along which movements can take place.

In transport geography, it is common to identify several types of transport structures that are linked with transportation networks. Network structure ranges from centripetal to centrifugal in terms of the accessibility they provide to locations. A centripetal network favors a limited number of locations while a centrifugal network does not convey any specific locational advantages. Recent decades have seen the emergence of transport hubs, a strongly centripetal form, as a privileged network structure for many types of transport services, notably for air transportation. Although hub-and-spoke networks often result in improved network efficiency, they have drawbacks linked with their vulnerability to disruptions and delays at hubs, an outcome of the lack of direct connections.

Hubs, as a network structure, allow a greater flexibility within the transport system, through a concentration of flows. For instance, on Figure 1, a point-to-point network involves 16 independent connections, each to be serviced by vehicles and infrastructures. By using a hub-and-spoke structure, only 8 connections are required. The main advantages of hubs are:

• economies of scale on connectionsby offering a high frequency of services. For instance, instead of one service per day between any two pairs in a point-to-point network, four services per day could be possible;

• economies of scale at the hubs, enabling the potential development of an efficient distribution system since the hubs handle larger quantities of traffic;

• economies of scope in the use of shared transshipment facilities. This can take several dimensions such as lower costs for the users as well as higher quality infrastructures.

Many transportation services have adapted to include a hub-and-spoke structure. The most common examples involve air passenger and freight services which have developed such a structure at the global, national and regional levels, such as those used by UPS, FedEx and DHL. However, potential disadvantages may also occur such as additional transshipment as less point-to-point services are offered, which for some connections may involve delays and potential congestion as the hub becomes the major point of transshipment.

Figure 1. Transport networks


The efficiency of a network can be measured through graph theory and network analysis. These methods rest on the principle that the efficiency of a network depends partially on the lay-out of points and links. Obviously some network structures have a higher degree of accessibility than others, but careful consideration must be given to the basic relationship between the revenue and costs of specific transport networks. Rates thus tend to be influenced by the structure of transportation networks. Inequalities between locations can often be measured by the quantity of links between points and the related revenues generated by traffic flows. Many locations within a network have better accessibility and higher opportunities. However, economic integration processes tend to change inequalities between regions. This in turn has impacted on the structure and flows of transportation networks at the transnational level (Figure 2).

Figure 2. Impacts of integration processes on networks and flows


Prior to economic integration processes (such as a free trade agreement) networks tended to service their respective national economies with flows representing this structure. With economic integration, the structure of transportation network s is modified with new transnational linkages. Flows are also modified. In some cases, there could be a relative decline of national flows and a comparative growth of transnational flows.

Transportation networks, like many networks, are generally embodied as a set of locations and a set of links representing connections between those locations. The arrangement and connectivity of a network is known as its topology. Each transport network has consequently a specific topology indicating its structure. The most fundamental elements of such a structure are the network geometry and the level of connectivity. Transport networks can be classified in specific categories depending on a set of topological attributes that describe them. It is thus possible to establish a basic typology of a transport network that relates to its geographical setting, and its modal and structural characteristics.

There are many criteria that can be used to classify transportation networks (Figure 3). The level of abstraction can be considered with concrete network representations closely matching the reality (such as a road map) while conversely an abstract network would only be a symbolization of the nodes and flows (such as the network of an airline). Since transportation networks have a geographical setting, they can be defined according to their location relative to the main elements of a territory (such as the Rhine delta). Networks also have an orientation and an extent that approximates their geographical coverage or their market area. The numbers of nodes and edges are relevant to express the complexity and structure of transportation networks with a branch of mathematics, graph theory, developed to infer structural properties from these numbers. Since networks are the support of movements they can be considered from a modal perspective, their edges being an abstraction of routes (roads, rail links, maritime routes) and their nodes an abstraction of terminals (ports, railyards). Specific modes can further be classified in terms of types of road (highway, road, street, etc.) and level of control (speed limits, vehicle restrictions, etc.). Flows on a network have a volume and a direction, enabling to rank links by their importance and evaluate the general direction of flows (e.g. centripetal or centrifugal). Each segment and network has a physical capacity related to the volume it can support under normal conditions. The load (or volume to capacity) is the relation between the existing volume and the capacity. The closer it is to a full load (a ratio of 1), the more congested it is. The structure of some networks imposes a hierarchy reflecting the importance of each of its nodes and a pattern reflecting their spatial arrangement. Finally, networks have a dynamic where both their nodes and links can change due to new circumstances.

Figure 3. Typology of transportation networks


Further, three types of spaces on which transport networks are evolving are found. Each of these spaces represents a specific mode of territorial occupation:

• clearly defined and delimited. In this case the space occupied by the transport network is strictly reserved for its usage and can be identified on a map. Ownership can also be clearly established. Major examples include road, canal and railway networks;

• vaguely defined and delimited. The space of these networks may be shared with other modes and it is not the object of any particular ownership, only rights of way. Examples include air and maritime transportation networks;

without definition. The space has no tangible meaning, except for the distance it imposes. Little control and ownership are possible, but agreements must be reached for common usage. Examples are radio, television and cellular networks, which rely on specific frequencies granted by governing agencies. Networks provide a level of transport service which is related to its costs. An optimal network would be a network servicing all possible locations but would have high capital and operational costs. Transport infrastructures are established over discontinuous networks. Therefore, operational networks are not servicing every part of the territory directly. Some compromise must often be found among a set of alternatives, considering a variety of route combinations and level of service.


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