The Geography of Transport Systems
Jean-Paul Rodrigue (2017), New York: Routledge, 440 pages.
ISBN 978-1138669574
The Geography of Transportation Networks
Authors: Dr. Jean-Paul Rodrigue and Dr. Cesar Ducruet
1. Transport Networks
Transportation systems are commonly represented using networks as an analogy for their structure and flows. Transport networks belong to the wider category of spatial networks because their design and evolution are physically constrained as opposed with non-spatial networks such as social interactions, corporate organization, and biological systems, which are usually constrained by other factors.
The term network refers 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, investments are made and as conditions change. The setting of networks is the outcome of various strategies, such as providing access and mobility to a region, reinforcing a specific trade corridor or technological developments making a specific mode and its network more advantageous over others. A transport network denotes either a permanent track (e.g. roads, rail and canals) or a scheduled service (e.g. airline, public transit, train). It can be extended to cover various types of links between points along which movements can take place. The relevance of a network is related to its connectivity. Metcalfe's law states that the value of a network is proportional to the square of connected nodes.
In transport geography, it is common to identify several types of transport structures that are linked with transportation networks with key elements such as nodes, links, flows, hubs or corridors. 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 tends not convey any specific locational advantages. The 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. Evidence underlines that the emergence of hub-and-spoke networks is a transitional form of network development rationalizing limited volumes through a limited number of routes. When traffic becomes sufficient, direct point-to-point services tend to be established as they better reflect the preference of users.
Transport networks are better understood by the usage level (e.g. number of passengers, tons, vehicles, capacity) than by their sole topology based on a binary state (i.e. presence or absence of links). Inequalities between locations can often be measured by the quantity of links between nodes and the related revenues generated by traffic flows. Many locations within a network have higher accessibility, which is often related to better opportunities. However, economic integration processes tend to change inequalities between regions, mainly through a reorientation of the structure and flows within transportation networks at the transnational level.
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 nodes 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 since the hub-and-spoke structure, particularly, had a notable impact on transport costs, namely through economies of scale.
2. The Topology and Typology of Networks
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, with each transport network having a specific topology. 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 transport networks that relates to its geographical setting as well as its modal and structural characteristics.
The physical grounding of a network varies in relevance depending on the transport mode considered. Roads and railways are composed of track infrastructure while maritime and air transports remain vaguely defined due to their higher spatial flexibility except for the terminals, whereas maritime networks remain more constrained than airline networks due to the necessity bypassing coastline. River networks typically form basins and can be classified as trees or dendrograms. Therefore, there are three types of physical spaces on which transport networks are set and where each represents a specific mode of territorial occupation:
  • Clearly defined and delimited. The space occupied by the transport network is strictly reserved for its exclusive 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 is not the object of any particular ownership, only of rights of way. Examples include air and maritime transportation networks.
  • Without definition. The space has no tangible meaning, except for the distance it imposes with nodes being the core structure. Little control and ownership are possible, but agreements must be reached for common usage. Examples are radio, television, WiFi and cellular networks, which rely on the use of specific frequencies granted by regulatory agencies.
Networks provide a level of transport service which is related to their costs. An optimal network would be a network servicing all possible locations but such a service would have high capital and operational costs. Transport infrastructures are established over discontinuous networks since many were not built at the same time, by the same entity or with the same technology. Therefore, operational networks rarely service all parts 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. Networks are also labeled depending on their overall properties:
  • Regular network. A network where all nodes have the same number of edges. In the same vein, a random network is a network that is formed by random processes. While regular networks tend to be linked with high levels of spatial organization (e.g. a city grid), random networks tend to be linked with opportunistic development opportunities such as accessing a resource.
  • Small-world network. A network with dense connections among close neighbors and few but crucial connections among distant neighbors. Such networks are particularly vulnerable to catastrophic failures around large hubs.
  • Scale-free network. A network having a strong hierarchical dimension, with few vertices having many connections and many vertices having few connections. Such networks evolve through the dynamic of preferential attachment by which new nodes added to the network will primarily connect larger nodes instead of being connected randomly.
Investigating the interdependencies among different transport networks, notably when those are of different nature and structure, is challenging. Some crucial aspects and problems related with inter-network relations may be as follows:
  • Coevolution. Different transport networks might follow similar or different paths based on spatial proximity and path-dependence of economic development, with a wider variety of networks at core regions than at remote regions.
  • Complementarity. Some locations may be central in one network but peripheral in another, depending on their specialization and function and on the scale of analysis (terminal, city, region, country); the complementarity between networks can be measured based on the number of common nodes and links.
  • Interoperability. Typically, cargo flows from a maritime network to a road network shift from a scale-free structure to a regular structure, thus following different topologies that are not easily combined; air and sea terminals remain few in the world due to the difficulty combining and integrating technically air and sea networks physically at the same locations.
  • Vulnerability. How do changes in one network affect the other network, on a global level (entire network) or local level (single node or region)? This is particularly important for two networks sharing common nodes, such as global cities, logistics platforms, and multilayered hubs in the case of abrupt conjunctures (e.g. natural disasters, targeted attacks, labor disputes, security and geopolitical tensions), thus posing the problem of rerouting flows through alternative routes and locations.
3. Networks and Space
Transportation networks underline the territorial organization of economic activities and the efforts incurred to overcome distance. These efforts can be measured in absolute (distance) or relative terms (time) and are proportional to the efficiency and the structure of the networks they represent. The relationships transportation networks establish with space are related to their continuity, their topographic space and the spatial cohesion they establish. The territory is a topological space having two or three dimensions depending on the transport mode considered (roads are roughly set over a two dimensional space while air transport is set over a three dimensional space). However, flows and infrastructures are linear; having one dimension since they conceptually link two points. The establishment of a network is thus a logical outcome for a one dimensional feature to service a territory by forming a lattice of nodes and links. In order to have such a spatial continuity in a transport network, three conditions are necessary:
  • Ubiquity. The possibility to reach any location from any other location on the network thus providing a general access. Access can be a simple matter of vehicle ownership or bidding on the market to purchase a thoroughfare from one location to another. Some networks are continuous, implying that they can be accessed at any location they service. Roads are the most salient example of a continuous network. Other networks are discrete, implying that they can only be accessed at specific locations, commonly at a terminal. Rail, maritime and rail networks are considered discrete networks since they can only be accessed through their terminals.
  • Fractionalization. The possibility for a traveler or a unit of freight to be transported without depending on a group. It becomes a balance between the price advantages of economies of scale and the convenience of a dedicated service.
  • Instantaneity. The possibility to undertake transportation at the desired or most convenient moment. There is a direct relationship between fractionalization and instantaneity since the more fractionalized a transport system is, the more likely time convenience can be accommodated.
These three conditions are never perfectly met as some transport modes fulfill them better than others. For instance, the automobile is the most flexible and ubiquitous mode for passenger transportation, but has important constraints such as low capacity and high levels of space and energy consumption. In comparison, public transit is more limited in the spatial coverage of its service, implies batch movements (bus loads, train loads, etc.) and follow specific schedules (limited instantaneity), but is more cost and energy efficient. Freight transportation also varies in its spatial continuity, ranging from massive loads of raw materials (oil and ores) that can be handled only in a limited number of ports to highly flexible parcels movements. Containerization has been a remarkable attempt to address the issue of ubiquity (the system permits intermodal movements), fractionalization (each container is a load unit) and instantaneity (units can be loaded by trucks at any time of the day and containerships make frequent port calls).
An important cause of discontinuity is linked to the spatial distribution of economic activities, notably industrial and urban, which tend to agglomerate. Congestion may also alter these conditions. Road congestion in a metropolitan area may impair ubiquity as some locations may be very difficult to reach since their accessibility is reduced. Fractionalization may also be reduced under such circumstances as people would consider public transit and carpooling and would thus move as batches. Further, as commuters cope with increasing congestion, several trips may be delayed or cancelled altogether reducing instantaneity.
Transportation networks have always been a tool for spatial cohesion and occupation. The Roman and Chinese empires relied on transportation networks to control their respective territories, mainly to collect taxes and move commodities and military forces. During the colonial era, maritime networks became a significant tool of trade, exploitation and political control, which was later on expanded by the development of modern transportation networks within colonies. In the 19th century, transportation networks also became a tool of nation building and political control. For instance, the extension of railways in the American hinterland had the purpose to organize the territory, extend settlements and distribute resources to new markets. In the 20th century, road and highways systems (such as the Interstate system in the United States and the autobahn in Germany) were built to reinforce this purpose. In the later part of the 20th century, air transportation networks played a significant role in weaving the global economy. For the early 21st century, telecommunication networks have become means of spatial cohesion and interactions abiding well to the requirements of global supply chains.
4. Network Expansion
The co-evolution of roads, canals and ports during the industrial revolution in England reveals noticeable interdependencies among the different nodes and networks over time, based on spatial and functional proximity. Initial network developments are often done to support and complement an existing network. Then, the new network competes with the existing network by expanding geographically and topologically in ways unavailable to the prior network. As transport networks expand, existing transport infrastructures are being upgraded to cope with spatial changes. Airports and ports are being transformed, expanded or relocated. In the air transport sector, emphasis is being given to integrate airports within fully-fledged multimodal transport systems, networking air with rail and road transport. In maritime transport, networks are also being modified with increasing attention paid to the expansion of the Panama and Suez Canals, to the increasing traffic on inland waterways and to creating new inland passages between semi-enclosed or enclosed seas.
The growing competition between the sea and land corridors are not only reducing transport costs and encouraging international trade but prompting many governments to reassess their land-based connections and seek shorter transit routes. Existing land routes are also being extended. Passages through difficult terrain are being investigated with a view to create fully-fledged land-based continental connections, notably through railways. These land network expansions are driven by economic globalization and inter-regional cooperation and eventually become multimodal transcontinental corridors for rail, road, pipelines and trunk telecommunications routes. But the impact of increasing world trade on land network expansion, notably over railways is scale specific. The expansion of railways has permitted inter and intra-continental connections, namely landbridges.
In recent years, new rail routes in North America, Eurasia, Latin America and Africa have been developed or are being considered. There is scope for shippers to increase their trade through these new routes, particularly if rising insurance premiums, charter rates and shipping risks prompt them to opt for a land route instead of the sea route through the Suez or Panama canals. These developments linked to the integration of the regional economies to the world market are part of a rationalization and specialization process of rail traffic. But the success of these rail network expansions depends on the speed of movement and the unitization of general cargo by containerization. Railways servicing ports tend to consolidate container flows, which allows an increase in capacity and the establishment of door-to-door services through a better distribution of goods among different transport modes.
New links are establishing and reshaping new trade flows, underpinning outward cargo movements and the distribution of goods. As some coastal gateways are emerging as critical logistics services centers that rationalize distribution systems to fit new trading patterns, the land network development and cross-border crossings throughout the world have far-reaching geopolitical implications.