Jean-Paul Rodrigue (2013), New York:
Routledge, 416 pages.
The Geography of Transportation Networks
Authors: Dr. Jean-Paul Rodrigue and Dr. Cesar Ducruet
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.
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
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
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
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
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
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
- 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
- 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.
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:
- 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
- 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.
Transportation networks underline the territorial organization of
economic activities and the efforts incurred to overcome distance. These
efforts can be measured in
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
- 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
- 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
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
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.
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 been 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
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
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 presently
occurring around the world. 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
New links are establishing and reshaping new trade flows underpinning outward cargo movements and the distribution of goods.
As some coastal gateways are now 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.
- 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.