1. Physical Constraints

Transport geography is concerned with movements that take place over space. The physical features of this space impose major constraints on transportation systems, in terms of what mode can be used, the extent of the service, its costs, capacity and reliability. Three basic spatial constraints of the terrestrial space can be identified:

• Topography: Features such as mountains and valleys have strongly influenced the structure of networks, the cost and feasibility of transportation projects. The main land transport infrastructures are built usually where there are the least physical impediments, such as on plains, along valleys, or through mountain passes. Water transport is influenced by water depths and the location of obstacles such as reefs. Coastlines exert an influence on the location of port infrastructure. Aircraft require airfields of considerable size for take off and landing. Topography can impose a natural convergence of routes that will create a certain degree of centrality and may assist a location in becoming a trade centre as a collector and distributor of goods. Topography can complicate, postpone or prevent the activities of the transport industry. Physical constraints fundamentally act as absolute and relative barriers to movements. Land transportation networks are notably influenced by the topography, as highways and railways tend to be impeded by grades higher than 3% and 1% respectively. Under such circumstances, land transportation tends to be of higher density in areas of limited topography.
• Hydrography: The properties, distribution and circulation of water play an important role in the transport industry. Maritime transport is influenced greatly by the availability of navigable channels through rivers, lakes and shallow seas. Several rivers such as the Mississippi, the St. Lawrence, the Rhine, the Mekong or the Yangtze are important navigable routeways into the heart of continents and historically have been the focus of human activities that have taken advantage of the transport opportunities. Port sites are also highly influenced by the physical attributes of the site where natural features (bays, sand dunes, and fjords) protect port installations. Since it is at these installations that traffic is transshipped, the location of ports is a dominant element in the structure of maritime networks. Where barriers exist, such as narrows, rapids, or land breaks, water transport can only overcome these obstacles with heavy investments in canals or dredging. Conversely waterways serve as barriers to land transportation necessitating the construction of bridges, tunnels and detours etc.
• Climate: Its major components include temperature, wind and precipitation. Their impacts on transportation modes and infrastructure range from negligible to severe. Freight and passenger movement can seriously be curtailed by hazardous conditions such as snow, heavy rainfall, ice or fog. Jet streams are also a major physical component that international air carriers must take into consideration. For an aircraft, the speed of wind can affect travel costs. When the wind is pushing the airplane towards its destination, it can reduce flight time up to several hours for intercontinental flights. Climate is also an influence over transportation networks by influencing construction and maintenance costs.

From a geometrical standpoint, the sphericity of the earth determines the great circle distance (Great Circle Distance Calculator); the least distance line between two points on a sphere. This feature explains the paths followed by major intercontinental maritime and air routes.

2. Transportation and the Spatial Structure

The concepts of site and situation are fundamental to geography and to transportation. While the site refers to the geographical characteristics of a specific location, its situation concerns its relationships in regard to other locations. Thus, all locations are relative to one another but situation is not a constant attribute as transportation developments change levels of accessibility, and thus the relations between locations. The development of a location reflects the cumulative relationships between transport infrastructure, economic activities and the built-environment. The following factors are particularly important in shaping the spatial structure:

• Costs. The spatial distribution of activities is related to factors of distance, namely its friction. Locational decisions are taken in an attempt to minimize costs, often related to transportation.
• Accessibility. All locations have a level of accessibility, but some are more accessible than others. Thus, because of transportation, some locations are perceived as more valuable than others.
• Agglomeration. There is a tendency for activities to agglomerate to take advantage of the value of specific locations. The more valuable a location, the more likely agglomeration will take place. The organization of activities is essentially hierarchical, resulting from the relationships between agglomeration and accessibility at the local, regional and global levels.

Many contemporary transportation networks are inherited from the past, notably transport infrastructures. Even if over the last two hundred years new technologies have revolutionized transportation in terms of speed, capacity and efficiency, the spatial structure of many networks has not much changed. This inertia in the spatial structure of some transportation networks can be explained by two major factors:

• Physical attributes. Natural conditions can be modified and adapted to suit human uses, but they are a very difficult constraint to escape, notably for land transportation. It is thus not surprising to find that most networks follow the easiest (least cost) paths, which generally follow valleys and plains. Considerations that affected road construction a few hundred years ago are still in force today, although they are sometimes easier to circumscribe.
• Historical considerations. New infrastructures generally reinforce historical patterns of exchange, notably at the regional level. For instance, the current highway network of France has mainly followed the patterns set by the national roads network built early in the 20th century. This network was established over the Royal roads network, itself mainly following roads built by the Romans. At the urban level, the pattern of streets is often inherited from an older pattern, which itself may have been influenced by the pre-existing rural structure (lot pattern and rural roads).

While physical and historical considerations are at play, the introduction of new transport technology or the addition of new transport infrastructure are leading to a transformation of existing networks. Recent developments in transport systems such as container shipping, jumbo aircrafts and the extensive application of information technology to transport management have created a new transport environment and a new spatial structure. These transport infrastructures have intensified global interactions and modified the relative location of places. In this highly dynamic context, two processes are taking place at the same time:

• Specialization. Linked geographical entities are able to specialize in the production of commodities for which they have an advantage, and trading for what they do not produce. As a result, efficient transportation systems are generally linked with higher levels of regional specialization. The globalization of production clearly underlines this process as specialization occurs as long as the incurred saving in production costs are higher than the incurred additional transport costs.
• Segregation. Linked geographical entities may see the reinforcement of one at the expense of others, notably through economies of scale. This outcome often contradicts regional development policies aiming at providing uniform accessibility levels within a region.

The continuous evolution of transportation technology may not necessarily have expected effects on the spatial structure, as two forces are at play; concentration and dispersion. A common myth tends to relate transportation solely as a force of dispersion, favoring the spread of activities in space. This is not always the case. In numerous instances, transportation is a force of concentration, notably for business activities. Since transport infrastructures are generally expensive to build, they are established first to service the most important locations. Even if it was a strong factor of dispersion, the automobile has also favored the concentration of several activities at specific places and in large volumes. Shopping centers are a relevant example of this process where central locations emerge in a dispersed setting.

3. Space / Time Relationships

One of the most basic relationships of transportation involves how much space can be overcome within a given amount of time. The faster the mode, the larger the distance that can be overcome within the same amount of time. Transportation, notably improvements in transport systems, changes the relationship between time and space. When this relationship involves easier, faster and cheaper access between places, this result is defined as a space / time convergence because the amount of space that can be overcome for a similar amount of time increases significantly. Significant regional and continental gains were achieved during the 18th and 19th centuries with the establishment of national and continental railway systems as well as with the growth of maritime shipping, a process which continued into the 20th century with air and road transport systems. The outcome has been significant differences in space / time relationships, mainly between developed and developing countries, reflecting differences in the efficiency of transport systems.

At the international level, globalization processes have been supported by improvements in transport technology. The result of more than 200 years of technological improvements has been a space / time collapse of global proportions in addition to the regional and continental processes previously mentioned. This enabled the extended exploitation of the advantages of the global market, notably in terms of resources and labor. Significant reductions in transport and communication costs occurred concomitantly. There is thus a relationship between the rate of a space / time collapse on the integration of a region in global trade. Five major factors are of particular relevance in this process:

• Speed. The most straightforward factor relates to the increasing speed of many transport modes, a condition that particularly prevailed in the first half of the 20th century. More recently, speed has played a less significant role as many modes are not going much faster. For instance, an automobile has a similar operating speed today than it had 60 years ago and a commercial jet plane operates at a similar speed than one was 30 years ago.
• Economies of scale. Being able to transport larger amounts of freight and passengers at lower costs has improved considerably the capacity and efficiency of transport systems.
• Expansion of transport infrastructures. Transport infrastructures have expanded considerably to service areas that were not previously serviced or were insufficiently serviced. A paradox of this feature is that although the expansion of transport infrastructures may have enabled distribution systems to expand, it also increased the average distance over which passengers and freight are being carried.
• Efficiency of transport terminals. Terminals, such as ports and airports, have shown a growing capacity to handle large quantities in a timely manner. Thus, even if the speed of many transport modes has not increased, more efficient transport terminals may have helped reduce transport time.
• Substitution of transportation by telecommunications. Enabled several economic activities to bypass spatial constraints in a very significant manner. Electronic mail is an example where the transmission of information does not have a physical form (outside electrons or photons) once the supporting infrastructure is established. There is obviously a limit to this substitution, but several corporations are trying to use the advantages of telecommuting as much as they can because of the important savings involved.

However, space / time convergence can also be inverted under specific circumstances, which means that a process of space / time divergence takes place. For instance, congestion is increasing in many metropolitan areas, implying additional delays for activities such as commuting. Traffic in congested urban areas is moving at the same speed that it did one hundred years ago on horse carriages. Air transportation, despite having dramatically contributed to the space / time convergence is also experiencing growing delays. Flight times are getting longer between many destinations, mainly because of takeoff, landing and gate access delays. Airlines are simply posting longer flight times to factor in congestion. An express mail package flown from Washington to Boston in about an hour (excluding delays at takeoff and landing due to airport congestion) can have an extra one hour delay as it is carried from Logan Airport to downtown Boston, a distance of only two miles. More stringent security measures at airports have also imposed additional delays, which tends to penalize more short distance flights. The "last mile" can be the longest in many transport segments.