Distance is an important concept in transport geography as it is associated with a cost or effort to reach a location at the local, regional, national, and global levels, each representing an isochrone. From a given location, distance and its effects can be represented in three fundamental ways:
- Linear. The effect of distance (also known as distance decay) increases proportionally from the origin, which can represent fuel cost or time spent. It tends to apply to land transportation services going in one direction. Such effects can be apparent both at the urban and international levels.
- Logarithmic/Nonlinear. Applies well to commuting and retail activities as most of the interactions are for short distances from a location with a return trip. The cost of undertaking travel over long distances for such a purpose quickly becomes prohibitive, implying a “lens-like” look of the effect where short distances are dominant in this representation. The Swedish geographer Hagerstrand was the first to articulate this logarithmic distance-decay effect when looking at migration patterns in Sweden during the 1950s. For specific retail and service activities, the effect of distance is more nonlinear, implying the willingness to travel over longer distances. Still, these representations are strongly impacted by the effect of distance.
- Inverse. Although this relation may appear counterintuitive, since what is close is distant and what is distant can be closer. Long-distance transportation services such as air travel and maritime shipping have prohibitively high short-distance costs in part attributed to high loading and unloading costs and the related inability to compete with other modes such as cars, trucks, or rail for short distances. They will be unwilling to offer impractical local services, implying a “tunnel-like” look of the effect of distance. The longer the travel distance, the less the cost per unit carried, implying an inverse effect of distance up to a range that is usually intercontinental.