Urban Gråsjö and Charlie Karlsson
Accessibility has for many years been a widely used tool in transportation research. Many definitions have been suggested and researchers have constructed numerous mathematical formulations to measure its value in order to be able to evaluate the relationships between the nature of the transport systems and the patterns of land use. Such correlations have been used especially in assessing existing transport systems and forecasting their performance to provide decision-makers with ideas about the need for investments in the transport systems. However, accessibility measures can be regarded as the spatial counterparts of discounting. The measures represent the spatial distribution of economic agents and their activities in a simple way that imposes a very clear structure upon the relationship between these agents and their activities and their environment. Various frictional effects arising from geographical distance between economic agents determine their interaction options, that is, their options to trade, to cooperate, to learn, to commute, and so on. Observing that the time sensitivities of the economic agents vary between different spatial scales (and between different economic activities) we may impose a spatial structure (for example, local, intra-regional, interregional and international) which offers opportunities to define variables in such a way that spatial dependencies can be accommodated. These newly defined variables can then be used in empirical explanations of various spatial phenomena, such as patent output, new firm formation, the emergence of new export products, and economic growth in different spatial units. We will in this chapter against this background show that accessibility is an underused analytical and empirical tool in regional science with an underestimated potential.