Spatial concentration analysis of italian domestic air transport network

The first part of this paper addresses the problem of measuring spatial concentration of air transport networks. A high spatial concentration level indicates a radial shape of the network, which is polarised by one or more major airports. On the other hand, a low spatial concentration level corresponds to a uniform distribution of the air services within the network. Spatial concentration measures may prove very useful in order to compare the spatial configuration of two or more different air transport networks and to analyse the temporal evolution of one network as well. For this purpose, several statistical indices can be applied. In this paper both the “Gini concentration ratio” and the “Herfindahl-Hirschman index are taken into account, which are the most commonly used indeces. The Gini concentration ratio, which can be represented through the so-called “Lorenz curve”, is considered by several researchers to be he most suitable spatial concentration measure for air transport networks. The Herfindahl-Hirschman index is easier to be calculated compared to the Gini index, but it is size dependent and thus it is not suitable for comparing air transport networks with a significantly different number of nodes and links. Furthermore, the Herfindahl index is sensitive only to modifications regarding the largest airports of the network. Past studies on airline network development typically consider airline networks exhibiting a high grade of spatial concentration as equivalents to hub-and-spoke networks. In principle, this is not correct, because spatial concentration is a condition that is necessary but not sufficient to classify one air transport network as a “hub-and-spoke” network. Indeed, an air transport network can be defined as an hub-and-spoke network if, and only if, (1) the spatial pattern is highly concentrated, (2) inbound and outbound flights at hub airports are temporally co-ordinated in “waves” or “banks” of flights (according to the so-called ideal wave scheme) and (3) hub functions and services are integrated (“effective hubbing”). For example, it can be noted that low-cost airline networks, such as those offered by Southwest and Ryanair, are characterised by a high level of spatial concentration even if they are based on point-to-point services and they do not operate any hubbing system. Generally speaking, changes in the spatial pattern of air transport networks are of main interest for the Regulator of an airport system. In deregulated markets, the presence and the frequency of air links modify air transport accessibility levels, which can affect regional economic development. Thus, the spatial concentration level of a regional air transport network should be estimated by the Regulator, in order to monitor the uniformity of the air transport service distribution and of the opportunities of economic development within the region. Finally, in the last part of this paper, the spatial concentration of Italian domestic scheduled air transport network has been evaluated in detail for the period 1986-2005. The analysis demonstrates that, during the last two decades, the Italian domestic air transport network has not showed any relevant change in the spatial concentration level of the flight frequencies. Thus, it can be concluded that no clear trend towards a more or less star-shaped distribution of domestic services within the Italian airport system seems to emerge up to now.