The impact of measuring internal travel distances on selfpotentials and accessibility
Internal travel distances are fundamental in accessibility measurement, as they affect the weight of the intra-regional interactions, especially when using a gravity formulation. The contribution of the internal accessibility of each zone to its overall accessibility is known as self-potential. Several studies demonstrate its importance in accessibility analyses, especially in the most urbanized regions. It is precisely in urban regions where internal travel distances are more difficult to estimate due to congestion, which in turn may be influenced by factors such as urban density, urban morphology, network infrastructure, etc. Accessibility analyses usually use coarse estimates of internal distances, generally based on the regions' area and in some cases considering its level of urbanization. In this study we explore different forms of estimating internal travel distances in accessibility analysis and reflect on their advantages and drawbacks. One of the main difficulties that arise when measuring internal travel distances is the lack of data. However, the growing potential of ICTs in providing new sources of data can be used to improve representativeness of data. In this study we used speed profiles data from TeleAtlas/TomTom to calculate internal travel distances for European NUTS-3 regions and we compare this measure with three other metrics traditionally used in the literature. Following this exercise, we discuss the conditions under which it is advantageous to use more complex measures of internal travel distance. Finally, we test the sensitivity of potential accessibility indicators to the combined effect of different internal distance metrics and distance decay factors.