Estimating distance decay of intra-urban trips using mobile phone data: The case of Bratislava, Slovakia

The distance decay function has been attracting attention in diverse disciplines including transportation studies, spatial planning and urban geography. In particular, much discussion has concentrated on the measurement of distance decay on the regional scale, since the emphasis of the model utilisation has been on explaining inter-regional mobility (mainly commuting). The intra-urban context makes the estimation more complicated and the fundamental questions, such as how far people travel within the city to reach a variety of urban destinations, and whether there are potentially significant differences between these types of destinations, are still not answered in a satisfactory way. In response to this challenge, the paper attempts to reveal the spatial variations of the distance decay effect on movements in urban space through the utilisation of mobile phone data. The signalling data from all major mobile network operators in Slovakia represent new opportunities with high accuracy of measurement and complexity of representation. The methodological procedure for deriving data about human daily move-ments from the mobile network at the level of 1 x 1 km statistical grid cells is presented. The objective is to estimate the appropriate distance decay functions for urban grids and demonstrate the variation of decay curves within the Bratislava city. The findings relativise the decay law of human mobility. For the approximation of daily mobility within the urban area, the polynomial-exponential function - the decreasing function with a small increase of interaction intensity at a greater distance - describes the impedance of travel distance more pref-erably. However, a significant proportion of urban grids have recorded weak or even no decay. A question worth investigating is whether the resulting friction of distance is a result of a specific functional structure of the study area, or it could reflect an advanced stage of urban evolution.