Using GIS to explore the effects of road network specifications on the external validity of accessibility measures

This paper describes the use of Geographical Information Systems to calculate accessibility measures. Special attention is paid to the specification of the underlying road network characteristics and the available tools to incorporate these characteristics into the calculation of accessibility measures. Networks, as used in this paper, consist of lanes and intersections with accompanying characteristics that influence traffic flows. Traffic flows in lanes are influenced by characteristics such as length, average speed, and capacity. Flows at intersections are directed by turning movements and transitions. The paper explores the influence of network specification on the external validity of accessibility measures.Three different networks for the city of Eindhoven, the Netherlands, are specified and compared. The first network is based on the distance of the various road links. The second network is specified with the average travel time that is needed to pass road links. The third network is based on the average travel time of links and nodes extended with extra time needed to turn at intersections or to transfer from one link type to another. Two simple accessibility measures are calculated for these networks: the Mean Separation Index and the Gravity Type Index. The results of these calculations are compared with residents’ evaluations scores for the accessibility of their neighbourhood. The study is placed in the context of shopping facilities. It appears that the detailed specifications of the road network, as suggested in this paper, result into marginal improvements in the external validity of accessibility measures