Inter-contact time between moving vehicles is oneof the key metrics in vehicular ad hoc networks (VANETs) andcentral to forwarding algorithms and the end-to-end delay. Dueto prohibitive costs, little work has conducted experimental studyon inter-contact time in urban vehicular environments. In thispaper, we carry out an extensive experiment involving thousandsof operational taxies in Shanghai city. Studying the taxi tracedata on the frequency and duration of transfer opportunitiesbetween taxies, we observe that the tail distribution of the inter-contact time, that is the time gap separating two contacts ofthe same pair of taxies, exhibits a light tail such as one ofan exponential distribution, over a large range of timescale.This observation is in sharp contrast to recent empirical datastudies based on human mobility, in which the distribution ofthe inter-contact time obeys a power law. By performing a leastsquares fit, we establish an exponential model that can accuratelydepict the tail behavior of the inter-contact time in VANETs.Our results thus provide fundamental guidelines on design ofnew vehicular mobility models in urban scenarios, new dataforwarding protocols and their performance analysis.

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