Since the 1940s, planners needing to predict population movement, transport-network usage and even epidemics have turned to a model based on the 'gravity law'. This assumes that the number of individuals travelling between two locations is proportional to the population at the source and destination, and decays with distance. This approach has its limitations, because it looks at the flow between two specific points only. Here, Albert-László Barabási and colleagues present an alternative model that takes into account population density at all intermediate points. Their parameter-free radiation model predicts a range of phenomena — from commuting and migrations to phone calls — much more accurately than the gravity model. Needing only data on population densities, which are easy to measure, the system can be used to predict commuting and transport patterns even in areas where data are not collected systematically.
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