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It is demonstrated that the minimum number of sensors required to know all link flows in a traffic network can be determined only if path information is available. However, not all paths need to be enumerated but, at most, a small subset defining the rank r w of the link-path incidence matrix W. If this rank for a reduced subset of paths is already m − n, where m and n are the number of links and noncentroid nodes, respectively, we can conclude that m − n sensors are sufficient. It is also shown that the formulas providing the dependent link flows in terms of the independent link flows can be obtained by the node-based or path-based approaches with the same results only when r w = m − n. Finally, an algorithm to obtain the small subsets of linearly independent path vectors is given. The methods are shown by a parallel network example and the Ciudad Real and Cuenca networks, for which the savings in link counts with respect to the m − n bound are larger than 16%. The corresponding savings in path enumeration are larger than 80%.Index Terms-Link flow estimation, optimal sensor location.

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