During the last few decades, substantial interests have been devoted to determining the optimal number and location of sensors for route flow observability. However, the robustness of the observability system has received minimal attention. This paper develops a technique for obtaining the lower bound of route flow observability under a specific sensor budget, i.e., the formulation of a budget-constrained minimum route observability problem. The proposed method is illustrated through the Nguyen-Dupuis network. Comparative analysis with the conventional optimal sensor location model is implemented. The results show that even when the sensor budget is the same, the observability level can fluctuate greatly. It implies that the route observation system is not robust if the flow pattern is changed. In the future, the factors that affect robustness can be further studied.

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