In [Bianco, L., Giuseppe C., and P. Reverberi. 2001. "A network based model
for traffic sensor location with implications on O/D matrix estimates".
Transportation Science 35(1):50-60.], the authors present the Sensor Location
Problem: that of locating the minimum number of traffic sensors at
intersections of a road network such that the traffic flow on the entire
network can be determined. They offer a necessary and sufficient condition on
the set of monitored nodes in order for the flow everywhere to be determined.
In this paper, we present a counterexample that demonstrates that the condition
is not actually sufficient (though it is still necessary). We present a
stronger necessary condition for flow calculability, and show that it is a
sufficient condition in a large class of graphs in which a particular subgraph
is a tree. Many typical road networks are included in this category, and we
show how our condition can be used to inform traffic sensor placement.Comment: Submitted for peer review on October 3, 201
