Application of the Maximum Flow Problem to Sensor Placement on Urban Road Networks for Homeland Security

The use of sensors in the detection of terrorists, weapons, or dangerous materials transported along the roadways of the United States could contribute to the protection of urban population centers from attack. Two critical issues in designing a system for detecting such attacks before they occur are determining how many sensors would be needed and where they should be located. This article discusses a methodology developed at the Institute for Defense Analyses (IDA) to help find optimal sensor locations on urban road networks. This methodology uses network theory to identify a minimum cut set (a smallest set of road segments necessary to cut to completely block flow through the road network). We applied this methodology to the road network of the New York City metropolitan area and it found that the minimum cut set is about 104 times smaller than the number of road segments in the network. This work was sponsored by the Department of Homeland Security.