Defining a Freeway Mobility Index for Roadway Navigation

Abstract

Roadway navigation systems have become increasingly popular, providing more efficient routing through a roadway network. In many cases, multiple routes are possible, and a specific route can be selected by minimizing some cost function. However, the number of possible routes varies by roadway network topology and location within the network. It is often useful to know how many possible routes exist; therefore, the authors developed a Navigational Mobility Index (NMI) with an initial focus on freeway networks that can be extended to other roadway facility types. NMI can be based on the number of possible routes weighted by shared segments among routes from a source to a destination (i.e., node-to-node pair NMI), as a single node to all other nodes (i.e., node-NMI), or on an average of nodes in a specific area (i.e., area-NMI). This article first reviews several of the existing accessibility or mobility measures that have been developed for other purposes and discusses how these are not well suited for navigational route selection. Then, NMI is defined in detail, highlighting its advantages for navigational purposes. Next, the authors present applications of NMI, including its ability to (a) measure the degree of freedom in which drivers can choose routes from a route choice perspective, (b) determine the potential effectiveness of navigation systems, (c) determine the overall connectivity level of an area, (d) guide the movement of people during evacuation in a disastrous event, and (e) determine a route on the basis of the least generalized cost that incorporates both distance and NMI. The author selected and evaluated two of California's freeway networks as case studies. Examples of node-NMI and area-NMI are calculated and plotted using data from the Census 2000 TIGER/LineShapefiles (2000). Last, the authors present experimental results to show the effectiveness of the proposed concept and algorithm.