The Influence of Network Structure on Travel Distance

The objective of this research is to identify the role of network architecture in influencing individual travel behavior using travel survey data from two urban areas in Florida: Fort Lauderdale and Miami. Various measures of network structure, compiled from existing sources, are used to quantify roadway networks, capture the arrangement and connectivity of nodes and links in the networks and the temporal and spatial variations that exist among and within networks. The results from the regression models estimated show that network design influences how people travel and make decisions. Results from this analysis can be used to understand how changes in network can be used to bring about desired changes in travel behavior. Introduction Planners have traditionally shown keen interest in the use of land use and urban design strategies not only to bring about changes in travel behavior but as a way of providing a better quality of life for residents. Traditional neighborhood designs are asserted to be better than post-war suburban developments in terms of land use mix and greater accessibility to a variety of commercial establishments, grid-like arrangement of streets, traffic calming strategies, availability of sidewalks and other amenities suited to nonmotorized travel. While the use of urban design and associated accessibility to influence travel makes intuitive sense, researchers have found it extremely difficult to provide clear evidence on the existence much less the extent of this complex relationship. Review of the body of literature in this area shows many differences in the modeling methodologies used (Crane, 2000; Krizek, 2003). Apart from the differences in modeling methodologies, the research community has also been divided on the actual impact of urban design on travel patterns. On the one extreme, we have researchers who argue the existence of a significant relationship between urban form and travel (Cervero and Radisch, 1996; Frank and Pivo, 1994; Kockelman, 1997) and on the other extreme, we have researchers who counter that the impact of urban form on travel is weak at best (Boarnet and Crane, 2001; Crane, 2000). Researchers like Kitamura et al. (1997) and McNally and Kulkarni (1997) argue that attitudinal and socio-economic factors are greater indicators of travel patterns than land use variables and land use and urban design policies might not necessarily bring about measurable changes in travel behavior. While many modeling methodologies and approaches have been proposed by researchers to analyze the relationship between urban form and travel behavior, consideration of the structure of the actual transportation network has been largely missing. The transport system, specifically the street system, plays the role of the primary structural element of any city. For example, as Marshall (2005) points out, the differences in modern cities such as New York or Los Angeles traces back to the transportation system in place during critical phases of growth for each city. An in-depth analysis of urban design and travel hence needs to explicitly consider the transportation network in terms of the structure, the actual layout of streets and routes. The traditional interest in understanding transportation network structure has been limited to geographers who view the spatial nature of the transportation network as a vital input to the regional development. Transportation planners acknowledge the importance of the transport system in influencing urban form. However most studies 2 looking at the influence of urban form only consider a coarse representation of easily measured metrics of the actual transportation network such as the density of the road network, the number of 3-way or 4-way intersections, cul-de-sacs, lineal length of street network etc. While these descriptive measures of roadway network structure are important, they don’t consider the arrangement and connectivity of nodes and links in the network and the impact of these aspects on the performance of the transportation system. The question of how travel behavior varies systematically with network structure is particularly important as network architecture is perhaps the slowest changing urban system. For that reason it is the most important to get right, as design of the network persists for centuries and is difficult to adjust, much less optimize. This paper aims to continue on the research interest in understanding travel behavior while explicitly accounting for the underlying highway network structure, using data from two urban areas (UA) in Florida: Fort Lauderdale and Miami. The results from this analysis are expected to throw light on how a transportation network influences travel behavior and how changes in network design can be used to bring about desired changes in travel behavior. The rest of the paper is organized as follows: The next section provides a brief review of relevant literature in this area. This is followed by the section on modeling methodology detailing the data and estimation of measures of roadway network structure. The statistical analyses conducted and the results are presented in the next section. The paper concludes with key findings from the study and future extensions to the current research. Literature review Kissling (1969) refers to network structure as a measure of the layout of the network and characteristics of individual elements in his analysis of the influence of network structure on linkage importance and nodal accessibility levels in the Nova Scotia region. Xie and Levinson (2007) provides a similar definition of network topology as the arrangement and connectivity of the network. Gauthier (1966) classifies measures of network structure into two broad levels: an aggregate level, referring to measures of overall network structure and a disaggregate level referring to measures of relationships between the individual elements in the network. Geographers have traditionally been interested in understanding the structure of a transportation network as an aspect of the geographical area. The reason for the interest is the complex and temporal nature of the spatial processes in transportation network, characterized by the nodes and their linkages along with the hierarchical relationships and associated flows. A topological approach based on graph-theoretic network analysis has typically been employed by geographers to understand the spatial aspects of transportation along with the underlying processes that created them (Haggett and Chorley, 1969; Rodrigue et al., 2006; Taaffe et al., 1996; Taaffe and Gauthier Jr., 1973). Garrison (1960) utilized measures of graph theory to measure the connectivity of the Interstate Highway system, analyze the system as a whole and understand the individual components that make up the system. One of the earliest studies on utilizing network measures to understand metropolitan settlement patterns was conducted by Borchert (1961). In this study, the number of road and street intersections per square