Urban Transit Accessibility Analysis Using a GIS: A Case Study of Florida's Tri-Rail System

Southeastern Geographer Vol. XXXI, No. 1, May 1991, pp. 15-30 URBAN TRANSIT ACCESSIBILITY ANALYSIS USING A GIS: A CASE STUDY OF FLORIDA'S TRI-RAIL SYSTEM* Shih-Lung Shaw INTRODUCTION. Geographers have studied transportation systems in both intercity and intracity contexts using models of spatial interaction, flow allocation, travel demand, and network analysis. These models can be classified as either areal or network oriented. For example, areal unit characteristics are used to estimate trip productions and attractions in a trip generation model while network characteristics are not explicitly considered. Most gravity model applications also use aggregate areal unit characteristics as attracting forces and some distance meaures as impeding forces to estimate amounts of interactions between regions. On the other hand, when network flow models are employed to solve problems of traffic assignment, maximum flow, or minimum cost flow, the characteristics of individual linkages in a network must be included explicitly in the problem-solving environment. (J) This dichotomy makes linking network characteristics with areal unit characteristics in an integrated problem-solving environment desirable. It is a common practice in transportation studies to represent areal data at centroids and to connect isolated points to a transportation network by creating dummy linkages. This approach effectively generalizes areal data into point data and links point data to network linkages. Areal unit characteristics therefore are linked with network characteristics for transportation network analysis. This generalization procedure, however , requires extra steps to prepare data for analysis and tends to remove some important information on spatial dimensions from analysis. What is needed in the examination of a specific transportation problem is a method or tool that combines the strengths of standard approaches to both areal and network-oriented analyses. This paper uses a case study of accessibility from/to a commuter rail system to examine the capabili- * This research was supported by an internal research grant from Florida Atlantic University. I would like to thank Ronald Schultz and the reviewers for their comments on an earlier version of the manuscript. Dr. Shaw is Assistant Professor ofGeography at Florida Atlantic University in Boca Raton, FL 33431 . 16Southeastern Geographer ties and advantages of performing such analysis within the framework of a geographic information system (GIS). RACKGROUND AND ORJECTIVES. Southeast Florida, comprising Palm Beach, Broward, and Dade counties, is one of the fastest growing regions in the nation. Interstate Highway 95 (1-95), the major northsouth arterial in the region, has been carrying an excessive amount of traffic generated by this rapid growth. In order to mitigate traffic congestion on 1-95, the Tri-County Commuter Rail (also known as the Tri-Rail) system was put into service in January 1989; it extends 67 miles from West Palm Beach to Miami International Airport along the 1-95 corridor (Fig. 1). The Florida Department of Transportation expected that 14,192 passengers per day would use the Tri-Rail service, but the actual daily ridership totaled only about 3,000 passengers when service began. Utilization rose to about 4,000 riders per day after mid-day train service was implemented in January 1990. (2) Even though Tri-Rail offers a potential alternative to handling the constantly growing travel demand in southeast Florida, the Florida Legislature requires Tri-Rail to increase its ridership to 9,300 passengers per day in order to generate revenues equal to 40% of its operating costs. Otherwise, the state could discontinue the service. (3) In order to increase utilization of Tri-Rail, this study focuses on the connections between the rail stations and trip origins and destinations by different travel modes. Commuter rail systems have fixed routes and only a few distantly located stations. They rely heavily on other travel modes to transport train riders between rail stations and their trip origins and destinations. At the origin end, train riders can choose between private automobiles and other travel modes to get to the boarding stations. At the destination end, however, they often are dependent on transit modes to get them to their final destinations. An on-board questionnaire survey of Tri-Rail riders indicated that 79% of the respondents identified "park and ride" as their major travel means to get from their residences to the boarding...