The Relationship Between Land Use and Trip Internalization Behaviors: Evidence and Implications

This paper addresses the relationship between land use and destination selection, and the question of destination selection on travel mode choice. Specifically, this work focuses on internalized trips, a sub-category of trip making where both trip origin and trip destination are contained in the same geographic unit of analysis. This investigation uses data from the 1994 Household Activity and Travel Diary Survey conducted by Portland Metro. Using multinomial logit and binary logistic models to measure travel mode choice and decision to internalize trips, the evidence here supports three conclusions: 1.) urban design elements do more to alter travel mode choice than alter trip destination; 2.) there is a threshold effect in the ability of mixed use to alter travel behavior; and 3.) greater emphasis to destinations within the area where the home is located needs to be given in trip distribution models. TRB 2006 Annual Meeting CD-ROM Paper revised from original submittal. Greenwald Page 1 of 18 INTRODUCTION Assumptions about travel behavior feed directly into the travel modeling process, explaining why better understanding of land use impacts on travel behavior, particularly on travel making decisions, has generated so much interest recently. The research up to now focused on questions of trip generation (e.g., mode choice, VMT, travel time, etc.) and the associated consequences (e.g., environmental quality, economic efficiency, public expenditure, etc.). That is the first step. The underlying land use theories on which these investigations are based emphasize both the ability of land use practices to eliminate or alter trips, and to alter the choices of trip ends. The current approach relating urban form to travel behavior is neotraditional design. These designs rest on the assumption smaller scale urban design, rectilinear street layout and variety of smaller scale economic activity closer to one’s residence will inspire changes in travel patterns which simultaneously cause substitution in activity location (locations closer to home substituted for activity centers further away), and substitution of travel mode (travelers choose to walk to closer locations rather than drive, due to increased convenience) (1). This paper addresses the relationship between land use and destination selection, and the question of destination selection on travel mode choice. Internalized trips are the focus of this paper. Internalized trips are a sub-category of general trip making behavior, where both trip origin and trip destination are contained in the same area. This definition is both intuitively simple, and identifies issues requiring special attention. The next section describes current methods of travel modeling, including a brief discussion of difficulties with these approaches as they relate to understanding internalized trips. Then we consider what land use/transportation policy research related to internalized trips already exists. Next, summary information and discrete choice models related to trip internalization are presented, helping put internalized trip making into a behavioral context. Finally, policy and modeling implications are considered. CURRENT STATE OF KNOWLEDGE The need for better understanding of land use influences on trip internalization was discussed by Ewing, et.al. in their investigation of rates of internal trip capture for 20 neighborhoods in South Florida (2). The authors found that land use mix and regional accessibility accounted for a significant proportion of trips which started and ended within a specific community. They also called for greater understanding of internal capture in traffic impact research. The question is, how does the issue of internal capture fit conceptually in travel modeling? The most commonly used method of regional travel modeling is the Four Step Model (FSM). According to Bates, FSM consists of separate models which calculate and distribute travel behaviors by transportation analysis zone (TAZ). TAZs are not defined by consistent geographic scales, but zone borders often correspond to major streets in a particular region under analysis. The models applied to these TAZs are 1.) trip attraction and generation; 2.) distribution of trips between zones; 3.) travel mode split for distributed trips; and 4.) route assignment (2). The models are linked sequentially, with the assumption that the inputs to each model are independent. This independence assumption, particularly on the connection between trip generation/attraction and distribution, has been the subject of serious scrutiny. According to Bates the problem lies not with the logic underlying the relationships, rather the specification and execution of those relationships (3). Trip internalization is most relevant to the trip distribution model, since by definition internalized trips should not be distributed. Bates describes two methods of trip distribution in FSM travel modeling. The first is a variant of Newton’s gravity model, where the number of TRB 2006 Annual Meeting CD-ROM Paper revised from original submittal. Greenwald Page 2 of 18 trips distributed between any two zones is a function of trip generating elements in the originating zone (e.g., housing), trip attracting elements in the destination zone (e.g. employment), and is inversely proportional to some measure of travel impedance between the zones (either travel time or distance). When both trip generation and attraction forces are well specified and trip origins and destinations fixed (e.g., distributing work commutes based on housing and employment figures by zone) distribution devolves into correctly specifying the impedance (3). These requirements also limit the usefulness of this method. Newer models integrating transportation and land use forecasting use discrete choice models to assign trips between zones. Examples include the TRANUS and MEPLAN packages described by Hunt et. al., comparing performance of network models for the Sacramento area (4). The advantage of discrete choice is only the origin must be predetermined; the choice of end point is allowed to vary based on characteristics of the traveler and the potential destinations. Borrowing Bates’ description, these discrete choice destination selection models are generalized as: ) , : ... ( ] : [ } { } { j k k