Potential of converting short car trips to active trips: The role of the built environment in tour-based travel
Introduction
New Urbanism and Transit-Oriented Development (TOD) strategies, which are a recent American reform approach to development, attempt to encourage physical activity and reduce personal vehicle use. Despite the growing interest in such strategies, the policies that must be implemented to encourage drivers to find alternatives to short automobile trips are often ignored (Gärling et al., 2000). The 2009 National Household Travel Survey for California Add-on (NHTS-CA) shows that about 40 percent of all trips taken in the Los Angeles County were under 5 miles in round-trip length, and that driving was the mode for about 60 percent of these short trips. Some argue that we should target short trips in cars to reduce vehicle use (Hillman, 1998, Loukopoulos and Gärling, 2005) because short trips in cars cause disproportionate environmental impacts. Short trips in cars often take place before the engine has had time to warm up, and such trips generate additional emissions, such as carbon monoxide (CO) and volatile organic compounds (VOCs) (de Nazelle et al., 2010).
In addition to the associated environmental effects, reducing short trips in cars may be a viable way to reduce traffic congestion on local roads. For example, about 40% of all short trips started during peak traffic times and a half of these trips were taken by private vehicles in Los Angeles area (NHTS-CA, 2009).
Furthermore, these trips in cars can be replaced with walking and cycling, thereby providing needed physical exercise in the course of daily life (Handy, 1996, Greenwald and Boarnet, 2001, Cervero and Duncan, 2003). However, few studies have assessed the determinants that lead people to use cars for short trips or the neighborhood-scale interventions that could be designed and implemented to encourage changing the mode of such trips (Rodrı́guez and Joo, 2004). Taken together, these are all compelling reasons to focus on short trips because benefits will accrue as short, emission-intensive trips are replaced by non-automobile trips.
People use their cars for short trips for several reasons. A British study (Mackett, 2003) investigated short trips through in-depth interviews with 377 travelers who had taken short trips in their cars. His results attributed the use of cars for short trips to the transport of heavy goods and of children to school, the shortage of time, and the need for the car in a subsequent trip. Another study (Forward, 1998) reported that convenience made cars the dominant mode of travel and that the main disadvantage of walking was time constraints. In their surveys, Walton and Sunseri (2010) found that weather was the most common factor influencing the decision to drive a short distance. The additional factors that affect the choice of travel mode for short trips include the traveler's socio-economic and demographic status and preferences, the availability of a car, activity patterns, the purpose of the trip, the availability and quality of alternative modes, and the quality of the built environment of the neighborhoods that they live in or travel to (Kim and Ulfarsson, 2008, Loukopoulos and Gärling, 2005).
Among these factors, the built environment has been identified as a key factor that affects non-motorized travel (Badoe and Miller, 2000, Brownstone, 2008, Handy, 2005). The phrase “built environment” encompasses many factors, including residential density, the mix of land uses, the connectivity and scale of the streets, aesthetic qualities, and the transportation system. These factors are related to the basic strategies of New Urbanism and TOD that attempt to foster more compact development near transit stations by providing retail and employment centers within walking distance of high-density housing. Many urban designers and planners believe that neighborhood-level urban characteristics are strongly related to transportation modes. In particular, it is believed that the right urban form will promote alternatives to personal car use. This belief has been supported by the empirical literature using a mode choice model that hypothesizes that environmental characteristics act as incentives for travel behavior (Cervero, 2002, Chen et al., 2008, He, 2011, Kockelman, 1997, Zhang, 2004). These studies have provided useful insights in analyzing travel demand based on the traveler's preferences and the benefits obtained from the travel, as well as the costs, by examining individual trips, which is a trip-based approach.
Research into the fundamental influences on travel behavior has addressed the weaknesses and limitations of the trip-based approach, as findings have seldom reflected the linked nature of most travel, even though the choice of mode may be affected by both the outbound and return portions of the trip, and have raised key concerns regarding 'trip chaining' (i.e., travel involving multiple purposes and multiple destinations). Furthermore, research has indicated that trip chaining has been a growing phenomenon over the past decade and is becoming a significant part of people's daily travel because people increasingly tend to economize their amount of travel, given their limited time budgets and the high value of travel time savings (Hensher and Reyes, 2000).
Given this increased interest in trip chaining behavior, recent efforts have examined travel behavior through observations of the sequence of trip segments, which is called the tour-based approach. A "tour" links individual trips, including all the stops made along the way. Tour-based modeling can offer a more insightful understanding of the impact of land-use strategies on various travel behavior decisions by analyzing the sequence and combinations of all trips and activity patterns (Rasouli and Timmermans, 2014). For example, it is possible that land use diversity influences a person's decision to drive or walk short distances for a grocery shopping trip from home as a single outing that does not involve trip chaining. In that case, trip-based models can be used to show that the potential for shopping near the residential area may contribute to shifts in mode. However, when commuting by car to work and doing grocery shopping on the way, the likelihood of changing modes is very low. This difference can be revealed by using tour-based modeling that analyzes whole activity patterns.
This paper builds on previous studies and draws together behavioral and practical aspects of modeling individual tour-based mode choice. With the relative paucity of research investigating the effects of activity patterns and tour formation on mode choice, especially for short travel, this study was conducted to obtain a better understanding of the effects of tours to test the hypothesis that compact urban forms reduce short car travel. In so doing, the built environment characteristics of the origin (home) and destination (work or other activities location) areas of the tours, measured on different spatial scales, were considered.
The next section provides an overview of the literature reviewing the context of the theory of activity and travel decisions, and the tour-based approach to investigate the relationship between travel behavior and land use. This is followed by a description of the research methodology used to conduct the study and the main results. The next section presents the results and discussion. The paper ends with the conclusion section, which includes the study limitations and directions for further research.
Section snippets
Literature review
The tour-based approach is drawn from the activity-based approach that views travel as a derived demand for activities (Jones et al., 1990, Axhausen and Gärling, 1992). The approach focuses on sequences of travel behavior as the unit of analysis. It emphasizes the effects of employment status, gender and socio-economic characteristics of travelers, transport network and locational division on the spatial and temporal constraints of individual movement (Hägerstrand, 1970, Hanson and Huff, 1982,
Research design
In this study, a ‘tour’ is defined as a home-to-home loop that represents a chained trip, including all links within the loop, starting and ending at home. As discussed earlier, the choice of mode may be affected by both the outbound and return portions of the tour. Thus, the distance used in this study is the length of a tour that included multi-stop journeys within a single traveler's home-to-home loop. For this analysis, only home-based tours were considered, because these tours are likely
Results and discussions
In this paper, several formulations were tested with all the explanatory variables and the possible interaction terms in the MNL models. The results from the MNL models for a tour taker's choice of tour mode for a short home-based tour are presented in Table 4. The unit of analysis is the tour, not the individual. Originally the data set included 1624 tours under 2 miles and 2504 tours under 5 miles. Because of missing values, mostly due to land use variables, some observations had to be
Conclusions
The literature is mixed regarding the potential effect on trip-chaining patterns and mode choice of various urban forms, specifically those under the concept of Smart Growth and New Urbanism. Researchers have debated the feasibility of alternative modes to car travel, but they have focused less on short round trips. Activity patterns and attributes of destination locations have often been ignored when assessing the travel impacts on mode choice. Thus, this study adds to the current body of
Acknowledgements
This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education (NRF-2016R1D1A3B03936226).
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