Improving Trip Generation Methods for Livable Communities

Kelly Clifton, Portland State University



This NITC report offers a multimodal framework for transportation impact analysis – a welcome tool for professionals in many cities seeking more detailed data about non-drivers. Headed by Kelly Clifton of Portland State University and Nico Larco of the University of Oregon, this project is the latest effort in an ongoing collaboration to create more open sourced, widely available data about non-motorized road users. 

Over the last decades, cities have become more invested in fostering the conditions to support walking, biking and public transit. The land development process presents a unique challenge. Prior to a zoning change or new development, someone has to determine what its impact on the transportation system will be, and whether upgrades will be necessary to accommodate travelers to the new destination. Trip generation is the first step in the conventional transportation forecasting process. Current trip generation methods used by engineers across the country tend to focus on motorized modes. Without reliable trip generation rates for anyone but drivers, the transportation impact is difficult to predict. Certain land uses will draw far more walkers, cyclists and transit riders than drivers. Thus, cities lack enough information to create planning and design requirements that support non-automobile modes, and to create spaces that will meet the needs of the people who want to use them. 

In previous research, Clifton examined contextual influences on trip generation such as the surrounding built environment and urban density, employing a method which has since made its way into the ITE handbook. She has also looked into the connections between consumer behavior and travel modes. She cautions not to lose sight of sociodemographic factors, but to remember to put the user in “land use.” 

“One establishment may fill different needs for different people, and the social context matters as much as the built environment,” Clifton said. 

A failure to account for the characteristics of site visitors or the potential market for businesses can lead to large errors in the estimation of the number of trips. The research has shown that seemingly similar land uses can have very different travel outcomes, even within the same type of area, because the users interact with the site activity in different ways. 

“We are challenged to define land use in a way that considers the nature of human activities and interactions on the site, has a strong theoretical link to travel behavior, places the end-user at the center, does not over-prescribe, and allows for flexibility and re-classification over time. This is a tall order, and more research will be needed to frame and develop such a methodology,” Clifton said. This project offers critical tools for planners and practitioners, including an approach for site-level and area-wide analysis. 

Recent efforts to improve trip generation data available for transportation impact analysis of new development include the collection of multimodal trip generation data, development of models that account for the built environment, and new recommendations for practice. Building on a long line of research on transportation and built environment, many studies have identified important features of the surrounding built environment that most impact trip rates and mode shares, building on a long line of research on transportation and the built environment. Despite these improvements in data and methods, less attention is placed on identifying the conditions of the site itself and the immediate surrounding environment that influence trip generation and mode choice. To fill this gap, this study builds upon previous work (Contextual Influences on Trip Generation (Project Number: OTREC 2011-407), 2012), re-examines the information collected in that study, and includes new site-level observations. The objective of this study is to examine establishment multimodal trip generation more closely from a finer-grained scale and identify site-level attributes of the built environment that help explain multimodal trip generation. From this, we have the additional objective of developing a framework for trip generation analysis that takes findings from this study into consideration. We placed emphasis on those sites in our previous study with a discord between the expected travel patterns, based upon the larger urban context and built environment of the site, and the observed. The research approach will make use of mixed methods. In addition to using archived data from the previous study, site visits provided direct observation of the overall performance of the site, including travel patterns on and around the site as well as specific site configuration, urban design details and traffic operations. The analysis of this combination of data provided a more complete picture of site-level trip generation and our findings highlight the influence of: people living nearby and using the site; the nature of the land use on the site; the development along arterial roadways; site permeability and access; and the local culture around walking and cycling. Finally, this report ends by reflecting on the numerous concerns identified from practice, the research findings from various recent studies, and the need for a sustainable process for evaluating the transportation impacts of new land development. We present a potential framework to advance the methods for how site plans fit into neighborhood and regional planning, using locally defined standards and goals. Here, we de-emphasize the site and its immediate environs as the primary (and only) scale of analysis and lessen the reliance on the problematic methodologies for estimating site-level travel demand. Rather, we argue that transportation impact analysis would benefit by first taking a district, neighborhood or area-wide approach with attention to the urban context—the built and social environment—where a site is located. At this larger scale, there is a better ability to understand the various elements that work together to shape travel demand and allows for a better assessment of how a specific site proposal will integrate into this larger context. This effort involves several faculty from across the country, including Kelly Clifton (PSU Engineering) and Nico Larco (UO Architecture) from our consortium and Susan Handy (UC Davis Environmental Studies) and Robert Schneider (UW Milwaukee Planning) as consultants/advisors.

Project Details

Project Type:
Project Status:
End Date:
June 30,2016
UTC Grant Cycle:
Natl Round 1
UTC Funding: