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Suggested Citation:"Summary ." National Academies of Sciences, Engineering, and Medicine. 2013. Trip Generation Rates for Transportation Impact Analyses of Infill Developments. Washington, DC: The National Academies Press. doi: 10.17226/22458.
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Suggested Citation:"Summary ." National Academies of Sciences, Engineering, and Medicine. 2013. Trip Generation Rates for Transportation Impact Analyses of Infill Developments. Washington, DC: The National Academies Press. doi: 10.17226/22458.
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1 S U M M A R Y The objective of this research was to develop an easily applied methodology to estimate automobile trip generation and mode shares of non-vehicular trips that can be used in the preparation of site-specific transportation impact analyses of infill development projects located within existing higher-density built-up areas. The primary source of data and methods for estimating automobile trips in preparing transportation impact analyses is the Institute of Transportation Engineers’ (ITE) Trip Gen- eration Manual. The majority of the data making up the manual are from automobile counts conducted at single-use, suburban, and exurban sites with limited pedestrian, bicycle, and transit activity. Although the manual is accepted as valid for sites in the same context from which the data were collected, it is less applicable to urban and urbanizing sites with moderate to extensive use of non-automobile modes of transportation. Urban and suburban places have important differences in trip generation because of sig- nificant contrasts in density of development, street networks, building types and height, and availability of multimodal transportation options. Regardless of these differences, the ITE Trip Generation Manual remains the primary, and often mandatory, resource for estimating the trip generation of infill development for transportation impact analyses. To achieve the research objective stated in the first paragraph, this study initially focused on the current state of the practice in trip generation for site impact analyses, as well as recent efforts to develop infill or smart growth trip generation estimation methods. Key findings from the initial review include: • There is no standardized and nationally accepted infill trip estimation methodology. • There are limited options for agencies and practitioners when estimating the site-specific traffic impacts of infill development. • Recently published and unpublished research on estimating infill trip generation provides an array of approaches from which can be developed an estimation methodology that is compatible with current impact analysis practice. This study recommends an approach that adjusts trip generation estimates based on data in the ITE Trip Generation Manual using mode share and vehicle occupancy as adjustment factors to more accurately reflect the travel characteristics of the context in which the pro- posed project is located. The research team divided the approach into two methods for deriving the adjustment factors that are applied to conventional trip generation data: 1. Proxy site method – Adjustment factors are derived from data collected from a site or sites that serve as a proxy for the proposed project’s land use in the context of urban infill development. There are two variants of the proxy site method: (a) Minimum data collection – Derives adjustment factors more quickly and less expensively than the other methods by collecting only the essential minimum data and using basic techniques to survey a proxy site or sites within contexts similar to the proposed project.

2(b) Comprehensive data collection – Derives adjustment factors from data collected using multiple techniques to survey a proxy site or sites within contexts similar to the proposed project. This variation is used when the complexity of the site or its surrounding context precludes the minimum data collection variant, or when more detailed traveler, site, or demographic information is desired. 2. Travel survey method – Extracts mode share and vehicle occupancy adjustment factors for a particular land use and context from regional household travel survey data for the metropolitan region within which the practitioner is preparing a study. This method has applications and limitations different from those of the proxy site method. While it can be used to estimate infill trip generation, this method is best suited to broader, more macroscopic applications. The recommended approach uses person trips as the common denominator between con- ventional and infill land uses. This concept—that a particular land use generates an equal number of person trips whether it is located in a suburban context or an urban context— is supported by common practice for infill trip generation. Based on this relationship, the remainder of the approach is a simple exercise in conversion: from conventional automobile trips to person trips, and from person trips to infill vehicle trips. The approach can be applied to any of the land use categories in the ITE Trip Generation Manual, making it compatible with current practice in preparing impact analyses. This study documents the typical forms of confirmation used to evaluate and demonstrate a methodology’s ability to predict urban infill trip generation: verification and validation. The verification process demonstrates that the methodology was appropriately developed and that there are no gross errors or oversights in the underlying theory. Verification and validation tests of the proxy site method used empirical data for sites in the Washington, D.C., area collected as part of this study as well as data obtained from a prior California Department of Transportation study on urban infill trip generation (1). The travel survey method was tested using adjustment factors derived from household travel survey data obtained from the Metropolitan Washington Council of Governments for the Washington, D.C., region and from the Metropolitan Transportation Commission data for the San Francisco Bay Area. Although the comparison of the method’s predicted infill trips to actual trips was incon- clusive due to small sample sizes, the research team did note that the method predicts con- sistently lower vehicle-trip estimates than the trips estimated using conventional ITE data, ranging from two-thirds to three-quarters of ITE estimates, a finding supported by other research on infill trip generation methods (2). Validation of the recommended methodology is anticipated to occur over time as trans- portation professionals contribute data from their testing of the methodology or as a result of their work on infill development projects. This report includes ideas regarding the collection of site data, based on the lessons learned in this study, to validate the methodology and for applying the method in impact analyses, and recommendations for future research to demonstrate the validity of the underlying assumption that the person trips generated by sites of similar size and land use type are equal regardless of context. Furthermore, this report presents a method of extracting mode share and vehicle occupancy from readily available household travel survey data—a methodological resource that has application in many areas of transportation planning.

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 758: Trip Generation Rates for Transportation Impact Analyses of Infill Developments details a procedure for analyzing potential vehicular trip generation impacts in urban and urbanizing locales.

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