Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
5 This chapter provides an overview of the knowledge and understanding on which this research study was undertaken. Specifically, this chapter includes discussion on (a) the impor- tance of trip generation, (b) defining infill development for the purpose of this study, (c) current methods for estimat- ing infill trip generation, and (d) recent research on infill trip generation estimation methods. 2.1 The Importance of Trip Generation Estimated vehicular trips are important input for many analysis and decision-making processes. Because subtle changes in estimates can have significant impacts, trip genera- tion is a pivotal concern in planning for a development and to urban planning in general. The results of trip generation are commonly used in many analysis capacities, including those to: ⢠Inform decision makers on the impacts or benefits of a development through mandated environmental impact statements or reports, ⢠Determine or evaluate short- or long-range capital improve- ment investments, ⢠Identify the traffic impacts and required mitigation mea- sures as part of the development entitlement process, ⢠Predict the effects of congestion management policies such as congestion pricing and travel demand management (TDM) programs, ⢠Determine site vehicular access needs as well as associated roadway and site-plan modifications and improvements, and ⢠Assess impact fees on new development to fund infrastruc- ture improvements. The decisions that result from these activities can result in a variety of impacts, including those that can affect local government, private investment, and the public. These effects can be far reaching and affect a variety of interests, includ- ing those related to fiscal, environmental, quality of life, local circulation and access, and related economic development concerns. Impacts can also affect other transportation modes either directly or indirectly. Increasingly, there is concern that required mitigations can result in unintended consequences such as adversely affecting the ability of people to walk, bicycle, or use public transportation as increasingly larger vehicular facilities reduce or negatively affect opportunities for nonmo- torized modes. Transportation facility layout or size can also cause significant direct or indirect effects that often extend beyond the immediate area of a development. Because the magnitude of trips directly affects analysis results that can have far-reaching implications, the accuracy of estimating trip generation is an important consideration. It is understood by the transportation profession that the trip generation rates published by ITE are representative of their underlying data. Accordingly, the trip generation rates can be biased by the context in which the underlying data collection locations existed. In the case of the ITE Trip Gen- eration Manual, much of the data have been collected at pri- marily single-use suburban developments that lack transit service and that are either difficult to access by walking or bicycling or are in areas where these modes are not exten- sively used. The recognition of this data bias has resulted in a common belief among practitioners that ITE trip generation rates overpredict vehicular traffic impacts for infill develop- ment projects, particularly for those in urban areas served by transit with good pedestrian and bicycle access. 2.2 Defining Infill Development In order to properly establish the current state of the practice of trip generation for infill development, a common definition of infill first needed to be established. For the purposes of this study, the definition was determined to need to be both under- standable and definable in an objective manner. The defini- tion also needed to address the two ways practitioners were C H A P T E R 2 State of the Practice
6envisioned to use it, namely: (a) to identify or qualify the con- text in which the subject of their study would be developed, or (b) to locate an existing context that represents the future environment in which the subject of their study would exist. Initial efforts to establish this definition by the research team took the form of a survey of professionals and a literature review. 2.2.1 Survey of Professional Organizations As an early part of this project, a questionnaire was sent to a cross-section of practitioners and agencies. Participants were asked to provide a definition of infill development. While answers varied from person to person, the majority of the responses were a variation of one of the following definitions: ⢠Development/redevelopment of vacant or underutilized parcels within a primarily developed or built-out area. ⢠Use of vacant land within a predominately developed area that contains existing public services and infrastructure but may require improvements to meet current development standards. ⢠Development/redevelopment of a nearly built-out area generally focused in moderate- to high-density urban, sub- urban, or former industrial areas. ⢠Adaptive reuse of existing structures or infrastructure, often resulting in greater intensity of use through higher densities. Survey responses stated that transit proximity, TOD, and mixed-use development are common attributes associated with infill development. Sources of infill definitions cited by the survey participants included those of municipal agencies and professional organizations, including the Urban Land Institute, Congress for the New Urbanism, and the American Planning Association. Most survey respondents (78%) cited their own professional or personal perspective and experience as the source of their definition of infill development. 2.2.2 Literature Review of Infill Development Definitions and Context Within the urban planning and transportation planning communities and among more mainstream literature there exist a multitude of definitions for the terms urban infill and infill development. However, in order to best relate to the application a typical practitioner might encounter, the litera- ture review focused on typical written definitions as defined by statute, agencies, and practitioners. This review resulted in the finding that even among similar agencies there can be sig- nificant differences in the definition of infill. As demonstrated by the example definitions in Table 2.1, definitions are often created for a specific purpose that may relate to the circum- stances of a specific geographic area. As a result, definitions often include quantifiable criteria that may not be transfer- able to other locales without substantial modification. 2.2.3 Definition of Infill Development Based on the survey responses, literature review, and the study objective, the research team selected the following defi- nition of infill development for the proposed methodology (4): Infill development or redevelopment is located in fully built areas, often in and around business districts; is walkable; is served by con- venient/ frequent transit; is commonly served by designated bicycle facilities; and generates significant non-automobile mode shares. Because of the prevalence of non-automobile trips in these areas, adjustments to data in the ITE Trip Generation Manual are appropriate. 2.3 Current Infill Trip Generation Practices Of the agency guidelines identified over the course of this study, nearly all require or support the use of ITE trip gen- eration estimates in preparing traffic impact analyses. Some of these agencies allow trip generation reductions for types Table 2.1. Example definitions of urban infill. Urban infill is the practice of developing vacant or underutilized properties within an urban area rather than undeveloped land in more rural areas (greenfield). Infill helps to prevent sprawl and can aid in economic revitalization. Source: United States Environmental Protection Agency. Urban infill means the development of vacant parcels in otherwise built-up areas where public facilities such as sewer systems, roads, schools, and recreation areas are already in place, and the average residential density is at least five dwelling units per acre, the average nonresidential intensity is at least a floor area ratio (FAR) of 1.0, and vacant, developable land does not constitute more than 10% of the area. Source: State of Florida, 2007 Statutes and Constitution. An infill opportunity zone is designated by a city or county and zoned for new compact residential or mixed-use development within 1/3 mile of specified transportation sites in counties with a population of over 400,000. The mixed-use development zoning consists of three or more land uses that facilitate significant human interaction in close proximity, with residential use as the primary land use supported by other land uses such as office, hotel, health care, hospital, entertainment, restaurant, retail, and service uses. The transit service serving the site has maximum scheduled headways of 15 min. for at least 5 hours per day. Source: State of California, Senate Bill 1636, Congestion Management: Infill Opportunity Zones, 2002.
7 of development typically found in urban infill areas (such as high-density housing, mixed use, and TOD), but only a few of the agencies provide quantitative adjustment factors or meth- ods for adjusting trip generation estimates for infill devel- opment. Even in urban infill areas, adjustment to ITE trip generation estimates is considered an exception and typically requires approval from the agency overseeing the preparation of the traffic impact analysis. In general, guidelines are provided for the following techniques: ⢠Adjusting standard trip generation rates based on transit use or mixed-use internal capture. ⢠Using or developing local trip generation or mode split data. ⢠Creating exceptions to, or revising, current transportation standards. ⢠Developing multimodal approaches to conventional level- of-service analysis. The survey of professional organizations and practitioners mirrored these findings, with the majority of respondents (68%) stating that trip generation adjustments must be justified and approved by the reviewing agency on a case-by-case basis. 2.4 Research in Estimating the Trip Generation of Infill Development The research team reviewed recently published and unpub- lished releases of methods for estimating infill trip genera- tion to identify the underlying approach for each estimation methodology. The six estimation methods the research team reviewed were determined to be based on one of the following approaches: ⢠Direct estimation based on regression analysis â Methods that estimate trip generation with derived regression equa- tions using a single type or multiple types of data as inde- pendent variables. ⢠Direct estimation based on empirical data â Methods that directly determine trip generation or derive rates through collection of data at sites with similar characteristics to a proposed development site. ⢠ITE rate adjustment based on regression â Methods that use various types of equations and data to develop factors that are applied to ITE trip generation rates or directly to trip estimates. ⢠ITE rate adjustment based on empirical data â Methods that apply factors to ITE trip generation rates or directly to trip estimates derived from data collected at sites with similar characteristics to a proposed development site. Table 2.2 summarizes the key attributes of the six methods reviewed for each of these approaches to estimating infill trip generation. As shown, four of the six methods use adjust- ments to ITE trip generation rates or directly adjust estimates derived from ITE trip generation rates, while the remaining two perform direct estimations of infill trip generation. Each method, even within the same approach category, employs a different technique using a unique set of variables. One commonality of the methods is that they all consider the context in which the study site is located. Methods using regression analysis correlate contextual characteristics with site trip generation, while methods based on empirical data select sites for data collection that are located in contexts with similar characteristics to the subject site. 2.5 Trends in Estimating Trip Generation Over the past two decades, the practice of estimating trip generation for new development has gradually shifted from focusing almost entirely on vehicle trips to estimating multi- modal travel based on context. This change in the practice is in response to a shift in land use planning focus in the United States, which now increasingly emphasizes smart growth, transit-oriented, infill, and traditional neighborhood devel- opment. This is resulting in development in many locations that is higher in density, more diverse in the mix of land uses, more compact, increasing in the quality and frequency of public transportation, and more walkable and bicycle friendly than the suburban pattern of segregated single land use development on which the ITE Trip Generation Manual is primarily based. Many agencies have made changes to their zoning ordinances to allow or encourage some of these development types. As the primary source of trip generation data and meth- ods in the United States, ITE has significant influence over the practice of estimating trip generation through publication of its recommended practices and informational reports. In 2012, ITE formed subcommittees to develop new chapters for the third edition update of the Trip Generation Handbook (5). The handbook is ITEâs recommended practice on estimating trip generation. The subcommittees are considering a variety of content updates, including those related to improved methods for estimating (a) multi-use internal capture, (b) pedestrian and bicycle travel, and (c) trip generation of infill development. Additionally, the subcommittee will be considering data col- lection techniques and the provision of further guidance on conducting local trip generation studies. The upcoming update is also expected to address whether the concept of person trips will become the fundamental basis for evaluating site and transportation impacts. Person trips, combined with the percentage breakdown of travel by mode, produce a broader spectrum of travel data that can be used to facilitate more comprehensive analysis than is typically
Table 2.2. Assessment of recent research in methods of estimating trip generation of infill development. Approach Category Example Method(s)/Source How Method Works Data Requirements Advantages Disadvantages Direct estimation based on regression analysis Trip generation for smart growth (7). Originally developed for the Environmental Protection Agency for smart growth and mixed- use development. Regression analysis based on numerous context and other descriptive variables. Estimates probability of non-automobile travel and adjusts project trips calculated with standard trip generation rates. Developed from household travel survey (HTS) data from eight metropolitan planning organizations (MPOs). Validated using mixed-use areas in the San Diego area. Vehicle-trip generation of study site based on ITE rates. Substantial area-wide data generally available from multiple sources. Some data requires significant research or computation of values. Calculates reductions for mixed-use as well as mode-share reductions in infill areas. A validated methodology for the San Diego region. Estimates area-wide trip generation. Not applicable to site-level (unless site is a large, mixed-use site) or specific land use analysis. Highly data intensive. Validated only for the San Diego region. Direct estimation based on empirical data California infill trip generation study (1). Derives trip generation rates for specific site land uses from person-trip cordon counts plus intercept surveys to determine mode-share and other data. Comprehensive cordon counts and intercept surveys for a minimum of 4 hours per day, person counts at all site entries. Vehicle counts (if feasible). Ability to collect multiple types of data, including travel, demographics, parking, cost, and policy related. Transparent process with high credibility. If trip rate database developed by researchers, negligible cost to use method. Highly labor intensive and disruptive to site, so permission to survey tenants is challenging. Relatively high cost per site; best if sponsored through funding source. No standardized data collection procedure. Many years before database is usable. ITE rate adjustment based on regression analysis Urban context-based trip rate adjustment (8). Uses regression analysis and the Portland-region Urban Living Infrastructure indicator (a measure of context) to develop adjustments to ITE rates for high- turnover (sit-down) restaurants, 24-hour convenience markets, and drinking establishments. Data for application requires determination of the Urban Living Infrastructure indicator requiring eight readily available variables, or use of a lookup table with less accuracy. Method correlates well with numerous easily obtained measures of context. Performs better than ITE rates in predicting trip generation of the subject uses in the Portland region. Method based on data collected at 78 sites and verified using data from 34 additional sites. Urban Living Infrastructure indicator or other measures of urban context may be used with similar results. Substantial data collection required to expand approach for additional land uses; currently only valid for three categories. Further, method needs evidence of transferability to other geographic regions, or requires data collection and validation in areas outside the Portland region. ITE rate adjustment based on regression analysis or data extracted from surveys HTS-based trip rate adjustment. Portland State University Dept. of Civil and Environmental Method uses data from travel surveys in a regional-scale model for predicting context-based adjustment factor (mode share) applied to ITE trip generation at Data for application requires determination of vehicle mode share for the urban context under study using equations, or using a lookup The use of HTS data to develop mode share and vehicle occupancy factors is valid within its stated limitations and within the geographic constraints of the HTS. A valid method for broader professional use if context-based factors derived from Seattle region HTS data are found to be transferable to other regions.
Engineering (8). the site level of development. The method divides HTS mode- share data into eight context classifications that serve as the variable for the model. table with less accuracy. Eight context classifications provide broader context variation than other methods. Context classifications control for socio-demographic variations. Method currently only valid in the Seattle region where the HTS data were obtained. ITE rate adjustment based on regression analysis Smart growth trip generation (9). Method uses limited site person- trip travel survey data to develop multivariate regression equations to adjust ITE rates. Adjustment factors are based on characteristics of the proposed development and its surrounding context. Uses a linear regression equation with the adjustment as the dependent variable and site and context characteristics as the independent variables. Requires person counts and intercept surveys. Method validated for, and applicable to, multifamily residential, office, and limited restaurant and retail (not yet available). The method uses vehicle-trip counts and site/context data from 50 smart growth sites in California. According to the model developers, even though the model is based on, and validated with, California person-trip generation data, it is believed to be transferrable to other urban locations. The model is limited to the uses defined in the validation process (multifamily residential, office, and limited restaurant and retail). Applicable study sites and surrounding context must conform to prescriptive criteria defining smart growth. For use with other land uses, data collection is required to expand the model. The model may not be transferable to regions outside of California. ITE rate adjustment based on empirical data Rate adjustment method based on mode share and vehicle occupancy factors (4). Uses limited vehicle and person- trip cordon counts and vehicle occupancy count equations to adjust ITE rates. Uses data collected to develop mode share and occupancy adjustment factors to apply to baseline ITE trip rates for any land use category (LUC) in the ITE Trip Generation Manual. Minimum data required include average vehicle occupancy and percentage automobile mode share. Requires data collection at (1) site(s) representing baseline ITE LUC, and (2) a site located in a similar context as proposed infill development. Depending on site, context, and type of data needed, data collection may require cordon counts of person trips and traveler surveys. Method is easy to comprehend, and computational procedure is transparent. With minimum data collection, the method is a low-effort/low-cost procedure to derive mode share and vehicle occupancy factors. Complex sites in more urban contexts may require intercept surveys. Method is consistent with ITEâs recommended procedures for developing local trip rates. Method in its simplest form is applicable to sites with exclusive on-site parking. Where off-site and on-street parking is used, or where a detailed mode-share breakdown is desired, the method requires traveler surveys that increase effort and cost. Selection of sites relies on user judgment, so risks associated with the method include collecting too little data or data at poorly selected sites, as well the potential for collecting data in an inappropriate context resulting in over- or underprediction of trips.
10 carried out under current typical traffic impact analysis pro- cedures. Ultimately, this could lead the profession to develop a person-trip generation database similar to the TRICS sys- tem in use in the United Kingdom and Ireland (6). 2.6 Summary and Conclusions on the State of the Practice The following is a summary of the key findings of the review of the state of the practice in estimating infill trip generation: ⢠While some public agencies employ methods to evaluate traffic impacts of infill development in urban areas, there is no standardized and nationally accepted methodology, and there is only limited guidance or data specifically attribut- able to infill development. ⢠A number of agencies permit trip generation reductions for development in urban areas, but they require support- ing information to justify reductions based on proximity to transit or mixed-use development. Supporting informa- tion often takes the form of local trip generation or mode- share studies. ⢠A small number of agencies provide local trip generation rates or provide specific methodologies for estimating trip generation. ⢠The number of recent research projects that have devel- oped methods of estimating trip generation for infill or smart growth development is an indicator of the trans- portation professionâs desire for methods to evaluate the impactsâand the benefitsâof current trends in land use development. ⢠Most jurisdictions in the United States take one of three approaches for evaluating site-specific traffic impacts of infill development (or associated forms such as transit-ori- ented and mixed-use development): (a) use the established trip generation data in the ITE Trip Generation Manual, with reductions for internal capture based on the method- ology in the ITE Trip Generation Handbook; (b) allow the application of pre-established maximum trip reductions for mixed-use internal capture and proximity to transit subject to the approval of the reviewing agency; and (c) allow devel- opment of an alternative approach to estimate traffic gen- eration on a case-by-case basis along with the provision of supporting data.