National Academies Press: OpenBook

Enhancing Internal Trip Capture Estimation for Mixed-Use Developments (2011)

Chapter: Chapter 4 - Conclusions, Recommendations, and Suggested Research

« Previous: Chapter 3 - Findings and Applications
Page 104
Suggested Citation:"Chapter 4 - Conclusions, Recommendations, and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2011. Enhancing Internal Trip Capture Estimation for Mixed-Use Developments. Washington, DC: The National Academies Press. doi: 10.17226/14489.
×
Page 104
Page 105
Suggested Citation:"Chapter 4 - Conclusions, Recommendations, and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2011. Enhancing Internal Trip Capture Estimation for Mixed-Use Developments. Washington, DC: The National Academies Press. doi: 10.17226/14489.
×
Page 105
Page 106
Suggested Citation:"Chapter 4 - Conclusions, Recommendations, and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2011. Enhancing Internal Trip Capture Estimation for Mixed-Use Developments. Washington, DC: The National Academies Press. doi: 10.17226/14489.
×
Page 106
Page 107
Suggested Citation:"Chapter 4 - Conclusions, Recommendations, and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2011. Enhancing Internal Trip Capture Estimation for Mixed-Use Developments. Washington, DC: The National Academies Press. doi: 10.17226/14489.
×
Page 107
Page 108
Suggested Citation:"Chapter 4 - Conclusions, Recommendations, and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2011. Enhancing Internal Trip Capture Estimation for Mixed-Use Developments. Washington, DC: The National Academies Press. doi: 10.17226/14489.
×
Page 108

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.

104 This chapter pulls together the conclusions, recommenda- tions, and lessons learned during this project. Statements made herein are intended to help practitioners use the results of this project either to estimate internal capture for MXDs or to add to the database and perhaps further refine the methodology and tools provided. Existing Practice Internal capture for MXDs is of most interest to those who either prepare or review TIAs for such developments, but transportation planners and developer consultants are also interested in internal capture and the resulting external trip generation. Some additional uses include planning for TODs and preparing environmental impact statements or assessments. ITE provides a recommended practice for estimating inter- nal capture and associated external trip generation for what it calls “multi-use developments.” As described, those devel- opments have characteristics similar to the common defini- tion used for MXDs. The research team accepted the two terms as used as being essentially equal. The ITE method doc- umented in their Trip Generation Handbook (1) is the most widely used technical method. The other widely used approach is a policy-determined flat percentage reduction in external trips. Such percentages are established by local planning, zoning, or transportation engi- neering officials for use in TIAs prepared to support applica- tions for zoning, subdivision, site plan approval, or access permits. The percentages are usually arbitrarily selected for use throughout the jurisdiction. These percentages are most typi- cally in the range of 10%, but were found to range from less than 5% to as much as 25%. Most percentages are conservative compared with internal capture data found in past research and this project. Other approaches found included tables of applicable rates and a formula to modify ITE estimates. The ITE method covers only trips among the three most fre- quent components of MXDs—office, retail, and residential. Data are available for the weekday P.M. peak hour, for midday, and for what is called “daily,” but which is drawn from data collected between noon and 6:30 P.M. The ITE method has nothing for the A.M. peak hour. The policy percentages men- tioned above are applied to each analysis period used. There is some limited use of invalid applications for internal capture estimation. The two found most frequently were use of shared parking reduction percentages and metropolitan area travel forecast model intrazonal trip percentages. Shared park- ing reductions apply only to parking accumulations in a park- ing facility serving multiple uses; the percentage reduction applies only to parking accumulation, not trip generation. Intrazonal trips apply to complete traffic analysis zones used in regional travel forecast models. Zones may range from a block to a square mile. Intrazonal trips are for the complete zone and are not applicable to portions of a zone. Estimates are also accurate only to a regional level, not a development site level. Neither method should be used for estimating inter- nal capture for MXDs. Six land uses are the most frequent components of MXDs— office, retail, restaurant, residential, cinema, and hotel. Most major MXDs have all of these. Most other MXDs have at least four. MXDs come in all sizes and layouts: some are vertically integrated and developed in one block, some are spread over several or many blocks with land uses well mixed or concen- trated in interconnected single-use areas. Six MXDs analyzed in this project ranged in size from 7 and 300 acres. All were single developments from one master plan developed either to integrate fully all land uses or otherwise to promote interaction between onsite land uses. There are other larger MXD types of developments such as SACs and even new towns or very large self-contained urban sections. These last two types were not covered in this project because it was felt that they act differ- ently than does the MXD of 300 acres or less and because they are far less frequently found in most states of the country. C H A P T E R 4 Conclusions, Recommendations, and Suggested Research

105 With the increase in emphasis on livability, compact cities, and smart growth in general, MXDs have become more pop- ular. Many are found in midtown-type urban areas (i.e., the central portion of a city or urban area that is outside the CBD but has higher densities than suburban or general urban and may include an outlying business district). Others are found in suburban locations and a few in urban peripheries. The research team did not include downtowns because they would be very difficult to survey and do not develop as one project or development and, therefore, would not need a TIA for the downtown. During the period this project was active, the research team received dozens of calls asking for internal capture data for land uses and time periods not included in the ITE method. Requests were most frequently received for • A.M. peak-hour internal capture rates; • Land uses not included in the ITE method—most notably hotels, cinemas, and restaurants; and • Very large MXDs in outlying areas. Available Data There are very limited data available that are capable of sup- porting internal capture rate estimation methodology that can use information that is available at the time of zoning. Three Florida surveys plus three pilot studies conducted for this project were the only surveys with enough detail to develop internal capture methodology • For both A.M. and P.M. peak hours; • For use with information that is available at the time of zoning requests and can be reliably projected; • That provides the ability to analyze the effect of proximity of land uses to each other; and • That is sensitive to differences in land use mix. Some cordon counts have been completed for various peri- ods and could be used for validation testing, but, by themselves with land use information, they do not provide what is needed to develop a sensitive procedure. More data are needed. Internal Capture Estimation Methodology Expanded ITE Methodology This project expanded the database from three to six devel- opments and, after considering options, expanded the ITE method to • Add the weekday A.M. peak hour; • Add restaurant, cinema, and hotel land uses; • Create a land use classification structure that would permit disaggregation of the six land uses to more detailed cate- gories should enough data become available; • Include the effects of proximity (i.e., convenient walking distance) among interacting land uses to represent both compactness and design; and • Provide a method that could easily be put in spreadsheet form. This method was tested for its ability to estimate external vehicle trip generation. The existing ITE method estimates produce about one-half of the estimation error that raw ITE trip generation rates produce. The method developed in this project cuts the estimation error in half again, or roughly to about one-fourth of the raw trip generation rates. The recommended method is described in Chapter 3. The researchers recommend its use for developments of up to 300 acres. Additional data and/or further testing could vali- date its use for larger developments, but that has not yet been attempted. The researchers do not recommend use of this method for downtowns, SACs, or new town types of devel- opment; the researchers do not believe it will be applicable. The method produced has a component that estimates the effects of proximity. Unfortunately, the database is small enough for the P.M. period that factors could only be devel- oped for some land use pairs. Absence of A.M. peak-hour data from the Florida studies precluded any A.M. proximity factors from being developed. This project’s estimation method gen- erally produced slightly closer P.M. estimates with the prox- imity factor included. It is recommended for use, but it is also recommended that when additional data becomes available, attempts should be made to develop proximity factors for more land use pairs. Suggested Modifications to Existing ITE Procedures As mentioned previously, the recommended estimation method builds on the current ITE internal trip capture proce- dures contained in the second edition of the Trip Generation Handbook (1). Incorporation of this project’s recommenda- tions could be accomplished by performing the following: • Expanding Tables 7.1 and 7.2 of the Trip Generation Hand- book (1) to include all six land uses covered in this report; and • Adding the proximity adjustment to be made after the unconstrained internal capture estimates are performed but before the balancing process. The data collection procedures could be modified to include those recommended in this project, including the next section.

106 Data-Collection Methodology A methodology and procedural instructions were devel- oped for the selection of data-collection sites and for the data collection itself. Those procedures were used to conduct sur- veys at three MXDs. The procedures were refined as a result of the experiences and lessons learned. Appendix C describes the recommended method. The researchers recommend that additional data be col- lected. The researchers suggest that MXDs selected meet at least the following criteria: • Be representative of typical MXDs being developed or being planned so the data will be of use for years to come; the area in which the MXD is located should also be representative; • Have at least four land uses so that most land use pairs are included; • Have owners or managers who will permit the needed sur- veys to be conducted; • Be easy to conduct a large enough sample for an affordable cost (in 2006 dollars, each survey cost about $50,000 to set up and conduct and to summarize the resulting data); • Be generally in the range of 300 to 500 acres or less; and • Be economically successful (by appearance), be mature (i.e., fully occupied for at least a year), and be in an area that is mostly developed. It is expected that NCHRP will turn over the results of this project and its data to ITE for inclusion in its database. ITE is also the body that issues recommended practices for this type of methodology. Organizations that collect additional inter- nal capture data are encouraged to provide a copy of the data and analyses to ITE for further use and future refinement to what was produced in this project. Recommended Changes to the Procedures Used in This Project Based on the experience of collecting and using data follow- ing procedures initially recommended for this project, four changes are recommended for consideration to improve the quality and content of data: 1. Conduct inbound interviews in addition to exit inter- views. Although there is developer/manager resistance to inbound interviews, they would increase the accuracy of the survey data. Questions about the previous trip before the one being interviewed drew some illogical results and included many trips made before the peak period of inter- est. If the development owner/manager resists giving per- mission, attempt to conduct inbound interviews in loca- tions where business will not be impeded. Office building and residential building lobbies are good places for con- ducting inbound interviews. 2. Delete the questions about the previous trip if inbound interviews can be conducted. The researchers found incon- sistencies and confusion associated with responses to those questions. 3. Add a time for the previous trip’s arrival at the interview building if no inbound interviews can be conducted. The time is needed to determine whether it was made during the A.M. or P.M. peak period. 4. Consider adding a question seeking induced trip infor- mation. An add-on question to attempt to determine induced trips was asked as part of the Legacy Town Cen- ter interviews. Respondent understanding about the ques- tion was inconsistent, so the results were not reported herein. However, it was evident that some of the internal trips made may have been induced or resulted from hav- ing proximate interacting land uses. The question asked whether the respondent would have made the trip being discussed if the selected destination did not exist within Legacy Town Center. A substantial number of respondents answered affirmatively—that is, they said they would not have made the trip had it required travel outside Legacy Town Center. Hence, such a trip would not represent a reduction in external trips: it would be an addition—made internally. Lessons Learned Several lessons were learned in this project that will be of interest to researchers and practitioners in this field. • Detailed data are very scarce and expensive to produce. • There has been little willingness of sponsors to fund data- collection efforts since the advent of the current ITE esti- mation method and adoption of flat reduction percentages by numerous agencies. • Due to the shortage of data, there is significant apprehen- sion on the part of development review agencies about whether trip generation at MXDs is actually less than the sum of its free-standing components—that is, if there is actually internal capture. • Owners/managers of some developments are reluctant to permit surveys. They have concerns about the interviews discouraging patrons from doing business at the MXD. In all three pilot studies conducted for this project, permission was gained to conduct only exit interviews—not inbound interviews. This was based on owner/manager belief that patrons would not be bothered after they had already done their business in a particular establishment where the inter- views would be conducted. • Data clearly show that there is internal capture in the ranges previously documented.

• The recommended method did produce estimates of exter- nal vehicle trips that represent counted volumes quite closely. While results varied in accuracy among the develop- ments tested, the recommended method was the closest of the methods tested in four of five cases for which directional volumes were available for both A.M. and P.M. peak hours. For two other sites with partial data, the recommended method was clearly the best for one and it was approxi- mately equal to the existing ITE method for the other. In total, the recommended method displayed about half the estimation error of the existing ITE method (13% versus 23%, respectively), both of which are well below the error using just raw ITE trip generation rates (53%). Suggested Research Although this project has made progress in estimation of internal capture, the database is still sparse and much that is thought to be logical about MXD travel characteristics is still unproven and even largely untested. The research team recom- mends that the following additional research be performed: • Collect more data at MXDs. Data are needed from at least six more sites that have five to six land uses. • Test the applicability of the existing methodology for MXDs of different sizes, character, and land use compo- nents independent of the additional data collection. Use validation tests similar to those used in this project. The only data needed are a complete directional cordon count for the A.M. and P.M. peak hours plus development data and a good site plan from which to estimate proximities. • Attempt to determine what differences design character- istics of MXDs have on external travel. Parking availabil- ity, degree of direct interconnection, and vertical versus horizontal integration are three such characteristics. • Ascertain the effect of off-site competing opportunities on internal capture. There may be a method of using GIS data and external trip data from the recommended surveys to ascertain these effects. • Devise and add a survey question to ascertain induced trip information. This would permit an assessment of whether MXDs result in induced trips because of the internal oppor- tunities. Note that such a question was asked during one of the three pilot studies, but respondents frequently had a dif- ficult time grasping the concept of an “extra trip that might otherwise not have been made.” Application in Practice Estimation Methodology and Data-Collection Framework This research project developed an improved estimation methodology and data-collection framework for use in esti- mating internal trip capture in MXDs during weekday A.M. and P.M. peak periods. The estimation methodology is based on weekday A.M. and P.M. peak-period survey data from three MXDs in Texas and Georgia (conducted as part of this project) plus similar weekday P.M. peak-period data from three devel- opments in Florida (conducted prior to this project). The six developments surveyed ranged from about 7 to 300 acres in size and had between four and six primary land uses each. This report presents a technical advancement beyond the internal capture method published in the Trip Generation Handbook, second edition, published by the Institute of Trans- portation Engineers (1). The researchers believe that the limited validations conducted for the proposed estimation method confirm that the results provide accurate approxima- tions of external trip generation for typical MXDs consisting of typical office, retail, restaurant, residential, cinema, and hotel land uses, consistent with the accuracy of trip generation esti- mates for single-use developments as portrayed in such refer- ences as Trip Generation, eighth edition (2). User Instructions and Cautions At the time of publication of this report, the approach devel- oped in this research had not yet been advanced through the ITE process for development of recommended practices and, therefore, it should not yet be considered as an ITE–approved methodology. This report presents information in Chapter 3 on how to use the proposed estimation procedure, but the researchers and the overseeing NCHRP project panel felt it is important to encourage users to adhere to the following instructions and cautions in using the proposed estimation methodology: • Identify specific land use components of the MXD and classify them into the six classifications—office, retail, restaurant, residential, cinema, and hotel—covered by the estimation methodology. Any component land uses that do not fit into those six classifications or are too unique to be considered normal for a classification should be kept separate. No internal capture is estimated in the proposed methodology for trips between uses within each of these categories (e.g., two or more different retail uses). • For each land use within the MXD, estimate single-use trip generation individually. Then, sum the individual estimates into the six aggregated classifications: office, retail, restaurant, residential, cinema, and hotel. Do not combine development units into the six classifications and then use one single-use trip generation rate or equation to esti- mate trip generation for the aggregated land use. • When applying the internal capture estimation method- ology, use the percentages suggested in Chapter 3 unless local data are available from developments similar to the development being analyzed. Users are cautioned that data 107

108 gathered in a method different than the data-collection methods described in this report may not be applicable and could produce inaccurate internal capture estimates. • Do not apply the internal capture percentages from this report to other land uses. Internal capture estimates were not developed for land uses beyond the six classifications provided herein. The extent of the internal capture for other land use pairs has not been tested as part of this project. The results presented in this report are based on surveys of six MXDs and validation was limited to seven such MXDs. As a result, some members of the project’s advisory panel strongly recommend that additional research, data collec- tion, and validation testing be conducted before the method is adopted for use in TIAs. Furthermore, caution should be exer- cised in the application of this methodology—for example, it cannot be concluded that the methodology will be appropri- ate for MXDs that differ significantly from those surveyed in this project in terms of • Regional context, including competing opportunities out- side the development; • Access and parking; • Scale of the development; • Complementary land uses, including specific pairs of busi- ness types; • Specific residence types, • Other component characteristics within each land use category; • Proximity and connectivity between each pair of land uses, especially the layout of the land uses relative to each other; • Other characteristics such as proximity to transit and pedes- trian access within and around the site; and • Colder locations that might limit or constrain pedestrian traffic. Request for Additional Data Users are encouraged to collect and contribute additional data using the data-collection procedures described in this report. Such data could be used to further enhance the accuracy of the proposed methodology and/or expand the number of land use classifications covered by the methodology. New data should be forwarded to the Institute of Transportation Engi- neers, 1627 I Street, Suite 610, Washington, D.C. 20006-4007 or by email to ite_staff@ite.org.

Next: References »
Enhancing Internal Trip Capture Estimation for Mixed-Use Developments Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

TRB’s National Cooperative Highway Research Program (NCHRP) Report 684: Enhancing Internal Trip Capture Estimation for Mixed-Use Developments explores an improved methodology to estimate how many internal trips will be generated in mixed-use developments—trips for which both the origin and destination are within the development.

The methodology estimates morning and afternoon peak–period trips to and from six specific land use categories: office, retail, restaurant, residential, cinema, and hotel. The research team analyzed existing data from prior surveys and collected new data at three mixed-use development sites. The resulting methodology is incorporated into a spreadsheet model, which is available online for download.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!