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26 practice (approved by its International Board of Direction) Conclusions contained the first endorsed internal trip capture estimation technique for use in TISs for MXDs. Based on this review of past work and the personal experi- Most public agencies and preparers of TISs use the ITE ence of the research team, the following were selected as being method (or a locally developed variation of the ITE method). a reasonable starting place for NCHRP Project 8-51 to de- The two other approaches that are also commonly used are velop an improved internal trip capture estimation method: (1) a local agency accepted or established internal trip capture To be of value, the project should address both mixed-use reduction percentage to apply to estimated site vehicle trip and multi-use developments (hereafter referred to in com- generation and (2) negotiations between the study preparer bination as MXDs). and agency reviewer. Activity synergy between the different uses within an MXD Developers, through payment for TIS, have typically funded is what captures trips internally. Other factors contribute most previous site trip generation research; however, since the to making this synergy and interaction both possible and appearance of the ITE Trip Generation Handbook that endorsed more or less attractive compared with other opportunities. an estimation method and provided some data on capture rates Land uses that are most frequently identified as having for the most frequent mixed uses, a combination of high cost synergy of the type that affects trip making and that are of internal trip capture data collection and an existing accepted commonly included in MXDs include residential; retail method have resulted in no new comprehensive data. (especially convenience); office; hotel; restaurant; and en- Since the late 1980s, there have been numerous studies of tertainment (theater). However, within each general land various census and regional travel survey databases, limited use classification, there will be a need for subclassifications site data collection, and studies and surveys of related travel if a method is to be easily and accurately applied. Chapter 3 and development characteristics that could contribute use- addresses land use categories. ful material for developing an improved estimation tech- The research team identified other characteristics most likely nique. Many studies were related to mode of access and find- to influence internal trip capture and be most readily devel- ing ways to promote transit usage, including through use of oped in actual practice. Table 30 lists these characteristics. land use and development tools such as TODs. Internal trip Trip capture has been studied at essentially three develop- capture rates found in the research vary widely depending on ment levels: single-site project, larger multi-site develop- conditions and land uses, but for developments with major ment and activity centers, and neighborhoods and subareas. commercial components, capture rates (percentage of trips The issues and challenges are similar, but some implications made from internal points to internal destinations) typically of internal trip capture are different and the extent and com- ranged up to more than 30%. For mixed-use neighborhoods plexity of data collection will be different. Findings at each and small communities, internal capture reached 50% and level may not be directly transferable, at least quantitatively. even higher. Interaction between individual pairs of land Specifically, there are more different scales of mixed devel- uses, in the proper balance, also was found in similar ranges; opment that may act somewhat differently or have to be however, it appears from the available data that few develop- treated or have data collected in different ways: ments (all uses combined) completed by about 2000 can typ- Single developments; ically be expected to have internal capture rates much above Blocks of separate interactive developments; 30%, and that percentage requires the right mixes and bal- Small areas of blocks containing interactive uses; ances of land use mix. Neighborhoods and districts with multiple interactive Besides land use mix, other factors were found to affect in- uses; ternal trip capture. These include connectivity and proxim- Mixed- and multi-use subdivisions; ity between interacting land uses and location within an Multi-use activity centers; and urban area (thought to reflect both competing opportunities Small communities. and modal options). Conflicting information was found on The sites for which travel data were used to develop the the effects of development density. Modal impacts found recommendations in this study are all single master-planned were attributed to proximity to transit (with good service). developments. Mockingbird Station is a single block. Atlantic Trip generation rates and mode split were found to be af- Station and Legacy Town Center are multiple block districts fected by such traveler characteristics as income and vehicle containing fully integrated and adjacent complementary availability. However, no site-internal travel data have been uses. Boca del Mar, Country Isles, and Village Commons collected that included those characteristics, and they would all contain pod-type mixtures of single-use development be hard or impossible to accurately project for a proposed within a single development to provide the mixed-use development at the zoning stage. interaction.

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27 Trip capture percentages vary greatly among land uses and these methods are specified in local agency TIS require- development types. They also vary by time of day and prob- ments or even ordinances. Both approaches are easy to use ably to some extent by the day of week and by season. Var- and require minimal data. ious studies have found internal trips make up as little as 0 Since the advent of the first edition of the Trip Generation and as much as more than 60% of total trips generated. Sev- Handbook in 1999 (55), there has been wide acceptance of eral studies included multiple developments or areas and internal trip capture percentages contained in the hand- were able to compute averages. book or lower values accepted by review agencies. The cost The extent to which trips are captured internally may also of internal trip data collection is high compared with other be influenced by other factors, such as TIS components, which has resulted in little incentive for Availability of personal vehicle during the stay at the pri- developers to fund collection of new data. Obtaining devel- mary destination (accounted for by mode of access); oper commitments to fund additional data collection may Match between traveler characteristics and characteris- be a challenge unless there is expectation of major increases tics of potential destinations (e.g., market position ver- in internal trip capture credit. sus income levels); Little detail was found in the literature on data collection Availability of competing onsite and off-site opportuni- methods. The research team's familiarity with data collec- ties; and tion for internal trips has revealed a relatively high cost Internal and external accessibility (including such fac- necessitated by interviews, a low return rate on intercept tors as proximity, connectivity, cost, comfort, attractive- mail-back surveys, and, most crucially, significant variabil- ness, convenience, parking availability, etc.) to desired ity in questions and the way they were asked--which affects activities. data stability and accuracy. A standard, low-cost method Local data or more diverse and representative data points for collecting data is needed. regarding internal trips associated with the different MXDs Travel forecast models have been used to provide the basis and multi-use-development types is needed to improve the for internal trip estimation and even directly to estimate accuracy of predicting trips for MXDs. internal trips. Given the absence of intrazonal trips on the Despite the availability of the method provided in the Trip model network and limits to traffic analysis zones, these Generation Handbook, several other methods are being used. travel models are not usable for estimating internal trips Some are arbitrary (e.g., set or maximum percentages), and for TIS or traffic impact fee use. a few are incorrect for application to transportation or TIA or studies (e.g., ULI shared parking percentages). It appears In conclusion, the estimation and data-collection meth- that only the ITE method balances internal trips based on ods developed by NCHRP Project 8-51 should be easily the amount of each interacting land use. used, explained, and understood so that they can be used in Two methods are most currently used for estimating inter- zoning cases and other TIS applications as well as for other nal trip capture: The ITE method contained in the Trip more sophisticated uses. They should also be as economical Generation Handbook, 2nd edition (1), and percentages as possible while supplying enough data to be reasonably that local agencies establish as acceptable. In many cases, reliable.