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