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F-1 APPENDIX F Validation of Estimation Procedure Estimation Procedure mate single-use vehicle trip generation for component land uses of the seven developments. Where businesses were closed The estimation procedure was applied to seven different during a peak period and there were no observed trips to or developments for which at least land use information, peak from the business (e.g., cinema during A.M. peak), no trips were hour cordon counts, and proximity information were avail- included in the validation estimate. able. Four of these developments provided data for this study; Southern Village had additional land uses (a school and the other three did not. The validation test was to see how well park-and-ride lot) that were not included in the internal cap- the estimation procedure could begin with ITE trip generation ture estimate; those were handled as additional land uses. data and reproduce the external vehicular cordon volumes. Information provided in the source document was used as Five of the developments had directional cordon traffic vol- the basis for the trip generation estimate. Table F-2 shows the umes available for both peaks. These developments included: results numerically. Figures F-1 through F-4 graphically com- pare the results for the five developments for which complete Mockingbird Station, data were available. Error comparisons were also made and Legacy Town Center, are shown in Table F-3. Atlantic Station, Table F-2 rows contain data as follows: Crocker Center (independent site, Boca Raton, Florida), and Mizner Center (independent site, Boca Raton, Florida). 1. Counted at cordon: vehicles(persons) counted using site driveways; Two developments had on non-directional P.M. peak period 2. Estimator output: directional volume of vehicles (persons) counts available. They were: estimated with recommended estimation method: First four columns: volumes as described, Boca del Mar and Last four columns: percent internal trips; Southern Village (independent site, Chapel Hill, North 3. From survey directional volume of vehicles (persons) Carolina). derived from survey: First four columns: volumes as described, The validation test compared four different estimation Last four columns: percent internal trips; methods to determine which method produced the results 4. Estimator/counted: ratio of estimated trips divided by closest to the cordon counts: counted trips in respective columns; and 5. Unadjusted/counted: estimate using raw ITE trip genera- the estimator described in this report, tion divided by counted trips the estimator, but without the proximity adjustment, the existing ITE estimation method, and Table entries for Southern Village contain additional rows unadjusted ITE trip generation. to account for land uses that do not qualify for internal cap- ture under the recommended procedure. Development data and approximations of surveyed mode Table F-2 shows comparison of external vehicle and person split and vehicle occupancies were input to the estimation pro- trips estimated by each method. Also shown are estimated cedure. Table F-1 shows the ITE land use codes used to esti- internal capture percentages. The most important results are

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F-2 Table F-1. ITE land use codes used in validation. Land Uses NCHRP Project 8-51 ITE Land Use Code Subgroup Classification Office - 710 Retail - 820 Quality sit down 931 High turnover 932 Restaurant Fast food, no drive-through 933 Fast food with drive-through 934 Cinema - 444 Hotel - 310 Single family detached 210 Residential Apartments 220 Townhomes 230 Additional Land Use Port and terminal Park-and-ride lot 090 the external trip estimates. Figures F-1 through F-4 show the yield approximately the same results for two MXDs. In this comparisons of vehicle trips for both A.M. and P.M. peak periods case, Boca Center is better estimated using the existing ITE and both inbound and outbound directions. In Figure F-1, it is method. evident that for the A.M. peak hour inbound vehicle trips, the In total, the recommended method--with or without the NCHRP estimation methods--both with and without the proximity adjustment--produces more reliable estimates for proximity adjustment--produce the best results for three of four of the five developments. the five developments; the current ITE method is closest for The results for the other two developments--Boca del Mar one site and slightly better than the NCHRP method for and Southern Village--show two different patterns. For Boca another site. Atlantic Station is more closely estimated by both del Mar, both the existing ITE and recommended methods unadjusted trip generation and the current ITE method. The produce significantly low estimates, but are closer than the rec- current ITE method is better than raw trip generation, but the ommended method without proximity adjustments or the ITE method developed in this project is even closer to the counts. method. The raw estimate is above the actual external count, Figure F-2 shows similar results for A.M. peak hour out- but it and the ITE method are the closest of the estimates bound vehicle trips with the recommended estimator (both (about 4 percent closer than the recommended method with with and without the proximity adjustment) producing the proximity adjustment). For Southern Village, the results are best results for four of the five developments. This time Mizner very different. The recommended method (both with and Center is better estimated by raw trip generation and the cur- without proximity adjustments) produce estimates very close rent ITE method. As with the previous comparison, the ITE to the counts. method is an improvement on raw trip generation. Table F-3 may quantify the degree of accuracy or error more The P.M. inbound comparison shown in Figure F-3 shows clearly, recognizing that the statistics presented represent the that the NCHRP method with proximity adjustment pro- sum of combined results. The average error shown is the sim- duces the closest estimates for two sites, with the methods ple sum of the percent deviations from the counts as derived in with and without proximity about equal for the two sites, and Table F-2. On average, as a group the estimates all exceed the the raw ITE trip generation closest for one site. Again, Mizner counts (for example, the recommended method with proxim- Center was better estimated by another method (this time ity adjustment is an average of 4 percent). This is very mislead- raw trip generation), but the other four are best estimated by ing and not relevant for single developments because overesti- the recommended method. mates and underestimates tend to cancel each other out. What Figure F-4 shows the comparison for P.M. peak hour in- may be of value in those percentages is that they could result in bound trips. As for the other time periods and directions, one the sum total trip generation of several developments in an or the other of the NCHRP methods produces the closest esti- area. However, that is not what is being validated here. mates in four of the five cases. The methods with and without More applicable is the absolute average error, which is the proximity adjustments are each best for one MXD while both sum of the magnitudes of the errors averaged over the five

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F-3 Table F-2. Summary of estimator validation comparisons. Vehicle Trip (Person Trips) Percent Internal Trips (Peak Period) A.M. Peak Hour P.M. Peak Hour A.M. P.M. Development/data In Out In Out In Out In Out Mockingbird Station Counted at cordon 272(385) 128(213) 367(595) 353(586) Estimator output 259(329) 107(165) 422(565) 412(588) 19% 32% 33% 33% From survey 35% 46% 36% 42% Estimator/counted 0.95(0.85) 0.84(0.77) 1.15(0.95) 1.17(1.00) Without proximity adjustment Estimator output Same Same 422(563) 411(586) Same Same 33% 33% Estimator/counted Same Same 1.15(0.95) 1.16(1.00) With ITE Trip Gen Handbook data Estimator output 322(409) 156(242) 537(715) 523(745) No data No data 15% 15% Estimator/counted 1.18(1.06) 1.22(1.14) 1.46(1.20) 1.48(1.27) Unadjusted ITE Trip Generation report Estimator output 399 233 798 832 0% 0% 0% 0% Unadjusted/counted 1.47 1.82 2.17 2.36 Atlantic Station With proximity adjustment Counted at cordon 962(1012) 455(502) 1023(1396) 1038(1260) Estimator output 796(843) 252(308) 962(1126) 1151(1342) 17% 37% 36% 34% From survey 40% 30% 41% 42% Estimator/counted 0.83(0.83) 0.55(0.61) 0.94(0.81) 1.10(1.07) Without proximity adjustment Estimator output Same Same 938(1097) 1124(1310) Same Same 38% 36% Estimator/counted Same Same 0.91(0.79) 1.08(1.04) With ITE Trip Gen Handbook data Estimator output 952(1130) 398(484) 1232(1445) 1604(1750) No data No data 16% 13% Estimator/counted 0.99(1.11) 0.87(0.96) 1.29(1.04) 1.55(1.39) Unadjusted ITE Trip Generation report Estimator output 1122 473 1690 1992 0% 0% 0% 0% Unadjusted/counted 1.17 1.03 1.65 1.92 Legacy Town Center Counted at cordon 734(819) 641(779) 933(1187) 955(1122) Estimator output 736(906) 690(850) 1003(1236) 912(1123) 15% 16% 34% 36% From survey 32% 25% 48% 44% Estimator/counted 1.00(1.11) 1.08(1.09) 0.95(1.04) 0.95(1.00) Without proximity adjustment Estimator output Same Same 923(1136) 831(1023) Same Same 39% 42% Estimator/counted Same Same 0.98(0.96) 0.87(0.91) With ITE Trip Gen Handbook data Estimator output 864(1065) 821(1009) 1231(1516) 1413(1740) No data No data 27% 24% Estimator/counted 1.18(1.30) 1.28(1.30) 1.32(1.28) 1.48(1.55) Unadjusted ITE Trip Generation report 909 862 1598 1502 0% 0% 0% 0% Unadjusted/counted 1.24 1.34 1.71 1.57 Boca (ex-Crocker) Center Counted at cordon 488 219 281 532 Estimator output 525 189 342 461 13% 26% 32% 31% From survey No data No data No data No data Estimator/counted 1.08 0.86 1.22 0.87 Without proximity adjustment Estimator output Same Same 342 461 Same Same 32% 31% Estimator/counted Same Same 1.22 0.87 With ITE Trip Gen Handbook data Estimator output 617 271 385 502 No data No data 26% 33% Estimator/counted 1.26 1.24 1.37 0.94 Unadjusted ITE Trip Generation report 655 295 566 678 0% 0% 0% 0% Unadjusted/counted 1.34 1.35 2.01 1.27 (continued on next page)

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F-4 Table F-2. (Continued). Vehicle Trip (Person Trips) Percent Internal Trips (Peak Period) A.M. Peak Hour P.M. Peak Hour A.M. P.M. Development/data In Out In Out In Out In Out Mizner Center Counted at cordon 220 145 547 328 Estimator output 239 99 417 388 13% 25% 29% 35% From survey No data No data No data No data Estimator/counted 1.09 0.68 0.76 1.18 Without proximity adjustment Estimator output Same Same 412 383 Same Same 30% 35% Estimator/counted Same Same 0.75 1.17 With ITE Trip Gen Handbook data Estimator output 267 134 425 402 No data No data 27% 32% Estimator/counted 1.21 0.99 0.78 1.23 Unadjusted ITE Trip Generation report 272 137 613 585 0% 0% 0% 0% Unadjusted/counted 1.24 0.94 1.12 1.78 Boca del Mar With proximity adjustment Counted at cordon - - 2187 2-way Estimator output - - 915 895 - - 26% 28% From survey No data No data 7% 8% Estimator/counted - - 0.83 2-way Without proximity adjustment Estimator output - - 689 676 - - 44% 47% Estimator/counted - - 0.62 2-way With ITE Trip Gen Handbook data Estimator output - - 839 831 - - 33% 35% Estimator/counted - - 0.76 2-way Unadjusted ITE Trip Generation report - - 1241 1209 - - 0% 0% Unadjusted/counted - - 1.12 2-way Southern Village Counted at cordon - - 1336 2-way Estimator output - - 546 438 Additional trips for non MXD uses - - 97 290 Total estimated - - 645 731 - - 11% 13% From survey No data No data No data No data Estimator/counted - - 1.03 2-way Without proximity adjustment Estimator output - - 537 429 No data No data N/Aa N/Aa Additional trips for non MXD uses - - 97 290 Total estimated - - 637 722 Estimator/counted 1.01 2-way With ITE Trip Gen Handbook data Estimator output 574 466 - - 6% 8% Additional trips for non MXD uses - - 97 290 Total estimated - - 671 756 Estimator/counted 0.99 2-way Unadjusted ITE Trip Generation report 633 512 - - 0% 0% Additional trips for non MXD uses - - 97 290 Total estimated - - 730 802 Unadjusted/counted 1.15 2-way a Person trips not known for non-MXD uses

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F-5 Figure F-1. Comparison of estimates to cordon counts: A.M. peak-hour inbound direction. developments. This shows more clearly what deviations-- ommended method brings the estimates significantly closer above or below actual--were found. Clearly, by examining to actual. Note that the difference between the actual and the figures and Table F-3, it is easy to determine that the raw absolute value of the errors shows that there are both over- trip generation greatly overestimates external vehicle trip estimates and underestimates occurring. generation for the validation sites. The existing ITE method The standard deviation shown in Table F-3 better repre- is a major improvement from raw trip generation. The rec- sents the estimated probable magnitude of error that might Figure F-2. Comparison of estimates to cordon counts: A.M. peak-hour outbound direction.

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F-6 Figure F-3. Comparison of estimates to cordon counts: P.M. peak-hour inbound direction. occur using these estimation methods. Again, the relative The standard deviations for the recommended method, both magnitudes of error among the methods place them consis- with and without proximity adjustment, are about 20 percent tently in the same order. of the actual external inbound and outbound volumes. This is It is clear that the recommended method provides more less than the variations in the raw ITE nondirectional trip gen- accurate estimates. Since the existing ITE method was devel- eration rates for the component land uses. For example, for the oped from data from three of the six developments used in land uses listed in Table F-1, the standard deviations for their this NCHRP project, the recommended method can only be A.M. and P.M. peak hour trip generation rates are all in excess viewed as being a further improvement. of 50 percent of the mean. Figure F-4. Comparison of estimates to cordon count: P.M. peak-hour outbound direction.

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F-7 Table F-3. Comparison of error statistics. Error Type Raw ITE Existing Recommended NCHRP Explanation Trip ITE Method Generation Method With No Proximity Proximity Adjustment Adjustment Average error +55% +26% -4% 7% Average error for sum of all sites Absolute average error 55 28 17 17 Average magnitude of error per site Standard deviation 68 34 20 19 Expect two-thirds of site estimates within this error range Not clear, however, is whether or not the proximity adjust- 2. The proximity adjustment, available at this time for the ment adds any current value. The validation results show no P.M. peak period, tends to make slightly more conservative significant statistical benefit. It has sufficient data only for the estimates but overall does not, at this time, improve accu- P.M. peak period (and less of that than would be desired). racy over a group of estimates. It can produce significant There is no A.M. proximity adjustment recommended at this effects for larger developments. time. On the other hand, the only examples for which the results were better without the proximity adjustment was It would be logical for ITE to consider the recommended when both variations of the new method were overestimat- method for inclusion in the next edition of its Trip Generation ing. In all cases the proximity adjustment either has no sig- Handbook. The researchers recommend this since it could nificant effect or renders the estimate more conservative increase trip generation estimation accuracy. The advisory (higher). committee that ITE uses to review potential new material may wish to test further both the existing method and the recom- mended method with more MXDs for which it can obtain the Conclusions needed data. This could help to determine if the proximity The validation supports two principal findings: adjustment shows enough added value in its current form to be included in the next edition. 1. The recommended method does produce noticeably more In addition, the research team confirmed the desirability accurate results than either raw ITE trip generation esti- and need for more surveys to expand the database. Six sam- mates from the ITE Trip Generation report or the existing ples are far better than three. Addition of several more could method described in the Trip Generation Handbook. This possibly provide the basis for confirming the value of the is true with or without the proximity adjustment. proximity adjustment.