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8 with other complementary locations, and the development Scope of Study must include land uses that generate activity that can be readily used by transit patrons. Specifically, the project had three objectives: to develop Internal Trip: An internal trip, as defined by ITE, is one 1. A classification system of MXDs that identifies site char- that is made without utilizing the major road system (2, acteristics, features, and context likely to influence trips p. 85). For the purposes of this project, the definition is subject to internal capture; expanded to include travel within a highly interactive area 2. A defensible improved methodology for estimating inter- containing complementary land uses and convenient in- nal trip capture with reasonable accuracy; and ternal on- or off-street connections that may use short 3. A data-collection framework to quantify the magnitude of segments of major streets. An example might be a one- travel associated with MXDs to determine appropriate re- block development consisting of residential, office, and ductions below single-use trip generation estimates. retail buildings with convenient sidewalk connections be- tween them and a single parking facility serving all three To accomplish these objectives, several tasks were completed: land uses. External Trip: An external trip is a trip made between land Compilation of a state-of-the-practice summary of meth- uses within the MXD and locations outside the boundaries ods in use to estimate internal trip capture for use in TIS; of the development. This excludes internal trips. Development of a prototypes methodology to guide the Internal Trip Capture (Site) Rate: Internal trip capture for subsequent work; a development site is the percentage of total trips (nor- Analysis of internal capture relationships; mally, but not always, vehicle trips when used for typical Determination of data needs; traffic impact studies) that are made internally to the de- Conduct of a pilot survey to test the data-collection method- velopment without using roads that are external to the site ology and provide additional data; being analyzed. The internal trip capture is most frequently Identification of data gaps and suggest data to be col- expressed in terms of a percentage or rate, but can be de- lected; and scribed in other forms such as equations. For example, if Documentation of the findings, conclusions, and recom- retail uses within an MXD generate 10 trips, 3 of which go mendations. to other land uses within the development and 7 of which go to external locations, the 3 internal trips are considered Following a review of available methods, it was determined internally captured. The internal capture is 3 out of 10 trips, that there were few methods and little data available that or 30%. MXDs addressed in this project may be a part of a could credibly be used to estimate internal capture for TIAs. major activity center. The level of internal connectivity and As a result, emphasis shifted from analyzing existing data to internalization of trips may be different for MXDs and ac- expanding the database through an additional pilot study. tivity centers. Only MXDs of less than 300 acres in size were Subsequently, a third pilot study was made possible through examined in this project. funding of a separate project by a different sponsor (Texas Internal Trip Capture (Area): This area can be defined to DOT). As a result, two additional tasks were added after the include all trips made internally to a defined area such that three pilot surveys: the trips do not use transportation facilities external to the area. For the purposes of estimating impact of such devel- 1. Analysis and compilation of data in combination with data opments and their internal trip capture on the transporta- available from other sources, and tion, care must be taken when considering the impact of 2. Refinement of the estimation methodology and factors and the internal trips on the (major) public road system pass- conduct a verification test. ing through the area. Trip Generation: Trips to or from a specific land use or a Past Research and Practice group of land uses constitute trip generation. Trips are inbound, outbound, or total. This portion of the chapter summarizes the state of the art Transportation or Traffic Impact Analyses (TIAs) or as it was at the time the background work was completed. Studies (TISs): TIAs are analyses of the impact of projected travel associated with existing or proposed land develop- Land Use Synergy ment and determination of needed access and transporta- tion system improvements to successfully accommodate Interaction of land uses has probably existed since the first the development without undue deterioration of travel settlements had people who performed different types of work. conditions. Older towns and cities had all different types of uses within

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9 walking distances since walking was the principal mode of system (10). When evaluating internal trip capture for an area, transportation. When suburbanization started to occur in the site, or activity center, the presence of safe facilities for pedes- late 1800s, there began to be separations of different land use trians and bicyclists can be a factor in the ability for a project types. By the mid 20th century, zoning and single-use areas to attract and internalize higher percentages of trips. had become the normal way to develop. The importance of pedestrian-based design is emphasized However, a new type of development began to be seen: the in many studies promoting connections between land uses, major shopping center, followed by regional malls with restau- but adding the transit component completes the overall pic- rants, theaters, and other uses. Next came the MXDs, which ture. TODs combine the MXD with good pedestrian connec- had combinations of uses. Developers found the mixed- or tions and direct access to transit. Portland's Land Use, Trans- multi-use developments appealing because such developments portation, and Air Quality (LUTRAQ ) approach to land use offered a way to capture several types of development in one and transportation planning worked to reduce vehicle-miles project that was larger than any single project they might cre- of travel (VMT), increase transit usage, increase walking and ate in the same place. Moreover, the interaction and sharing biking, and reduce trips overall. Internal trip capture was of facilities had the potential to reduce long-term develop- assumed to explain a portion of the VMT reduction based on ment costs and increase profitability. Trends in MXDs have the design, proximity of uses, and overall accessibility (11). In progressed through many phases--from early urban villages a later study by the Oregon DOT, Reiff and Kim identified to downtown complexes, early mixed-use towers, atrium de- several similar characteristics that may influence internal velopments, and open centers and, most recently, to town trip capture including density; land use dissimilarity; urban centers and urban villages (3, pp. 922). form; proximity to complementary uses (specifically retail- What made MXDs work then and now is the interaction residential); building coverage ratio (i.e., compactness); and and shared-use features. The key to success is synergy between local street connectivity (12). the land uses. Table 1 shows what ULI considers to be major Ewing and Cervero identified a number of potential in- land use combinations that have the most synergy. dependent variables that might be used to establish travel Several other factors that affect internal trip capture have characteristics of MXD: land use mix, availability of conven- been suggested by Steele (6)--mixing uses in proximity, clus- ience services, accessibility of services, perception of safety, tering, and siting buildings to promote interaction, connectiv- and pleasing aesthetics (13). Much of their quantitative ity between buildings and parcels, and proper time-phasing. findings were derived from regional transportation models To those Cervero added density, diversity, and other factors in and may not be directly adaptable for individual sites and design such as accessibility and high-quality pedestrian con- developments. venience and provisions (7). The Sacramento Transportation Kittelson & Associates listed key characteristics to be ana- and Air Quality Collaborative lists land use balance as one of lyzed for MXDs when determining internal capture rate, which the most crucial factors in reducing off-site trips (8). Filion were as follows (14, p.7-1): et al. found that the synergy works best if it is pedestrian- based to reduce the dependence on personal vehicle travel Site Characteristics and internalize the trips (9, p. 427). Development size; In their evaluation of multimodal areas, Guttenplan et al. Land uses and quantity of development for each use; discuss the importance of the infrastructure for walking and Parking spaces provided for each use; biking when assessing the performance of the transportation Density of development for each use; and Table 1. On-site support and synergy in mixed-use projects. Degree of Support/Synergy Retail/ Culture/Civic/ Land Use Residential Hotel1 Entertainment2 Recreation Office Residential Hotel Retail/Entertainment Cultural/Civic/Recreation Bullets: =very weak, =weak, =moderate, = strong, = very strong. 1 Synergy is strongest between high-end hotels and condominiums, less so for mid-priced hotels and residences. 2 Restaurants and food services are the main source of benefit for offices. Source: (3, p. 85.)

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10 Proximity of residential and non-residential develop- that a significant amount of survey data would be available ments within the development. based on responses to a 2004 ITE member survey; however, it Transit Characteristics was determined that respondents misinterpreted a question Bus or rail routes serving the development; regarding data in hand. Of the 77 persons interviewed, 12 Proximity of transit stops to the development; were able to provide data either directly or indirectly related Transit assistance provided to workers by employers; to internal trip capture. Some data had already been acquired and by the research team. No additional new survey data was On-site connectivity to transit stops. found. Some information related to regional travel modeling Non-Motorized Transportation Characteristics was discovered as was some general or limited findings that Internal connectivity among land uses (for pedestrians, may be usable as supporting information. bicyclists, and motorists); The interviews confirmed that the most frequently used Parking spaces designated for carpools or vanpools; resource for estimating internal trip capture is the ITE Trip Fee charged for employee parking spaces; and Generation Handbook (2, p. V-39). It contains summaries of Availability of on-site bicycle amenities. studies of internal trip capture for individual sites and devel- opments as available through 1998. With caveats, Chapter 7 Gordon and Peers noted that the jobs-to-housing balance of the report provides suggested capture rates and a recom- was a crucial component to internal capture of trips. People liv- mended procedure for use in TIS for proposed developments. ing near where they work were more likely to stay within the The recommended procedure permits estimates for several development area for daily activities (15, p. 144). The Florida different land uses and includes a procedure for balancing DOT (FDOT) cites the following factors to consider when eval- internalization of trips based on the size of the component uating internal trip capture: remoteness from other develop- land uses. The handbook also contains unconstrained inter- ments and areas, development phasing, income compatibility nal capture rates (that assume sufficient quantity of comple- between residents and patrons, competing opportunities, and mentary land use to accept internal trips) for office, retail, internal circulation (16). and residential land uses. These rates are based on surveys Other factors that have been discussed by the ITE Trip that had been made available to ITE by 1998.Capture rates Generation Committee during development of ITE's Trip for origins within a multi-use development range between Generation Handbook as affecting MXD synergy include 0% and 53%; for destinations, they range between 0% and competing opportunities and proximity, size of both the devel- 37%. Tables 3 and 4 provide the unconstrained internal cap- opment and the individual land uses, maturity and viability ture rates used in the ITE internal trip capture procedure. of the development and its components, and compatibility of The handbook also recommends procedures for data- patron/employee income levels with the development's uses. collection including interview questions. The handbook in- cludes several summaries of key quantitative and qualitative Trip Capture--Sites findings from previous studies of trip generation characteris- The research team reviewed websites and contacted repre- tics at mixed-use sites. For each study, available data are pre- sentatives of a cross section of organizations and agencies that sented on the mix and sizes of land uses within the site, the prepare or review traffic impact studies (TISs) to determine level of internalization of trips within the site, overall trip gen- what surveys or other data may have been completed in recent eration characteristics for the site, and the level of pass-by years. Table 2 summarizes the responses. It had been expected trips for the site. In most cases, the analyses use traditional Table 2. Summary of interview responses. Sources Have Completed Surveys or Other Suggested One Type Called Interviewed1 Information or More Others Agency Rep. 35 34 3 9 TIA Preparer 44 35 8 5 Researcher 7 3 0 1 Other 5 5 1 2 Total 91 77 12 17 1 Sources not interviewed were called at least twice and either declined interview or did not return calls.

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11 Table 3. Unconstrained internal trip capture rates for trip origins within an MXD. Weekday Percent Trips Captured Internally1 Midday Peak P.M. Peak Hour of From To Hour Adjacent Street Traffic Daily Office 2% 1% 2% Office Retail 20% 23% 22% Residential 0% 2% 2% Office 3% 3% 3% Retail Retail 29% 20% 30% Residential 7% 12% 11% Office NA NA NA Residential Retail 34% 53% 38% Residential NA NA NA 1 Based on limited data; NA = not available. Source: (2, p. 93) Table 4. Unconstrained internal trip capture rates for trip destinations within an MXD. Weekday Percent Trips Captured Internally1 Midday P.M. Peak Hour of From To Peak Hour Adjacent Street Traffic Daily Office 6% 6% 2% Office Retail 38% 31% 15% Residential 0% 0% NA Office 4% 2% 4% Retail Retail 31% 20% 28% Residential 5% 9% 9% Office 0% 2% 3% Residential Retail 37% 31% 33% Residential NA NA NA 1 Based on limited data; NA = not available. Source: (2, p. 94) ITE independent variables. In several cases, new variables are 250,000 and 1.3 million sq. ft. (with three of the sites having introduced. less than 300,000 sq ft). Districtwide Trip Generation Study, FDOT, District IV, Internal Trips. Table 6 lists the proportion of daily trips March 1995. This study sponsored by FDOT was to develop generated within the surveyed mixed-use sites, which were databases of internal capture rates for MXD sites and for pass- internal to the sites. The internal capture rates ranged be- by capture rates. Table 5 presents a summary of the character- tween 28% and 41% (average 36%). istics of six surveyed mixed-use sites (17 ). The sites range in Three of the mixed-use sites were further evaluated to de- area from 26 to 253 acres (with four of the sites being 72 acres termine the internal capture rates for different types of trip- or less). The office/commercial square footage ranges between makers. As listed in Table 7, the internal capture rates for trips Table 5. Characteristics of mixed-use sites surveyed by FDOT. Site Size Office Commercial Hotel Residential Mixed-Use Site (acres) (sq ft) (sq ft) (rooms) (units) Crocker Center 26 209,000 87,000 256 0 Mizner Park 30 88,000 163,000 0 136 Galleria Area 165 137,000 1,150,000 229 722 Country Isles 61 59,000 193,000 0 368 Village Commons 72 293,000 231,000 0 317 Boca Del Mar 253 303,000 198,000 0 1,144

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12 Table 6. Daily internal capture rates at rates were reported for trips to/from banks and sit-down FDOT sites. restaurants. Mixed-Use Development Site Internal Capture Rate Pass-By Trips. Table 9 lists the pass-by trip proportions Crocker Center 41% as determined through intercept surveys for the six study sites. Pass-by trips are made as intermediate stops on the way Mizner Park 40% along a street on the way from an origin to a primary trip des- Galleria Area 38% tination (2, p. 29). Four of the six sites have pass-by rates be- Country Isles 33% tween 26% and 29%. These rates appear to be high given the size and composition of the developments. Future surveys Village Commons 28% should attempt to verify these rates. Boca Del Mar 33% Average 36% FDOT Trip Characteristics Study of MXDs, FDOT Dis- trict IV, December 1993. This study was the predecessor of the March 1995 FDOT trip generation study (18). Much made by site workers is typically higher than rates found for of the data that were collected and many of the relationships visitors to the site (i.e., users of the mixed-use-site services). derived in this first study are included in the 1995 study re- The rates by trip-maker are consistent across all three sites. On sults described previously. The 1995 study did not report on average, 37% of user trips are internal and 47% of worker trips two relationships presented in the 1993 report: a procedure are internal to the mixed-use site. for estimating internal trips and internal trip capture by time Finally, three of the mixed-use sites were further evalu- of day. ated to determine the internal capture rates of individual land uses. Table 8 lists the reported internal capture rates by Internal Trip Estimation Method. Relationships were land use/trip purpose. In general, the higher internal capture developed for estimating internal trips as a function of the Table 7. Internal trip capture rates by type of trip-maker at FDOT sites. Trip-Maker Crocker Center Mizner Park Galleria Area Average Users 37% 38% 36% 37% Workers 46% 49% 46% 47% Total 41% 40% 38% 40% Table 8. Internal trip capture rates by land use type at FDOT sites. Land Use/Trip Purpose Crocker Center Mizner Park Galleria Area Office (General) 11% 11% 7% Office (Medical) 15% 12% Retail 36% 30% 42% Restaurant (SitDown) 54% 52% Restaurant (Fast) 26% 56% Hotel 30% 29% Bank 48% 62% Cinema 23% MultiFamily Housing 11% 50% Retail Mall 39%

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13 Table 9. Daily pass-by rates at Table 11. Comparison of internal trip FDOT sites. capture: estimation model vs. actual. MXD Site Daily Pass-By Rate Trip Capture MXD Crocker Center 26% Model Estimate Actual Mizner Park 29% Country Isles 24.5% 33.0% Galleria Area 40% Village Commons 31.9% 27.5% Country Isles 28% Boca Del Mar 35.0% 32.7% Village Commons 14% Source: (18, p. V-39) Boca Del Mar 29% Overall Average 28% estimates were within 15% of actual; the third differed from actual by about 25%. This study also collected information on internal capture combination of two interacting land use types in terms of de- rates by time of day. Table 12 shows the total internal capture velopment units (e.g., residential dwelling units and office/ rates for the three surveyed mixed-use sites. The estimated retail square footage). Good relationships were developed for daily, midday, and evening peak period internal capture rates two internal trip type categories: residential-retail and retail- are quite similar. The mean values for the entire survey period retail. The office-retail relationship was less definitive. shown in the table have a high degree of statistical validity; The study presented a working hypothesis that the number the maximum two-tailed errors calculated using the binomial of internal trips from one land use type (A) to another land distribution, with 90% confidence-level methodology, are all use (B) within a mixed-use site is directly proportional to the less than 5%. size of Land Use A and also proportional to the size of Land This report also identified the percentage of employees Use B. This suggests a functional relationship of the form who are also residents and vice versa (18, p. V-27). Table 13 shows the findings for each of the three developments (not Person Trips between A and B = Constant Land Use A included in ITE report [1]). The 16% to 19% of employees Land Use B being locally employed are possibly a major factor in the re- where: ported internal trip capture rates. Land Use A = total site land use of Type A in residential Trip Generation for MXDs, Technical Committee Report, units or 1,000 sq ft; Land Use B = total site land use of Type B in residential Colorado-Wyoming Section, ITE, January 1986. This study units or 1,000 sq ft; and included interviews to determine whether persons entering Constant = a value that is solely a function of the two and leaving mixed-use sites came there for multiple purposes land use types. (19 ). Table 14 lists the size and mix of land uses at the eight sites with interviews to ascertain internal trip-making. In the equation shown above, the constant can be derived from information collected on person trips between different Internal Trips. A key piece of information collected was land use types and on the sizes of these different land uses. the number of trip purposes that a respondent accomplished Table 10 shows the derived constants. on the particular trip to the mixed-use site. Overall, a major- Application of these coefficients was tested for the three ity (77%) of the interviewees indicated that their trip involved MXDs. Table 11 shows the results (not included in ITE Trip only a single stop within the mixed-use site, but this still left a Generation Handbook [1]) (16, p. V-39). Two of the three significant proportion (23%) who indicated they were making Table 10. Internal trip coefficients for paired land use types. Evening Peak Midday Peak Period Paired Land Uses Period Daily (12 noon2 P.M.) (4 P.M.6 P.M.) Residential/Retail 0.00082 0.00103 0.00557 Retail/Retail 0.01219 0.00995 0.07407 Office/Retail 0.00087 0.00024 0.00232

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14 Table 12. Internal person trip ends by time of day. Average Recorded Range Recorded Time Period at Three Sites at Three Sites Daily 31% 2833% Midday Peak Period (12 noon2 P.M.) 32% 3035% Evening Peak Period (4 P.M.6 P.M.) 30% 2832% Table 13. Percent locally employed residents and locally residing employees. MXD Country Isles Village Commons Boca Del Mar Residents employed within 3.9% NA 0.9% development Employees residing within 16.1% 16.8% 18.9% development two or more stops within the mixed-use site. Based on these occupied dwelling units, with 180 townhouse-style condo- interview results, the study authors estimated that 25% of an miniums and 2,120 single-family detached units. Commercial otherwise total number of trips generated by individual trips development consisted of an 82,600sq ft shopping center; a were eliminated with the linking of internal trips within the 63,000sq ft business park; a 14,000sq ft medical center; and eight surveyed mixed-use sites. a 4,400sq ft restaurant. There were also recreational facilities Table 15 presents the number of trip purposes/stops including a golf course, tennis courts, swimming facilities, reported by survey respondents. The responses are arrayed and several lakeside recreation facilities. Finally, there was a according to the primary destination. Office buildings and day-care center, a church, an elementary school, and a mid- a post office generated the greatest number of multi-stop dle school. The study had the overall goal of determining the trips. Theaters, restaurants, and banks tended to generate onsite (internal) and off-site (external) traffic generation at lower-than-average numbers of multi-stop trips within the Brandermill. mixed-use site. Internal Trips. Based on the various data collected, the The Brandermill Planned Unit Developments Traffic Gen- split between internal and external trips was estimated. As eration Study, Technical Report, JHK & Associates, Alexan- Table 16 shows, 51% of the daily trips, 55% of the P.M. peak- dria, Virginia, June 1984. Brandermill is a large, planned hour trips, and 45% of the A.M. peak-hour trips were inter- MXD (and, in many respects, is a small town/village) located nal to (or captured within) the mixed-use site. Additionally, approximately 10 miles southwest of Richmond, Virginia. At 46% of the persons employed in Brandermill also reside in the time of the study (20), there were approximately 2,300 Brandermill. Table 14. Characteristics of mixed-use sites with interviews. Size Site Land Uses (sq ft) 1 240,917 Retail, General Office, Government Office, Restaurants, Health Club, Bank 2 731,846 Retail, Office, Restaurants, Hotel 3 500,000 Retail, Office, Restaurants, Motel, Theaters 4 115,000 Retail, Restaurants, Hardware Store, Supermarket 5 1,000,000 Regional Mall, Retail, Restaurants, Banks, Office, Theaters 6 110,000 Retail, Theaters, Restaurants, Banks 7 95,104 Retail, Restaurants, Supermarket, Medical Office, Savings and Loan 8 300,000 Retail, Hardware, Restaurants, Supermarkets, Post Office

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15 Table 15. Percentages of persons within multi-sites by number of purposes (stops) and by primary destination. Number of Purposes/Stops Stated by Interviewee Primary Destination 1 Purpose 2 Purposes 3+ Purposes Bank/Savings and Loan 83% 8% 9% Hardware Store 76% 22% 2% Supermarket 77% 17% 6% Theater 93% 7% 0% Office/Work Site 68% 31% 1% Small Retail Shop 73% 14% 13% Restaurant 85% 12% 3% Health Club 71% 29% 0% Post Office 63% 24% 13% Total (Average) 77% 16% 7% Travel questionnaires were distributed to residences and worded, and asking how many trip purposes are being sat- used to measure the level of internal trip ends for home-based isfied on one trip to a development may not yield the same trips. As Table 17 shows, approximately 35% of the daily responses as asking how many stops or how many differ- home-based trips from Brandermill residences are linked ent businesses were visited within the development or how with trip ends within Brandermill. Over 39% of the daily trip many driving trips would have been needed otherwise. It also ends to Brandermill residences began within Brandermill. For demonstrates that the effect of a successful (financially) de- the shopping center trips within Brandermill, approximately velopment's additional trips may overshadow internal trip two-thirds of the trips originate within Brandermill during capture (this is also one reason why trip generation data are the midday and evening peak hours. These internal percent- so highly dispersed). For this project, the research team sought ages are higher than the Florida examples. out developments that appeared to be active and had low vacancy rates. Other Surveys. As previously mentioned, a study by the ITE recently conducted a member survey asking about avail- Colorado/Wyoming Section Technical Committee of ITE in- ability of additional studies on internal trip capture (21). The cluded surveys of eight MXDs ranging in size between about survey identified methods currently being used to estimate 95,000 and 1 million sq ft with varying combinations of com- internal trip capture. Unfortunately, a question that inquired ponent land uses (19). That study recommended that peak- about trip capture data was misunderstood, and responses in- hour trip generation rates be reduced by only 2.5% even dicating 48 sources for additional information were incorrect. though the surveys showed 25% internal trips. The reason is Other findings are described later in this section. that driveway counts showed a lower reduction below esti- In Transportation Research Record 1617, Steiner studied six mates based on ITE rates. While one of the most ambitious of shopping districts that were integrated within residential areas the early studies of internal trip capture, this study illustrates and found that in these districts walking was more prevalent, a key point: survey responses depend on how a question is ranging from 24% to 41% of users studied (22, p. 29). Steiner Table 16. Split between internal and external trip ends at Brandermill. Trips A.M. Peak Hour P.M. Peak Hour Daily Total Generated 2,570 2,935 33,540 External Trips 1,420 1,325 16,280 Internal Trips 1,150 (45%) 1,610 (55%) 17,260 (51%)

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16 Table 17. Internal trip ends linked with Brandermill residences and retail centers. Home-based trips with Home-based trips with origins Hours destinations within Brandermill within Brandermill 7 A.M. to 9 A.M. 18% 51% 9 A.M. to 4 P.M. 44% 50% 4 P.M. to 6 P.M. 55% 34% 6 P.M. to 7 A.M. 41% 34% Daily 35% 39% Shopping center trips with Shopping center trips with Hours destinations within Brandermill origins within Brandermill 11 A.M. to 1 P.M. 66% 65% 4 P.M. to 6 P.M. 66% 52% used the ITE rates for shopping centers, rather than for mixed largest U.S. suburban activity centers (SACs) (26). That re- use. Steiner compares trip rates from both ITE and NCHRP search developed a comprehensive procedure for determining Report 187 (23) with the local daily trips that occurred in the six travel patterns, including trips internal to the activity centers. shopping districts studied and found situations where the ITE Data were collected at the six SACs listed in Table 20. In the and NCHRP methods overestimate and underestimate trips following discussion, larger centers refer to the three centers when compared with the local data (22, p. 35). Kittelson & having at least 15 million sq ft of office/retail space in each; Associates conducted surveys for three mixed-use sites in smaller centers refer to the remaining three, which have less Florida: the Crocker Center, Mizner Park, and the Galleria than 8 million sq ft. area. They found that the rate of internalization of trips ranged For activity center residents, Hooper found that 13% to between 38% and 41% (14, pp. 57). 50% of employed residents work within the activity centers, Mehra and Keller reported relationships between the per- with the average being 27% to 33% based on activity center centage of internal trips and the ratio of office space to residen- size and whether they lived in owned or rented dwellings. An tial units and the ratio of commercial space to residential units average of 50% of office employees was found to make mid- (24). Based on a Richmond Regional Planning District Com- day trips outside their buildings; 20% to 33% of those trips mission Planned Unit Developments study they had reviewed, were internal to the activity centers. Work-related, eating, they reported finding that A.M. peak-period home-based work and shopping trips were the most common midday trips for trips were internalized at rates between 0% and about 15% office employees. The study also examined stops to and from and that midday home-based other trip internal percentages work during peak periods and found that such stops within ranged up to more than 40%. Both percentages increased as the activity centers were made on an average of 13% to 15% the ratio of office or other commercial space per dwelling unit of the trips. increased in ranges of more than 80 sq ft/dwelling unit. JHK & Associates conducted a shared parking study for Table 18. Internal trips by office workers to San Diego that included user surveys. Table 18 shows the re- onsite destinations. sults of surveys of office worker trips to internal destinations in two MXDs (25). For both developments, 6% of the mid- Internal Trip Purpose Internal Trips day trips made by office workers are to onsite locations. Marriott La Jolla Village Office work location to Table 19 shows the percentage of internal trips to restau- Mission Valley Professional Center rants and retail for five San Diego MXDs. Also shown are per- Business 6% centages of trips made by walking. Shopping 14% 13% Eat Meal 29% Trip Capture--Activity Centers Health Club In a comprehensive study of suburban activity centers, Other Hooper conducted interviews of employees, patrons, and vis- Total 6% 6% itors to office, retail, residential, and hotels within some of the

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17 Table 19. Percentage of internal trips to restaurants and retail. Origin Origin Component Land Uses Percent Percent Percent Percent MXD Walking Walking Restau- Gen'l Medical Resi- Internal Internal Retail Cinema Hotel To Restaurants To Retail rant Office Office dential La Jolla Village 23% 14% University Square 15% 14% 2% 10% Hazard Center 21% 6% 20% 18% La Mesa Village 25% 21% 13% 17% Point Loma Place 4% 25 Hooper found that internal trips involving retail centers Cervero found that the existence of a retail component in within activity centers were higher in larger activity centers. office buildings in major activity centers was associated with P.M. peak-hour internal trips averaged 24% (7% to 57% range) an 8% reduction in vehicle trip rates per employee (29). Filion while midday trips averaged 37% (7% to 68% range). In the et al. found that over 40% of office building employees make A.M. peak periods, hotel trips internal within the large and restaurant trips outside their buildings, but internal to the largest activity centers averaged 19% and 37%, respectively, activity center, averaging 2.2 such trips per week (9, pp. 420, and 27% and 36% in the P.M. peak period, respectively, with 428434). About one-third make similar trips for shopping, the internal percentage increasing with the amount of activity averaging about 1.6 trips per week. Four times as many retail center office space. customers said they shopped within the activity center due to Table 21 presents a summary of some relevant relation- location rather than because of specific retailers located there. ships reported by Hooper in NCHRP Report 323. Many of the About 55% of the internal trips are made on foot (compared internal trip percentages resemble the 30% order of magni- with 26% driving and 19% by transit), with preference being tude reported in some of the studies previously mentioned. given to "easy and pleasant" (pedestrian environment, no traf- From the information provided, it appears that the larger fic conflicts) walking experiences. The researchers noted that SACs have higher percentages of internal capture. This is log- more internalization of trips resulted from better balance, ical since larger activity centers (1) offer more opportunities proximity, and pedestrian connectivity of interacting uses. to meet traveler needs and (2) similarly offer more choices to meet a given need. Trip Capture--Neighborhoods, Small Communities, Zietsman and Joubert conducted extensive studies at three and Subareas MXDs in South Africa (27, 28). They distinguished between internal trips made out of pure convenience and planned in- Several studies have been conducted in neighborhoods and ternal trips that would have saved a trip on the external road subareas to assess the amount of trip internalization as well as network. Internal capture rates ranging from 5% to 33% were the differences in vehicle trip generation. Some have used observed depending on factors such as center size, types of regional travel modeling to compare characteristics of neigh- secondary land uses, and weekends versus weekdays. borhoods or areas with different design characteristics. The Table 20. Characteristics of NCHRP Report 323 study sites. Office Space Retail Space Hotel Residential Suburban Activity Center Gross Floor Area Employees Gross Leasable Area Employees Rooms Dwelling Units Bellevue (WA) 4.7 million 12,880 3 million 6,150 1,000 N/A South Coast Metro 3.5 million 10,465 4 million 6,865 1,800 2,300 (Orange Co., CA) Tysons Corner 17.0 million 35,020 7 million 13,355 3,100 15,000 (Fairfax Co., VA) Parkway Center 13.0 million 39,000 2 million 3,430 1,800 206 (Dallas, TX) Perimeter Center 13.0 million 32,500 3 million 5,150 910 2,000 (Atlanta, GA) Southdale 4.0 million 13,700 3 million 6,155 2,200 3,000 (Minneapolis, MN) Source: (2)

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18 Table 21. Internal trip-making characteristics at NCHRP Report 323 study sites. Average Range OFFICE EMPLOYEES Percent who make an intermediate stop within SAC 10% 7% to 15% on the way to work 11% 6% to 16% on the way home from work Percent who make midday trips internal to the activity center -- 29% to 33% SACs with high level of professional employment1 -- 20% to 23% SACs with low level of professional employment OFFICE VISITORS--Percent from within activity center A.M. peak period o all SACs -- 15% to 59% o small SACs 30% -- o large SACs 54% -- P.M. peak period o all SACs -- 15% to 68% o small SACs 33% -- o large SACs 58% -- REGIONAL MALLS--Percent trips which are internal to SACs Midday o all SACs 37% 7% to 68% o small SACs 23% -- o large SACs 47% -- P.M. peak period o all SACs 24% 7% to 57% o small SACs 14% -- o large SACs 31% -- EMPLOYED RESIDENTS--Percent who work within SACs all -- 13% to 50% small SACs 27% -- large SACs 33% -- HOTEL TRIPS--Percent internal to SACs A.M. peak period o all SACs -- 13% to53% o small SACs 19% -- o large SACs 37% -- P.M. peak period o all SACs -- 15% to 46% o small SACs 27% -- o large SACs 36% -- 1 Sites with at least 60% of the work force in professional, technical, managerial, or administrative positions. Source: 2, 26. research team chose not to include those here since the level reducing vehicle trip generation rates (31). She cautioned that of detail is insufficient for use for development sites and the other factors such as income, household size, and other fac- need is for primary data. tors affect transportation choices and highlighted the impor- In comparative surveys of Austin, Texas, neighborhoods, tance of separating the effects of those factors. Handy found that walkable neighborhoods with neighbor- Ewing et al. used regional travel surveys to identify internal hood shopping could generate 6.3 walking trips per (adult) travel within suburban communities in Florida that ranged in resident per month to internal neighborhood retail establish- size between about 600 to more than 15,000 acres (32). Al- ments and that 77% of those apparently substituted for driv- though this is not the development scale sought for this re- ing trips (30). This might correspond to a reduction in the search, it is interesting to note that within complete suburban residential vehicle trip rate of 3% to 5%. communities, internal trips averaged about 25% but ranged Steiner added that higher density puts destinations closer between 0% and 57%. Ewing et al. attributed the variation to together, making it possible to walk for some trips, thereby two factors: (1) larger population communities had higher