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87 this point the researchers had no evidence to verify the pos- thest), the points represent Mockingbird Station, Legacy sibility of higher percentages. Hence, for the purpose of this Town Center, Boca del Mar, Village Commons, Country Isles, research project, the researchers concluded that a conserva- and Atlantic Station. tive approach is to use the values of Tables 99 through 102 in A key premise about internal capture is that for a trip from the proposed estimation process developed in this project. At one land use to another at a mixed-use site, one direction of a future time, if subsequent surveys using similar procedures travel must be unconstrained (in terms of internal capture) show even higher percentages, those results could be incor- and the other must be constrained. In some instances, the porated into Tables 99 through 102. internal capture rates in both directions of travel are in per- fect balance and are, therefore, both constrained. Proximity Effects In Figure 21, the presumed unconstrained direction is des- ignated as a large dot and the presumed constrained direction Data collected in the pilot study and Florida surveys pro- as a small dot. If a site is constrained in the top chart, it must vided the basis for evaluating proximity effects on internal be unconstrained in the bottom chart; if a site is constrained capture. This analysis was performed to test the hypothesis that in the bottom chart, it must be unconstrained in the top travel distance between locations of interacting land uses chart. Each site must have an unconstrained value in one would affect the degree of interchange between those land uses. direction or the other. In addition, the unconstrained internal The analyses used surveyed interchanges and walking dis- capture values should exceed the constrained values on each tances between origin and destination. All three pilot study individual chart. developments had similar pedestrian environments--out- In the top chart, internal capture values at the unconstrained door sidewalks adjacent to buildings, mostly along internal sites decrease from around 16% at a proximity of 700 ft to two-lane streets or parking lots. Most sidewalks are land- around 5% at a proximity of 2,200 ft. In the bottom chart, scaped with trees, although in Mockingbird Station some internal capture of greater than 50% (at a proximity of 200 ft) sidewalks have no trees. In most cases, the sidewalks are at decreases to about 20% at a proximity of 1,500 ft. least 10-ft wide in commercial areas. No sidewalk was con- For many land use pairs, the database consists of only three sidered too narrow for people to walk or pass conveniently. data points representing unconstrained internal capture-- Mockingbird Station has an elevator and one main set and two two in one direction and one in the other. It is difficult to reach supplemental sets of stairs between the ground and second definitive conclusions about the effect of land use proximity levels serving the cinema, a few restaurants, and the DART on internal capture with so little data. To improve the likeli- rail station and bus transfer center. Although the elevator and hood of defining a reliable relationship between proximity and stairways undoubtedly impede some people in Mockingbird internal capture, data for various land use pairings with poten- Station, so few destinations required using the stairs that they tially common characteristics were grouped and examined. For were not considered further. example, trips to or from retail might have the same proximity- The three Florida sites are more spread out with most of the capture characteristics as trips to or from restaurants. As a land uses in pods. Each pod is conveniently walkable within. result of that analysis, two proximity relationships were iden- Many of the pods are not interconnected by sidewalks, but are accessible by walking or driving across parking aisles or lots. tified, as Figure 22 shows. However, each of these development pods is clearly designed Each point in the figure represents a measured uncon- to encourage internal interaction among land use activities. strained internal capture rate for a particular pair of land Data collected and compiled as part of this project provide uses at a single mixed-use site. The x-axis in the figure is the an indication of the effect of land use proximity on internal proximity distance. The y-axis is normalized to represent the capture. It was observed that as distance increases, the level of percent of the highest unconstrained value for the particular interaction (i.e., the internal capture) declines. To quantify this land use pair. relationship, internal capture rates derived from intercept sur- The square-shaped dots in the figure represent the proxim- veys were plotted against proximity of pairs of land uses. All ity and internal capture values for all land use pairs with res- land use pairs for the three newly surveyed pilot study sites and idential as the destination, for the origin end of the trip. In the three Florida sites surveyed in the mid-1990s were plotted. other words, these are a combination of the rates To illustrate this concept, the top chart in Figure 21 shows the internal capture observed at the six mixed-use sites for From office to residential, trips from retail/restaurant uses to residential uses. The bot- From retail to residential, tom chart shows internal capture observed to residential from From restaurant to residential, retail/restaurant. In the charts, each plot point represents a From hotel to residential, and single mixed-use site. From left to right (i.e., closest to far- From cinema to residential.

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88 from Retail/Restaurant to Residential 18.0% 16.0% 14.0% 12.0% 10.0% 8.0% 6.0% 4.0% 2.0% 0.0% 0 500 1000 1500 2000 2500 Distance (feet) Unconstrained Constrained to Residential from Retail/Restaurant 60.0% 50.0% 40.0% 30.0% 20.0% 10.0% 0.0% 0 500 1000 1500 2000 2500 Distance (feet) Unconstrained Constrained Figure 21. Example of relationship between internal capture percentage in unconstrained and constrained directions (between residential and retail/ restaurant land use pair).

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89 Figure 22. Relationship between proximity and unconstrained internal capture percentage for P.M. peak-period trips for land use pairs and directions with confirmed proximity effects. The best-fit curve for these data points is shown in the fig- the less-than-1,524-ft curve is 0.50. The data at longer dis- ure as Adjustment #1. The R-square for the curve is 0.58. tances does not track with that equation and was grouped to This curve is used in the estimation procedure described create a second intersecting line; that line connected from later in this chapter to account for land use pair proximity the extreme end points of the upper line to the midpoint adjustments to unconstrained internal rates. between the two points for the longest proximity distance in The triangular-shaped dots in the figure represent the prox- this data subset. imity and internal capture values for all land use pairs with The best-fit curve equations in Figure 22 intersect the x-axis either office or residential as the origin and retail or restaurant at proximity distances above which there would presumably as the destination, for both the origin and destination ends of be no internal capture. However, at the study sites, internal the trip. In other words, these are the rates capture was measured between land uses at the extreme lim- its of all six mixed-use sites where data were collected. To From office to retail, account for this assumed synergy between land uses no mat- From office to restaurant, ter how far apart as long as they are both within the mixed-use From residential to retail, center, both proximity adjustment lines in the figure are ter- From residential to restaurant, minated at an arbitrary minimum y-axis value of 0.10 (i.e., at To retail from office, 10% of the unconstrained values). This then leaves a minimal To restaurant from office, internal capture percentage at long distances. To retail from residential, and Note that these proximity adjustment relationships repre- To restaurant from residential. sent only a fraction of all potential land use pairs (only 13 prox- imity adjustment factors out of a total of 60 directional internal The best-fit curve for these data points is shown in Figure 22 capture rates for the 6 land uses). For the remaining 47 land use as Adjustment #2. The curve is actually two straight lines that pairs, a definitive relationship between proximity and internal intersect at a proximity distance of 1,524 ft. The R-square for capture rate could not be established with the available data.