Journal of Arboriculture 31(6): November 2005 307 that the median difference value was –0.017, while the mean was –0.039. Regression analysis showed a mild inverse relationship between TSI and alligator cracking (R2 was –0.002, P = 0.0358). Analysis for the other distresses (i.e., block cracking, distortions, longitudinal/transverse cracking, patching/utility patching, rutting/depressions, and weathering) found that the data were not normally distributed and, therefore, regression analysis was not conducted. Economic Analysis Thirty years after planting, the calculated TSI values were 41 for Hackberry Drive and 9 for Crape Myrtle Drive. Using these TSI values, as well as 0 (no street tree shade), and assuming these values remained constant, predicted PCIs were plotted for a 30-year period (Figure 3). Pavement management systems often determine a set of “break points” to effectively time maintenance activities. These points represent critical PCI values, signifying the need for pavement repair before conditions deteriorate to a level where a more expensive strategy is required. This scenario used a break point of 70 as an indicator for the need to resurface. It was assumed that resurfacing was performed with a slurry seal applica- tion, a typical and cost-effective maintenance strategy. It was also assumed that with each slurry seal treatment, the PCI value immediately increased to 100. By applying this break point to the graph, maintaining a PCI of at least 70 over 30 years on the unshaded street required a slurry seal applica- tion every 5 years, or approximately six slurry seals within 30 years. Maintaining a PCI of at least 70 on Crape Myrtle and Hackberry drives required slurry sealing every 6 and 12 years, respectively, or 5 and 2.5 applications within 30 years (Table 2). Assuming slurry seal applications cost $2.05/m2 ft2 ($0.19/ ) (Buus 2002), and this price remained fixed over the 30- year period, one application cost $829 per street segment. The cost of maintaining a PCI of 70 was $4,971 on the unshaded street segment, $4,142 on Crape Myrtle Drive, and $2,071 on Hackberry Drive (Table 2). Thus, shade on Hackberry Drive is projected to reduce costs for repaving by $2,900 (58%) over the 30-year period, or $7.13/m2 ($0.66/ft2 ) compared to the unshaded street. Shade from Figure 3. PCI degradation curves for the unshaded street (TSI = 0), Crape Myrtle Drive (TSI = 9 ), and Hackberry Drive (TSI = 41). the smaller crape myrtles is projected to save $829 (17%). The benefits from shade on Hackberry Drive compared to Crape Myrtle Drive illustrate the value of larger-stature trees. Projected savings from the larger hackberries ex- ceeded savings from the crape myrtles by $2,071. Also, the need for fewer, larger-stature trees would reduce tree planting costs by 50%. While this analysis does not include a potential higher tree care cost for the hackberry trees, it also does not consider greater environmental and property value benefits from the larger trees (McPherson 2003). Limitations and Future Research Table 2. Number of slurry seals required over 30 years, total costs, savings because of the need for fewer slurry seals, and savings per unit pavement surface for shaded compared to unshaded street segments (area = 406.5 m2 [4,375 ft2 ]). Scenario Unshaded Slurry seals Total cost ($) Savings ($) Savings ($/m2 6 Crape myrtle 5 Hackberry 2.5 4,971 4,142 2,071 829 2,900 2.04 7.13 0.19 0.66 The scope of this study extended only to residential asphalt concrete streets “typical” of those in the sample. The complex relationships among factors that affect pavement performance, such as environmental conditions, material composition, traffic loads, and the type/technique of preservation methods, are not well understood, and their confounding effects are not well quantified. A change in any one of these factors may considerably affect the relation- ships uncovered in this investigation. Results, therefore, are site specific and have limited applicability. In addition, the sample was comprised of 40- to 60-year-old streets that were maintained by surface preservation strategies, which limits the applicability of the results primarily to overlays and resurfacing treatments. Because the behavior and performance of Modesto street segments were used to create the deterioration curves, any changes ) Savings ($/ft2 ) ©2005 International Society of Arboriculture
November 2005
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