304 McPherson and Muchnick: Effects of Shade on Pavement Performance residential streets are inspected every 4 to 6 years, depend- ing on budget constraints. 1996 was the last year that residential streets were inspected, and over 70% of streets were in “good to very good/excellent” condition (City of Modesto 2001). ]) that came from the same roadway. The dimensions of a typical segment were 38.1 × 10.7 m (125 × 35 ft). Visual estimates of tree canopy size were used to pair low-shade control segments and nearby high-shade treat- ment segments. Sampling Data from Modesto’s street tree inventory, geographic information system, and street services records were used to identify street segments that met the following require- ments: (1) residential streets constructed in the 1950s and 1960s, (2) majority of originally planted trees still present, and (3) segment pairs that had similar maintenance histo- ries. Forty paired segments were sampled. Each pair consisted of identically sized segments (93 to 418 m2 to 4,500 ft2 An additional 16 segments (eight pairs) could not be paired with another from the same street but were paired with segments from nearby roadways. An asphalt core was extracted and material testing performed on one core from each of these segments (16 cores) to ensure that material properties of the matched segments were similar. Kleinfelder, Inc. collected one 10 cm (4 in.) core sample per street segment and conducted four tests that assessed asphalt content, air-void content, and gradation characteris- tics. Segments that met the following criteria were consid- ered analogous and were included in the sample: 1. No more than 3% difference in air-void content between segments. 2. Segments with multiple lifts (layers) had similar ratios of the top lift thickness to the underlying lift thickness. 3. Segment construction dates were no more than 4 years apart. 4. Mixes had similar aggregate gradations. Eight segments met these criteria and were matched into four pairs. The entire sample consisted of these four pairs plus 20 pairs that had segments from the same street, for a total of 24 pairs containing 48 segments. Data Collection Tree, site, and pavement condition data were collected for each sampling segment during summer 2001. Tree data collected included species, age (planting date from the inventory database), dbh (diameter at breast height), crown diameter, tree height, bole height (ground to crown bottom), shape (silhouette of entire tree, recorded as either ellipsoid or paraboloid), crown density (at one tree-length away compared the observed crown density with percentages ©2005 International Society of Arboriculture [1,000 shown on the crown density card to the nearest 5 percent class), and curb distance (setback distance of tree to curb). Site data included sampling unit dimensions (segment length and width), street orientation (recorded as E/W or N/ S), and distance from the relative origin (corner of the segment). A sketch of each segment recorded the corner marked as the relative origin and tree coordinates. Pavement condition was assessed using protocols defined by the Metropolitan Transportation Commission (MTC 1986). Pavement data included • street name; • segment unit number; • type of distress, assessed by visual estimation: (1) alligator cracking, (2) block cracking, (3) distortions, (4) longitudinal and transverse cracking, (5) patching/ utility cut patching, 6) rutting and depressions, and (7) weathering; • amount of distress (distance measured with measuring wheel); and • severity of distress (measured in levels of low, medium, or high). PCI and TSI Calculations and Statistical Analysis Two parameters, Pavement Condition Index (PCI) and Tree Shade Index (TSI), were calculated based on measurements and observations recorded for each street segment. PCI was calculated based on pavement distress types, amounts, and severities using MTC’s standard six-step protocol (Smith 2001). 1. The inspection unit was inspected using a distress identification guide, and the approximate amount of each distress type/severity combination was recorded as a percentage by dividing the distress type/severity combination quantities by the total area of the segment and multiplying by 100. 2. The deduct values for each distress type/severity combination were determined from deduct curves in the appendices. The PCI procedure uses a set of “deduct curves” to calculate the numerical impact of each distress type/severity combination on the overall PCI. They are called deduct curves because the value determined from the curves is deducted from the maximum score of 100. 3. The number of distress type/severity combinations with deduct values greater than 5 were counted. This is the q-value and was used later in the calculation to correct the curves because research found that if occurrences of small deduct values were included, the final value would be too small, or over-corrected. 4. The total deduct value was computed by summing all deduct values for the distress type/severity combinations. 5. When multiple distress type/severity combinations are present, the deduct units must be corrected because research found that as more distress type/severity
November 2005
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