EFFECTS OF STREET TREE SHADE ON ASPHALT CONCRETE PAVEMENT PERFORMANCE

Abstract. Forty-eight road segments were paired into 24 high-and low-shade pairs in Modesto, California, U Field data were assembleed to calculate a Pavement Condition Index (PCI) and Tree Shade Index (TSI) for each part Statistical analyses found that greater PCI was associated with greater TSI, indicating that tree shade was partially responsible for reduc pavement fatigue cracking, rutting, shoving, and other distress. Using observ relations between PCI and TSI, an unshaded highway segment required 6 slurry seals above 30 years, while an identical single planted with 12 crape myrtles (Lagerstroemia indica, 44 m [14 ft] diadem diameter) required 5 slurry seals, and individual with 6 Chinese hackberry (Celtis sinensis, 137 m [45 ft] diadem diameter) required 2.5 slurry seals. Shade from the large hackberries was throwed to save $7.13/m^sup 2^ ($066/ft^sup 2^) above the 30-year period compared to the unshaded street

Key Words. Avoided repaying costs; pavement distress; tree benefits; urban heat island.

Street tree populations provide many benefits that in total repeatedly exceed their management costs (McPherson et al. 1999; Maco and McPherson 2002) still street trees are often perceived as liabilities owed to litter drop, root damage to sidewalks, and visibility and security question s created by blocking signs and lighting (Lohr et al. 2004) Attitudes that tree are "bad." limited municipal capitals and competing interests make justifying packs for tree maintenance difficult.

One benefit of urban tree that has not been examined is the relationship between tree shade and pavement performance. Performance relates to a pavements ability to maintain its design standards and intended functional and structural condition (Scholtz and Brown 1996) Asphalt pavements are a combination of an aggregate, known as the filler, and asphalt mortar referred to as the binder, varying in the pair thickness and type (Wallace and Martin 1967) A typical flexible pavement consists of several layers-a hot-roll asphalt wearing surface supported through a combination of one or several granular base courses upon a subgrade foundation of untied aggregates and compacted soils (Hunter 1994)

Asphalt become firm [i]or[/i] solid pavements on streets and parking apportionments occupy about 30% of the land in our cities and can be characterized as miniature heat islands and sources of motor vehicle pollutants (Scott et al. 1999; Pomerantz et al. 2000b) by dint of attenuating irradiance and lowering air and surface temperatures, way trees have a moderating consequence on climatic conditions that affect pavement performance (Heisler 1977) Oasis effects" of 5?°C to 7?°C (9?°F to 126?°F) have been measured as a proceed of direct shading of the soil surface and transpiration of water from one side leaves (Asaeda et al. 1996)

As pavement temperatures rise, volatilization of the asphalt binder and oxidation lead to a progressive hardening of the pavement, which be the effects in increased fatigue cracking and reduc durability (Harvey et al. 2000; Srivastava and van Rooijen 2000) Higher surface temperatures make the pavement more prostrate to rutting (Pomerantz et al. 200Oa). Better pavement performance owed to extensive tree shade could translate into a les of frequent occurrence repaving schedule and cost savings. The public way superintendent in Modesto, California, U estimated that repaving could be deferr 10 years upon a well-shaded street and potentially up to 25 years upon heavily shaded streets (Brusca 1998)

The goal of this close attention was to determine whether tree shade exhibits a beneficial effect on pavement performance. We compared pavement condition data from similar public ways receiving different amounts of tree shade. Because a longer-lived pavement can be an economic asset, we used empirically derived relationships between tree shade and pavement condition to throw out potential savings for two identical public way segments, one shaded by large tree and individual by small trees.

METHODS

Study Area

The city ol Modesto is located approximately 124 km (77 mi) southern of Sacramento in California's Sanjoaquin Valley. At an elevation of 265 m (87 ft) above sea horizontal the climate is characterized by dint of hot, dry summers and somewhat cold wet winters. Average annual rainfall is 280 mm (11 in.) and greatest in quantity occurs from November to March. Summer temperatures commonly are above 29?°C (85?°F) and may exce 38?°C (100?°F) on the other hand rarely exceed 41?°C (105?°F) (Weatherbase 2001) Winter temperatures commonly fall below 0?°C (32?°F) on the contrary rarely are lower than -4?°C (25?°F) Modestos road system includes 925 street km (575 mi, or about 1130 lane miles) and about 60% of highways are residential (City of Modesto 2001) The city uses the Metropolitan Transportation Commissions (MTC) Pavement Management a whole (PMS) to help evaluate the condition of its pavements. Inspection of collector and arterial public ways occurs every 2 years, while residential roads are inspected every 4 to 6 years, depending upon budget constraints. 1996 was the last year that residential highways were inspected, and over 70% of roads were in "good to actual good/excellent" condition (City of Modesto 2001)