Davis, CA (November 1, 2007)- We would all prefer to walk down a tree-lined street to one without trees, but did you know that the street itself prefers to run under trees? This report examines the cost-saving benefits of having shaded streets. All other factors equal, the condition of pavement on tree-shaded streets is better than on unshaded streets. In fact, shaded roads require significantly less maintenance and can save up to 60% of repaving costs over 30 years.
After more than 100 years of road and highway building, the United States is now criss-crossed by nearly four million miles of roadways. Add in all the parking lots, private roads, driveways, and road shoulders, and the total amount of paved land comes to approximately one percent of the total area of the contiguous United States. The cost of maintaining this asphalt can be lowered through urban tree planting.
Asphalt streets are a combination of filler materials, known as aggregate, and a binder- asphalt cement- on top of one or more layers of gravel and compacted soil. As pavement temperatures rise, the binder evaporates and breaks down and the pavement begins to harden, making it easier for cracks to form. Tree planting along roads provides shade, thereby improving pavement conditions. According to research conducted by this study, 20% shade on a street improves pavement condition by 11%, which is a 60% savings for resurfacing over 30 years.
Tips for Street Shading Trees:
* Start by establishing very clear goals for your street trees including shade and other functions, longevity, stress tolerance, rainfall interception, air pollution uptake, level of maintenance, and infrastructure conflicts.
* Increase community-wide tree canopy by targeting shade for streets, as well as parking lots, and other paved surfaces.
* Large trees can shade a greater area than smaller trees can but should be used only where space permits. Remember that a tree needs space for both branches and roots.
* Avoid locating trees where they will block illumination from streetlights or views of street signs in parking lots, commercial areas, and along streets.
* Check with local transportation officials for sight visibility requirements. Keep trees at least 30 ft away from street intersections to ensure visibility.
* Avoid planting shallow-rooting species near sidewalks, curbs, and paving. Tree roots can heave pavement if planted too close to sidewalks and patios. Generally, avoid planting within 3 ft of pavement.
* Be aware of strategies to reduce infrastructure damage by tree roots such as meandering walks around trees and selecting deep-rooting species. (Costello and Jones 2003).
* Select only small trees (25 ft tall) for location under overhead power lines. Do not plant directly above underground water and sewer lines. * Match each tree to the site. Maintenance requirements and public safety issues influence the type of trees selected for public places. The ideal public tree is not susceptible to wind damage and branch drop, does not require frequent pruning, produces negligible litter, is deep-rooted, has few serious pest and disease problems, and tolerates a wide range of soil conditions, irrigation regimes, and air pollutants (SelecTree). * Provide adequate soil volume. For trees to deliver benefits over the long term, they require enough soil volume to grow and remain healthy. Matching tree species to the site's soil volume can reduce sidewalk and curb damage as well. Related Resources:
Why Shade Streets? The Unexpected Benefit
Center for Urban Forest Research