Urban Forestry and Transportation

By: Richard J. Magill, Magill & Associates, Inc.

This is the second in a series of six articles that will explore the various interactions and outcomes that result from human contact with our urban forests. This article will investigate the positive and negative impacts of our city forests on urban transportation systems.

“Urban forests (when properly designed and managed) can contribute to the enhanced safety, improved aesthetics and superior environmental conditions of our city transportation systems and our communities at large.” While this statement may seem contrary to commonly accepted transportation system planning and implementation guidelines, please consider the following.

Safety Issues

• Speed Reduction- It has been proven over many years of observation that tree-lined roadways provide “spatial definition” that makes the road appear to be narrower; therefore encouraging slower driving speeds.
• Stress Reduction- the psychological effect of trees along roads tends to reduce the occurrence of road rage and also improves the attention span of drivers.
• Physical Buffers- Vegetation can provide an effective barrier between opposing vehicular traffic, and between adjacent public spaces and pedestrian ways.
• Unwanted Obstacles- traditional road planning and design guidelines identify “clear zones” where no large objects should be allowed. Trees and other vegetation planted in these “zones” are thought of as hazards to visibility and vehicular safety. While there are certainly situations where this is absolutely true, often these guidelines are too strictly defined and applied. There are many instances where proper landscape design and plant selection would allow for trees or shrubs in or adjacent to these areas without compromising safety.

There have been a number of studies (Dumbaugh 2005, Naderi 2003 and Mok et al. 2006) that seem to indicate a decrease in crash rates on landscaped urban arterial roads and highways, versus roadways that did not have landscaped areas adjacent to them.

Aesthetic Issues

• Drivers believe that forested urban highways have higher visual quality and consequently, they have more favorable perceptions of communities that contain and promote green roadsides. This subjective perception leads to a better image of these communities and results in positive socio-economic benefits beyond purely aesthetic considerations.
• As alluded to in the section above, attractive trees and other vegetation planted adjacent to vehicular and pedestrian ways fosters a general sense of wellbeing and is conducive to more-positive human interaction in transportation corridors.

Environmental Issues
The Good

• Landscaped areas adjacent to roadways are critical to effective stormwater management, including snow storage and melting areas in colder climates. Vegetation in these areas serves to prevent unwanted erosion and helps to absorb the water that runs off of impervious roadways and associated infrastructure. Trees play a particularly important role in erosion control and absorption of surface runoff.
• Properly landscaped roadways serve to filter man-made pollutants after water runs off the roads. Filtering stormwater runoff helps to keep our rivers, lakes and aquifers cleaner for human use and consumption, and ultimately protects the health of our oceans.
• The shade provided by trees and other vegetation can significantly prolong the usable lifespan of roadway surfaces (particularly asphalt) by protecting them from harmful ultraviolet rays.

The “Not So Good”

• Invasive tree roots can be a problem, particularly for surface streets and sidewalks where landscaping is directly adjacent to the roadway. Solutions like the Structural Soil Cells designed and manufactured by Citygreen® serve to minimize damages caused by tree root systems by providing a defined area for healthy root growth while providing exceptional structural stability for heavy-traffic areas above. Structural soil cells also improve surface drainage and play an important part in efficient stormwater management systems.

• The leaves and branches from deciduous trees and shrubs can be an issue with the stormwater drainage systems of our roadways, particularly in the fall season. Regular maintenance, well-designed drainage infrastructure, and appropriate plant selection can dramatically minimize this hazard however.
• Trees and shrubs adjacent to roadways, bikeways, and railroad lines can hinder visibility of other vehicles, particularly at intersections. This problem can be mitigated by good landscape planning and design, attentive pruning practices, and the timely replacement of plants that outgrow their immediate surroundings.

By utilizing a balanced response to the functional needs of transportation systems and the overall health and welfare of urban dwellers, a thoughtfully planned and managed urban forest can be the catalyst that unifies these two seemingly disparate ideals, and going forward, play a crucial role in the world’s evolving urban environment.

Next article: “The Urban Forest and Local Economies”