Speed bumps, street markings, speed limits and other measures have all been used to create safer conditions for all users of the road. But what about trees?

Changes in transit design that aim to make roads and car traffic safer are one critical component of the complete streets movements underway across North America. Vehicle usage is responsible for staggering CO2 emissions, human injury and death, energy consumption, and more. Still, cars remain a part of the urban landscape, and street design that integrates them safely is imperative. Speed bumps, street markings, speed limits and other measures have all been used to create safer conditions for all users of the road. But what about trees?

Most of us like trees. These incredible organisms clean our air and water, provide valuable habitat for wildlife, increase our property values, and make us feel happier through their beauty. They also seem to make urban roads safer. Most of us don’t think of trees as infrastructure, but in an urban context they are just that. Research indicates that they can play a powerful role in traffic calming, especially through their impact on three vehicle-related risks: speeding, road rage, and pedestrian/bicyclist injury.

The Edge Effect

According to a 1999 report by the National Highway Traffic Safety Administration (NHTSA), speed was a factor in 30% of all traffic fatalities. That’s a scary statistic. Rumble strips, speed bumps, and speed limits are a few of the common measures taken to reduce traffic speeds to safer levels. It turns out that trees placed along streets can also play an important role in speed control. A 2006 study reported that using trees to line roadways brought down average speeds by up to eight miles per hour. Researchers theorize that streets with landscaped center medians or perimeter street trees may affect driver perception of lane width, causing something called the “edge effect.” This “edge effect” provides them with a psychological prompt to go slower.

Putting Safety First

Creating a safe environment for pedestrians and bicyclists who share the road with vehicles is also an important consideration in road design. Here, too, trees can help create safer conditions. A 2006 study of ten urban arterial and highly sites in Texas compared accident records before and after planting trees. Analysis showed a 46% decrease in crash rates across the sites after landscape improvements were installed. All types of roadside treatments – roadside landscaping, median landscaping, and sidewalk widening with tree planting – positively affected vehicle safety outcomes. A marked decrease in the number of pedestrian fatalities was also noted, from 18 to only two after landscape improvements. These findings contradict conventional street safety guidelines that maintain that increased numbers of objects in the road-side and constrained rights-of-way will increase accident rates.

Reducing Road Rage

Multiple studies have found that simply viewing nature in urban settings has a strongly restorative and calming effect. These findings also have applications on the road, in particular for preventing emotionally charged confrontations known as road rage. A healthy roadside tree canopy can help offset angry and aggressive reactions by keeping drivers calm and reducing stressful responses. A 2010 report from the School of Forest Resources at the University of Washington showed that drivers seeing natural roadside views exhibited lower levels of stress and frustration compared to those viewing all-built settings.

Trees may not be the right design solution for every site considering techniques for traffic calming. However, they are an extremely effective and low-cost tool with potential applications across many urban settings. Unlike other traffic calming devices, trees are multi-functional – in addition to helping make roads safer, they also increase property values, save energy, reduce flooding, and generally make our environments more comfortable and pleasant. Despite being completely out of their natural context, urban trees act as green utilities that have become an indispensable part of the urban fabric. We need to re-examine accepted assumptions about where it is appropriate to plant trees and develop street and road designs that utilize their powerful influence on human health and safety.

Leda Marritz works for DeepRoot Green Infrastructure and writes for Green Infrastructure for Your Community. Nathalie Shanstrom is a landscape architect specializing in sustainable landscape design at the Kestrel Design Group.

Landscape Architecture 


Nathalie Shanstrom