Introduction

Street trees can meaningfully contribute to slowing traffic and creating safer urban streets — an important benefit that is often overlooked. When thoughtfully integrated into a streetscape, trees can reinforce lower driving speeds, help define the roadway and create visual and spatial separation between vehicles, cyclists, and pedestrians.

Street Design Influences Driving Speed

A speed-limit sign is only one signal drivers receive. Lane widths, sight distances, intersection geometry and the treatment of the street edge all help communicate how fast it feels comfortable to travel.

For this reason, the National Association of City Transportation Officials recommends designing streets around a desired target speed. Its Urban Street Design Guide specifically identifies street trees as one element that can narrow a driver’s visual field and establish a rhythm along the roadway. Recent driving-simulator research has similarly found that roadside vegetation, particularly when combined with narrower lanes, can reduce speeds in certain street-transition designs.

Trees are not a replacement for narrower lanes, safer crossings, appropriate speed limits or other proven traffic-calming measures. They can, however, reinforce those measures by making a street feel more enclosed, active, and clearly intended for lower-speed urban travel.

Street Trees and Pedestrian Safety

Research increasingly supports the connection between greener streets and safer conditions. A 2022 study in Melbourne found that pedestrian casualties decreased as street-tree density and canopy increased. More recent studies in Bogotá and Austin have also associated greater numbers of street trees or streetscape greenery with fewer pedestrian deaths or crashes.

Trees can also help organize the space between different street users. When planted between the roadway and a sidewalk or bicycle facility, they establish a visual and spatial buffer from moving traffic. The Global Designing Cities Initiative recommends using trees and other streetscape elements in this way, while maintaining clear pedestrian routes and visibility at intersections, particularly along lower-speed urban streets where trees are incorporated into a broader people-centered design.

Designing Streets That Make Room for Trees

Traffic calming is only one of the many jobs street trees can perform. The same trees can provide shade, reduce heat, intercept stormwater and make walking or cycling more comfortable. Few pieces of urban infrastructure deliver that range of benefits from the same footprint.

But those benefits depend on trees having enough room and soil to mature. In dense streetscapes, systems such as Silva Cell can support sidewalks and other paved surfaces while providing lightly compacted soil beneath them. This allows trees to become a meaningful part of the street rather than an afterthought squeezed into whatever space remains.

A successful urban street should do more than move vehicles as quickly as possible. By designing streets that make space for healthy trees — and that prioritize pedestrians, cyclists, and other users alongside drivers — cities can create places that are safer, greener, and better suited to everyday life.

Sources

• Zhu, M., Sze, N.N. and Newnam, S. (2022). Effect of urban street trees on pedestrian safety: A micro-level pedestrian casualty model using multivariate Bayesian spatial approach. Accident Analysis & Prevention.
• Zewdie, H.Y. et al. (2024). Road Traffic Injuries and the Built Environment in Bogotá, Colombia, 2015–2019. Journal of Urban Health.
• Yu, C.Y. (2024). The Association Between Streetscape and Surrounding Environment and Pedestrian Crashes on Urban Arterials. Findings.
• Eisenman, T.S., Coleman, A.F. and LaBombard, G. (2021). Street Trees for Bicyclists, Pedestrians, and Vehicle Drivers: A Systematic Multimodal Review. Urban Science.
• National Association of City Transportation Officials. Design Speed and Speed Reduction Mechanisms.
• Jiang, Y. et al. (2024). Geometric Constraints and Visual Field Related to Speed Management. Purdue University and Indiana Department of Transportation.
• Global Designing Cities Initiative. Pedestrian Toolbox.