22 Reasons Trees in Cities Keep Us Healthy and Save Us Money
By Dan Burden, Blue Zones Director of Innovation and Inspiration, and the nation’s most recognized authority on walkability, bicycle & pedestrian programs, street corridor & intersection design, traffic flow and calming, and road diets.
[Editor’s note: This was first published on walkable.org in August 2006 and has been slightly adapted and shortened.]
Street trees are of great value to people living, working, shopping, sharing, walking, and motoring in and through urban places. For a planting cost of $250-$600 (including first three years of maintenance), a single street tree returns over $90,000 of direct benefits (not including aesthetic, social, and natural) in the lifetime of the tree. Street trees provide so many benefits to those streets they occupy, that they should always be considered as a default urban area street making feature.
Why Urban Trees are Smart Investments in Public Health
1. Urban trees reduce traffic speeds and reduce crashes.
Urban street trees create vertical walls framing streets, providing a defined edge and helping motorists guide their movement and assess their speed (leading to overall speed reductions). Street safety comparisons show reductions of run-off-the-road crashes and overall crash severity when street tree sections are compared with equivalent treeless streets (Texas A and M conducted simulation research which found people slow down while driving through a treescape). These observations are also seen in the real world when following motorists along first a treed portion of a street, and then a non-treed portion. Speed differentials of 3 mph to 15 mph are noted.
2. Trees help create safer visual walls for motorists.
By forming and framing visual walls, trees provide distinct edges to sidewalks so that motorists better distinguish between their environment and one shared with people. If a motorist were to significantly err in their urban driving task, street trees help deflect or fully stop the motorist from taking a human life.
3. Street trees increase safety with medians.
Trees call for placemaking planting strips and medians, which further separate motorists from one another, pedestrians, buildings, and other urban fabric. This green area adds significantly to aesthetics and placemaking. Urban area medians with trees are safer than those without trees (R. Ewing, Caltrans Study, circa 2003).
Medians reduce crashes by 50 percent or more.
4. Trees create increased security with more pleasant walking environments.
Trees create more pleasant walking environments, bringing about increased walking, talking, pride, care of place, association and therefore actual ownership and surveillance of homes, blocks, neighborhoods plazas, businesses and, other civic spaces.
5. Treescaped streets improve business.
Businesses on treescaped streets show 12 percent higher income streams, which is often the essential competitive edge needed for main street store success, versus competition from plaza discount store prices.
6. Trees improve drainage infrastructure.
Trees absorb the first 30 percent of most precipitation through their leaf system, allowing evaporation back into the atmosphere. This moisture never hits the ground. Another percentage (up to 30 percent) of precipitation is absorbed back into the ground and taken in and held onto by the root structure, then absorbed and then transpired back to the air. Some of this water also naturally percolates into the ground water and aquifer. Storm water runoff and flooding potential to urban properties is therefore reduced.
7. Trees provide rain, sun, heat, and skin protection.
For light or moderate rains, pedestrians find less need for rain protection. In cities with good tree coverage, there is less need for chemical sun blocking agents. Temperature differentials of 5-15+ degrees are felt when walking under tree-canopied streets.
8. Cities with trees reduce harm from tailpipe emissions.
Automobile and truck exhaust is a major public health concern and contains significant pollutants, including carbon monoxide (CO), volatile organic compounds (VOC), nitrogen oxides (NOx), and particulate matter (PM). Tailpipe emissions are adding to asthma, ozone, and other health impacts. Impacts are reduced significantly from proximity to trees.
9. Urban trees absorb harmful pollution.
Trees in street proximity absorb nine times more pollutants than more distant trees, converting harmful gasses back into oxygen and other useful and natural gasses.
10. Trees lower urban air temperatures.
Asphalt and concrete streets and parking lots are known to increase urban temperatures by 3-7 degrees. These temperature increases significantly impact energy costs to homeowners and consumers.
A properly shaded neighborhood, mostly from urban street trees, can reduce energy bills for a household from 15-35 percent.
11. Streets with trees help lower ozone levels.
Increases in urban street temperatures that hover directly above asphalt where tailpipe emissions occur dramatically increase creation of harmful ozone and other gasses into more noxious substances impacting health of people, animals, and surrounding agricultural lands.
12. Trees convert streets, parking and walls into more aesthetically pleasing environments.
There are few street-making elements that do as much to soften wide, grey visual wastelands created by wide streets, parking lots, and massive, but sometimes necessary, blank walls than trees.
13. Urban trees soften and screen necessary street features such as utility poles, light poles, and other needed street furniture.
Trees are highly effective at screening those other vertical features to roadways that are needed for many safety and functional reasons.
14. Trees help reduce blood pressure and improve overall emotional and psychological health.
People are impacted by ugly or attractive environments where they spend time. Kathlene Wolf, PhD, of the University of Washington, gave a presentation that said “the risk of treed streets was questionable compared to other types of accidents along with the increased benefit of trees on human behavior, health, pavement longevity, etc.” She noted that trees have a calming and healing effect on ADHD adults and teens.
15. Tree-lined streets improve time-in-travel perception.
Other research and observations confirm that motorists perceive the time it takes to get through treed versus non-treed environments has a significant differential. A treeless environment trip is perceived to be longer than one that is treed (Walter Kulash, P.E.; speech circa 1994, Glatting Jackson).
16. Trees help reduce road rage.
Although this may at first seem a stretch, there is strong, compelling research that motorist road rage is less in green urban versus stark suburban areas. Trees and aesthetics, which are known to reduce blood pressure, may handle some of this calming effect.
17. Street trees can improve visibility of vital traffic signs.
When properly positioned and maintained, the backdrop of street trees allow those features that should be dominant to be better seen, such as vital traffic regulatory signs. The absence of a well-developed greenscape allows the sickly grey mass of strip to dominate the visual world. At the same time, poorly placed signs, signals, or poorly maintained trees reduces this positive gain, and thus proper placement and maintenance must be rigidly adhered to.
18. Trees add value to adjacent homes, businesses and tax base.
Realtor-based estimates of street tree vs non-street tree comparable streets relate a $15,000-$25,000 increase in home or business value. This often adds to the tax base and operations budgets of a city allowing for added street maintenance. Future economic analysis may determine that this is a break-even for city maintenance budgets.
19. Trees make a natural barrier for safety and fun.
Road verges, or tree lawns, provide added green space for a splash and spray zone, storage of snow, driveway elevation transition and more. Tree lawns are an essential part of the operational side of a street.
20. Trees act as a filtering and screening agent.
Trees soften and screen utility poles, light poles, on-street and off-street parking and other features creating visual pollution to the street.
21. Trees lengthen pavement life.
Studies conducted in a variety of California environments show that the shade of urban street trees can add from 40-60 percent more life to costly asphalt. This factor is based on reduced daily heating and cooling (expansion/contraction) of asphalt. As peak oil pricing increases roadway overlays, this will become a significant cost reduction to maintaining a more affordable roadway system.
22. Trees create a connection to nature and the human senses.
Urban street trees provide a canopy, root structure, and setting for important insect and bacterial life below the surface; at-grade for pets and romantic people to pause for what pets and romantic people pause for; they act as essential lofty environments for song birds, seeds, nuts, squirrels, and other urban life. Indeed, street trees so well establish natural and comfortable urban life it is unlikely we will ever see any advertisement for any marketed urban product, including cars, to be featured without street trees making the ultimate dominant, bold visual statement about place.