Unpacking the New Rule of Thumb for Urban Greening

April 22, 2024 |  Alec Sabatini

In 2021 Cecil Konijnendijk, Director at the Nature Based Solutions Institute, proposed a new guideline for greener, healthier, more resilient cities: the 3+30+300 rule. The rule sets three minimum criteria for urban trees and green space:

• At least 3 trees in sight from every home, school, and workplace

• At least 30% tree canopy in every neighborhood

• No more than 300 meters between every home and the nearest park or green space

The 3+30+300 Rule

Evidence-based guidelines are a proven strategy for motivating decision-makers, planners, developers, and residents. All over the world, tree canopy goals have helped energize cities into action by distilling the complexities of their urban forest into a single target.

The 3+30+300 rule takes a similar approach but goes a few steps further by including components for green space visibility and accessibility. All three components offer vital contributions to our health and well-being. The objective of this rule was to strike a careful balance between being comprehensive, straightforward to implement and monitor, and easy to remember.

Below we’ll break down each piece of the rule and how cities can measure how well they meet the requirements. You can also watch the on-demand webinar: Mapping the 3+30+300 Rule for Actionable City Greening. Cecil and Joost Verhagen, whose team at Cobra Groenzicht is pioneering methods of mapping the rule, will discuss 3+30+300 and share visualizations of real-world scenarios.

3 Types of Nature Exposure

The rule covers three types of nature exposure: the ability to see urban nature from the windows of your home and workplace, living among trees and vegetation in your neighborhood, and accessing a park or green space for recreational purposes. Studies have shown that each type of nature exposure creates varied, positive health impacts.

30% Canopy

Tree canopy coverage is connected with a host of positive associations, such as reducing extreme urban heat, lower all-cause mortality, and improved childhood development. 30% urban tree canopy coverage has been found to be an important threshold for positive health effects. Around the world, from Barcelona to Seattle to Canberra, cities have set 30% canopy goals. The rule specifies that 30% canopy should be achieved at the neighborhood level, not a citywide average, to ensure lush, affluent areas don’t overshadow sparsely treed, disadvantaged neighborhoods.

How To Measure the 3+30+300 Rule

Measurement is what gives a benchmark traction. The efficacy of the 3+30+300 rule is linked to how effectively it can be measured. Once quantified, a community has a baseline by which they can set goals and track progress.

This three-pronged rule requires multiple measurement strategies. A 2023 study identified seven main measurement options: vegetation indices, street-level analyses, tree inventories, questionnaires, window view analyses, land cover maps, and urban green space property maps.

Land cover maps and geospatial analysis are well-established, accessible methods for calculating urban tree canopy. Measuring the nearest public green space can be accomplished using similar tools, though there are ways to add additional detail for greater accuracy. A straightforward analysis looks at the direct distance, i.e. “as the crow flies”. A more sophisticated calculation would factor in access points, crosswalks, and physical barriers.

“3 visible trees” is the trickiest component to measure. One route is to use a community science approach. Distribute surveys to a representative sample of people per neighborhood and ask them to report the number of trees they can see. They could also submit photos looking through their most viewed windows to be analyzed for trees. The benefit of the community science approach is it promotes citizen engagement and awareness of urban forestry efforts.

Building Momentum

The 3+30+300 rule has been gathering steam since its 2021 launch. The United Nations Economic Commission for Europe included the rule in a policy brief, and it’s making its way into municipal policies, such as the city masterplan of Malmo, Sweden.

Adoption of the 3+30+300 rule will accelerate once measurement strategies become standardized. The data sets from Cobra Groenzicht and the planned inclusion of the data in TreePlotter™ software will help propel the rule to a broader audience.

Cities may have a long road to meeting all three criteria. A study found only 4.7% of people in Barcelona met a surrogate version of the 3+30+300 rule. Even as a hard-to-reach target, there is immense value in getting cities to recognize the multiple aspects of urban forests needed to boost health and resilience, and to start moving in the right direction.