Most people don’t think of trees as medicine. But the science is hard to ignore. People who can see greenery from their homes show lower cortisol levels, the biological marker most closely tied to chronic stress. They are also significantly less likely to suffer from anxiety and depression than those living in grey, tree-sparse streets. This isn’t a peripheral finding. It shows up across populations, cities, and continents, consistently enough that urban health researchers have stopped asking whether greenery matters and started asking how much. The answer, it turns out, is a lot. And we even have a rule for it.
The uncomfortable gap between policy and reality
The 3-30-300 rule: a simple idea with profound implications
– 3 trees visible from every home and workplace
– 30% canopy cover in every neighborhood
– 300 meters maximum distance to the nearest green space
It’s a standard that, if met, would meaningfully improve the health, comfort, and mental wellbeing of millions of urban residents. Many cities (Oslo, Singapore, Barcelona, …) have adopted it as an official policy target.
A city government that commits to the rule is essentially making a promise to its residents. But when a sustainability officer or landscape architect then asks “where do we plant the next 500 trees?”, the answer comes down to available budgets, willing landowners, upcoming infrastructure projects, or sometimes just institutional memory.
This produces well-intentioned but deeply inefficient outcomes. Entire clusters of residential buildings, full of real people, many of them elderly or lower-income and less likely to travel to greener areas, remain invisible in the planning process.
The rule exists. The ambition is real. What’s missing is a method.
From Vision to Evidence: The data-driven approach
The process works in five steps:
Using geospatial analysis, we assess every residential and commercial building in a defined area and classify it as green (residents can see at least 3 trees) or red (they cannot).
The study area is divided into a spatial grid. For each cell in that grid, we calculate “red buildings” seeing the grid cell (those failing Rule 3). Separately, we assess which grid cells currently contain no trees, or very few. This filters out spaces where new planting would simply add to existing abundance. We want the gaps, not the highlights.
Combining the visibility scores with the green-deficiency data, we identify the highest-impact candidate cells and model the effect of planting trees there. How many red buildings would shift to green? By how much does Rule 3 compliance improve?
The final step extends this logic outward. Testing combinations of placements, evaluating neighboring cells, and iterating toward a solution that delivers the greatest aggregate improvement in Rule 3 compliance across the entire study zone.
The output is a ranked, evidence-based planting plan: specific locations, projected impact, and a clear before-and-after picture of how the city’s green access landscape changes.
Why this changes the conversation
3-30-300 Vision answers that question. When you can show a city council that a specific sequence of 200 tree placements will bring 4,800 households into Rule 3 compliance. Households that are disproportionately located in low-income or heat-vulnerable districts.This is no longer soft environmental policy. This is targeted urban infrastructure.
Beyond the 3-30-300 rule
The 3-30-300 rule has has given cities a shared vocabulary for urban greening ambition. But vocabulary isn’t strategy.
If your city has committed to the rule, the next question shouldn’t be “how many trees do we need?” It should be “which trees, planted where, will make the most difference to the most people and provide a long-lasting improvement?”
ASITIS works with city governments, urban planners, and sustainability teams to deliver evidence-based greening strategies built on the 3-30-300 framework. Whether you’re running a pilot district analysis or planning a citywide green infrastructure programme, we can help you move from commitment to coordinates.
With 3-30-300 Vision at Prague Innovation Marathon 2026.
Wish us luck!
