This article examines how Medellín used existing roads, waterways and movement corridors to create green infrastructure without requiring large new parcels of urban land. It analyses the environmental and mobility outcomes of the Green Corridors programme and identifies practical lessons for city authorities seeking to address heat, air quality, biodiversity, sustainable mobility and public-space needs within limited urban space.

The Pressure on Urban Land

The fundamental challenge that cities have is one that is simple and persistent: Limited land is being demanded and required at an ever increasing rate. There are a variety of other activities that need physical surface to cool the streets, clean the air, provide space for walking and cycling, protect biodiversity and provide gathering places. It is typically neglected to address these needs together in practice. The notion of parks is introduced on unused parcels; the addition of cycle lanes on a select few roads; the introduction of emission rules, and tree planting is considered an amenity. This leaves the results typically broken up. There are green, cooler neighbourhoods that are juxtaposed with hot polluted corridors which people are still forced to walk or cycle through on a daily basis.

The big question is how to allocate the scarce land space when it is being used by the environment, sustainable transport and public space. This is particularly acute in cities in tropical and subtropical regions with high population. The concrete and asphalt store heat for an extended period of time after the sun has set. Pollution does not disperse easily in valleys or when affected by local weather conditions. Increased car and motorcycle usage degrades air quality and creates space to be used by pedestrians and bicyclists. A park in a less traveled, cooler area can help to cool the area around it, and provide a place for people to rest, but not much else for the hottest and most polluted routes that make up daily travel. That exposure, travel and remaining infrastructure overlap is a serious issue that needs to be addressed in any serious response.

Surface temperature is not always in concert with air temperature. Trees will reduce the amount of particulate matter but do not mask the need to take action at the source of pollution. Ecological connections are best when there is a connection between green spaces and not isolated. The same section of street or stream bank may be utilized for more than one purpose, if carefully planned. The question is how planning systems are designed to take advantage of that option – or whether they are just continuing to view each need as a project.

The decisions lie clearly in the hands of governments, be they municipal, metropolitan planning or transport agencies or environment departments. They make decisions regarding the allocation of land, on which capital is invested in green and grey infrastructure and how public space is being cared for over time. The discussion which follows is addressed to those institutions.

A Proven Approach: Medellín’s Green Corridors

In the Colombian city of Medellín, from 2016 to 2019 roads and stretches of waterways were lined with green corridors throughout their length. The city didn’t purchase considerable tracts of land. Rather, it placed a series of vegetation in the linear spaces that were already utilized for movement, and in spots, on vertical garden walls, alongside trees and understorey plants, to accommodate the remaining infrastructure. There are approximately 30 corridors in total, which represent approximately 65-70ha of improved or newly planted ground and approximately 20km of shaded corridors (cycle lanes, footpaths).

The city has planted hundreds of thousands of trees over the years and over 2 million smaller items. Species representing structure and ecological value were selected, and arranged in low, medium and high layers, similar to a natural forest. Training of local people from the disadvantaged community as urban gardeners, thereby generating employment and capacity to continue with the care. The price of the capital is estimated to be approximately USD 16.3 million, and the annual maintenance is about USD 600,000 to 625,000.

The figure below displays the environmental outcomes that were made at the corridor locations from 2016 to 2019. Average surface temperatures fell from 40.5 °C to 30.2 °C; average air temperatures from 31.6 °C to 27.1 °C. Particulate levels also declined: PM10 from 46.04 µg/m³ to 40.4 µg/m³, and PM2.5 from 21.81 µg/m³ to 20.26 µg/m³. These are from the Sustainable Energy for All and are found in all institutional reports. The drop in surface temperatures was greater than the drop in air temperatures. That is expected. Paved surfaces are shaded and moisture is released, which allows the ground and air immediately above it to cool faster than the surrounding air, which is affected by the regional weather. On the local level that local cooling is more significant than a city average for the walking and cycling population.

Figure 1. Reported average values for surface temperature, air temperature, PM10 and PM2.5 in Medellín Green Corridor locations, 2016 and 2019, with percentage reductions.

Cycling in the city increased 34.6 per cent and walking increased by 4 per cent during the same time. This was accompanied by an extra 80 kilometres of new bike paths that connect to the broader programme. The mobility with the environmental changes is thus not separable in the time sense, cannot be interpreted as pure cause and effect. While vegetation can trap particles, it can’t replace measures to reduce emissions from vehicles and industry to make active transport more comfortable.

The cuts in fine particles were achieved but not very large, highlighting the need to use green infrastructure in conjunction with source control. There was a parallel decrease in the incidence of acute respiratory infections, from about 160 to 95 cases per thousand during the same period. This type of health outcome is a reflection of many factors, and cannot be attributed just to the corridors.

Figure 2. Percentage changes associated with the Green Corridors implementation period (2016–2019). Mobility figures are city-wide; temperature and particulate figures refer to corridor locations. These are reported associations for the project period, not pure causal estimates.

It is the design logic that is most noticeable. The programme addressed the issues of environmental quality and sustainable mobility in the same spatial extension as it planted vegetation at the busiest pollution and heat spots and as part of green and healthy walking and cycling routes. Residual habitats, waterways and greenery on the hillsides also enabled wildlife to move throughout the city again. The creation of separate parks was not the only way of enhancing public space, but also through the daily experience of taking the street to and from work, and strolling along the tree-lined, green streets. From the outset of its creation, care was added in – a dedicated maintenance budget and trained local gardeners realised that canopy and understorey can only continue to work if cared for.

What This Means for City Authorities

Medellín does not have a set recipe for municipal governments and planning teams—one that involves a number of trees or a set temperature. It provides a useful guideline: the environmental services could be incorporated into the linear spaces that already move people around the city. These metres of road verge or stream bank can also cool the air, collect particles, enhance walking and cycling infrastructure and create more biodiversity and enjoy everyday public space. This does not obviate the need for parks to be bigger, for more stringent emission standards and for a more compact urban form. It does provide cities with one more option to purchase large new green sites when it is politically and/or economically challenging to do so.

The table below sets the corridor approach against more conventional, segmented responses.

Some useful advice is then provided to authorities that select and design corridors. Routes where there are significant levels of pedestrians and cyclists, pollution and heavy use should be given first priority because these are the locations where exposure reduction will be most beneficial. Corridors should also be evaluated with respect to their functional connectivity to waterways, remaining habitats/areas of greenery, and the feasibility of installing continuous cycle and walking infrastructure without sacrificing remaining vehicle capacity where it is still needed. Budget and personnel for maintenance must be built up at the outset. Only good management can ensure trees and understorey continue to serve. Surface temperature and air temperature monitoring should be conducted separately and particulate reductions should be considered as complementary to controls on emissions.

When shortlisting corridors, teams may find it useful to work through a short checklist:

• How intense and prolonged is heat exposure along the route, and how little existing canopy is there?
• How heavy is traffic-related pollution, and how many people walk or cycle through it?
• How fragmented or missing is green space compared with surrounding neighbourhoods?
• Can the corridor help reconnect waterways, residual habitats and larger green areas?
• Can continuous cycle and pedestrian routes be added while keeping essential traffic function?
• Is there a realistic plan for long-term maintenance, including local jobs and a steady budget?

Evidence from Medellín only comes from a particular three-year period and is alongside other investments in bike infrastructure and social programmes. It would be inappropriate to attribute all of the percentage changes to vegetation. The record demonstrates that, when planners create a plan to green the hottest and most polluted corridors, to plant in layers that create ecological connections, and to integrate the greening with walking and biking routes, there are measurable environmental gains, as well as benefits for public space and mobility.

The key takeaway for cities that are constrained in their land availability in the face of increasing heat and air quality pressures is that: sustainable urban planning can tackle heat, environmental quality, mobility, biodiversity and public space by making coordinated use of corridors that already exist, not just the mere presence of green amenities.