The famed American landscape architect and Central Park designer Fredrick Law Olmsted said that parks are the “lungs of the city.” However, many cities around the world—from growing Addis Ababa to sprawling Mumbai and dense Sao Paulo—currently lack adequate public space and find it difficult to breath.

Indeed, public green spaces—from parks and plazas to natural areas, recreation areas, playgrounds, greenways or traditional squares—provide critical space for residents to breath and be active. Studies have shown they can improve air quality and reduce storm water runoff to provide ecosystem services and help raise property values. They also contain routes for walkers and cyclists and, along with other active uses, have been linked to greater physical activity and more active cities.

The new Sustainable Development Goals (SDGs) acknowledge the value of public space in creating sustainable cities, but the challenge now is to figure out how to define and measure public space—as well as access to it. By looking at existing methodologies and considering additional indicators on accessibility, cities may find an effective path for meeting their goals.

Public Space as a Development Goal

Target 11.7 of the SDG on cities is to “provide universal access to safe, inclusive and accessible, green and public spaces, particularly for women and children, older persons and persons with disabilities.” In February 2016, the Sustainable Development Goal Network proposed an indicator for this target as “the average share of the built-up area of cities that is open space for public use for all, disaggregated by age group, sex and persons with disabilities.”

So how would this get measured? The proposed indicator refers to “open space for public use” and not necessarily public green space, streets or privately-owned space. This makes the indicator difficult to track given the lack of a proper inventory, or a clear definition for what makes a private space open for public use. If a parking lot available to the public is “open space for public use for all,” does it count? “Open space” is actually a term used very generally, such as by architects for anything that is not built up. One option would be to change the indicator to “public space,” as this is the actual purpose of the target. In any case, on a practical level cities will need to determine what is public, or “public use” and what is not. Furthermore, it would be difficult to disaggregate this by age and sex, given that the indicator is about land, not people.

Most public space is actually a city’s streets, which UN-Habitat has documented nicely in a report on streets as public spaces. The rest is public green space: natural areas, parks, plazas and playgrounds. Public green spaces are hard to measure because they fall under the jurisdiction of numerous government entities—or as the proposed indicators’ “open space,” any kind of publicly-available open area.

This is why a common methodology for measuring public space will be necessary. Today, most cities lack a clear protocol or standard guide for how they might measure these spaces, let alone an existing inventory or understanding of the public agencies involved in public space (e.g. cities can have both city-owned parks and national parks). Google maps might have a better inventory of a city’s public space than the city itself.

Looking to Existing Standards

One model can be found in the United States, where information about public space has been extensively developed and improved over the years through the Trust for Public Land’s Center for City Park Excellence yearly survey of park systems. The survey includes all types of public spaces in over 75 of the country’s largest cities, from natural areas to traditional city parks and plazas. The Global City Indicators Facility also uses a similar classification, calling these spaces “green areas.”

While simply applying these existing methodologies to an entire globe of different cities may not be appropriate, there are some basic principles that cities can use to measure public space. Cities can inventory the spectrum of spaces, from natural areas to small neighborhood parks owned by different government entities. For example, in some cities, cemeteries are publicly available spaces run by the city park and recreation department. A basic guide is needed that can build on these methodologies, define and inventory spaces and be potentially used in a first round of selected indicator cities.

Another Complication: the Question of Genuine Access to Public Space

Cities vary considerably in size, history, development patterns and attitudes towards public space. Measuring how much public space a city has is only one part of measuring whether residents actually benefit from the space. For this reason, an additional indicator may be helpful on the accessibility of public green spaces. This could serve as a component of a greater indicator for cities (as the World Bank has suggested) on accessibility to destinations, including access jobs, goods and services.

For example, Sao Paulo, Brazil has approximately 12.4 square meters per resident of public green space, yet this space is not distributed equitably across the city. To remedy this, the city’s new Plano Direto seeks to create 167 new parks to get parks closer to people. Minneapolis, often considered the city with the best park system in the United States, has 14.9 percent of its land area dedicated to public green space, but is recognized for its connectivity and accessibility, as 94 percent of residents are within a ten-minute walk of a park. A simple indicator measuring the amount of public space does not capture this. Adding access to public spaces is the next step for the indicator.

Next month, officials and experts will gather in Barcelona to discuss the role of public space in the New Urban Agenda for the United Nations and UN-HABITAT. This will also coincide with further discussion on the SDG indicators late March in Mexico City. Creating good measures for public space will ensure that public space genuinely counts when it comes to residents’ quality of life.