The article has great potential, and as author of the upcoming IPCC special report on cities, I believe it is very useful for benchmarking cities globally and gathering clear evidence of the benefits of active mobility.

It also serves to emphasise that the dynamics and functioning of measures are not the same in all cities and that detailed studies are needed to understand how to implement mobility measures in the most efficient, effective, fair, sustainable and transformative way.

Global models have their limitations, as they use the same variables to compare more than 1,000 cities, and in many cases these may not be the parameters that explain the dynamics. However, in addition to being extremely useful for gaining a global and comparative overview with relatively little effort, these models are a definitive step towards legitimising regional and national policies that support active mobility in cities. It is therefore essential to carry out specific studies in each city to see how these measures can be implemented in the most appropriate way.

Finally, it should be emphasised that changing the mobility model in cities not only has benefits for reducing emissions and improving air quality, but also indirectly for improving urban adaptation to climate change by reducing paved and impermeable surfaces and by reducing heat sources such as vehicles, which increase the urban heat island effect. A change in the mobility model leads to changes in land use (with an increase in green infrastructure) with numerous benefits for the quality of life and well-being of the population.

The article presents a fascinating methodology that allows for the study of nearly 40% of the world's population. In addition, the results highlight specific factors at the city and country level that influence active transport, such as population density, kilometres of bike lanes built, and the coexistence of bicycles and cars on low-speed streets. Active transport policies could reduce greenhouse gas emissions by shifting motorised vehicles, reduce pollution in cities and improve health by increasing physical activity.

The article is very interesting from an urban planning and public transport perspective. In an article recently published in the journal Landscape and Urban Planning, we examined how the built environment, social structures, economic conditions and political context influence physical activity-related behaviour in three socio-economically diverse neighbourhoods in Bilbao. This participatory project highlighted how pedestrian infrastructure and walkability are fundamental and how traffic regulation was associated with perceived safety for pedestrians and cyclists.

[Regarding possible limitations] As this is a study of more than 11,500 cities, a lot of detail is lost. Even so, the study is able to identify different urban and transport models that have different ways of affecting the health of the inhabitants of these cities.

This study represents a significant milestone in global research on active transport. Its main strength lies in the use of the Google Environmental Insights Explorer (EIE) dataset, which for the first time allows for a methodologically consistent analysis of walking and cycling transport rates in more than 11,500 cities in 121 countries, covering 41% of the global urban population. From a scientific perspective, it is a methodologically rigorous study, transparent about causality limitations and with results supported by sensitivity analyses. The use of a Bayesian hierarchical model allows for the capture of both local (city) and national effects, integrating variables such as urban density, cycling infrastructure, topography, climate, fuel prices and GDP per capita.

The work is in line with the growing interest in promoting sustainable cities and provides new global evidence by demonstrating that urban density and street design positively influence active transport, even beyond traditional European cities, where this association has already been widely studied. However, the study has a number of limitations that should be noted. The first of these is the absence of individual-level socioeconomic data. This limits our understanding of how the effect of the environment varies depending on the effects of population inequality (income, gender, age, etc.). Furthermore, the aggregation of data at city level makes it difficult to capture intra-urban or seasonal dynamics. EIE data are not representative in countries with low smartphone access, which limits their validity in socially disadvantaged environments. The associations studied have no causal implication, a fact acknowledged by the authors themselves, which is a normal limitation of studies with such a broad global scope. Finally, associated with the characteristics of the data, a limitation of the study is the use of the length of cycle paths as the only proxy for urban design for its use, which oversimplifies the complexity of the urban environment.

This study reaffirms the strong interconnections between urban planning, transport, the environment and health. Encouraging walking and cycling in cities plays a crucial role in reducing CO₂ emissions, mitigating the effects of climate change and improving environmental quality and public health. However, for these measures to be effective, cities must incorporate local knowledge and take into account the specific social, cultural and spatial conditions they face. Tailored strategies are essential to address specific challenges and design successful interventions.

The authors highlight urban density as a key factor in promoting active transport and suggest, for example, relaxing height restrictions to allow taller buildings. While higher density can encourage greater cycling and walking, it is essential to maintain a human-scale urban environment and avoid excessive vertical expansion. From a health perspective, mid-rise buildings are often considered the optimal balance, as they promote active lifestyles without compromising liveability.

To maximise the environmental and health benefits of active transport, cities must strike a balance between sufficient density and compactness, diversity of services and facilities, connectivity and accessibility, necessary infrastructure and the existence of ample green spaces. Together, these elements promote conditions conducive to walking and cycling. All interventions should take into account the local context and be based on the cultural and behavioural norms of residents to ensure that strategies are effective and equitable.