Traffic Flow Relies on More Than Just Roads

If a city's suburban railway network is expanded, additional flats are likely to be built in the agglomeration that is better connected as a result. The opposite also holds true: If new buildings spring up like mushrooms in a suburb, this will call for an expansion of the transport infrastructure. Urban development and transport therefore have a mutual relationship.

"Our cities are becoming increasingly complex, while transport systems are under ever mounting pressure. Consequently, it is crucial to understand the relationship between mobility and cities, as this is the only way to develop and design urban centres sustainably," as Yatao Zhang emphasized. He is the first author of a study by ETH Zurich and the University of Wisconsin (USA), which has just been published in the journal external page Nature Communications . The study is based on the geoinformatics expert's doctoral thesis, which he completed in the autumn of 2025 at the Singapore-ETH Centre in the Asian city state of Singapore.

A comparison of 30 cities worldwide

In this study, Zhang analysed how urban development and traffic are mutually dependent and what cause-and-effect relationships occur. He and his colleagues compared a total of 30 major cities worldwide, including the city of Zurich.

The researchers focussed on road traffic and particularly on traffic jams on congested roads. They based their investigation on traffic data from Here Technologies. The Dutch company records the congestion situation around the globe using vehicle movement data with a time resolution of five minutes. For the city of Los Angeles alone, for example, the congestion values of over 18,000 road sections were included in the study.

The scientists correlated the congestion data with a variety of characteristics of the cities analysed. This included the structure of the road network, consisting of traffic junctions and road connections with different levels of traffic, as well as data on the shape of green spaces or districts and neighbourhoods, which allows conclusions to be drawn about the flow of traffic. The researchers also used data on the function of urban areas such as housing, shopping, sport, administration and education.

As their data source, the researchers mainly based their work on Open Street Map, a freely usable map database maintained by a community of volunteers. This resulted in a comprehensive collection of city characteristics and features for the 30 cities. The scientists correlated these with congestion data from the respective cities.

It's not just the road network that shapes and determines traffic

It is well known that urban features and traffic influence each other. Therefore, it only stands to reason that a city with a high building density or a good road infrastructure will have a lot of traffic. Zhang and his colleagues, however, went one step further: Together, they developed a new method with which they are able to describe the mutual influence of urban features and traffic over time and even establish cause-and-effect relationships, which was previously not possible.

Interesting in this context: There is a strong correlation between the expansion of the road network (urban feature structure) and traffic. The spatial arrangement of the city (urban characteristic of form), however, and the different building types (urban characteristic of function) are also determining factors for the traffic volumes.

A sprawling city tends to result in more traffic, and the accumulation of leisure activities in a neighbourhood can increase weekend traffic. Mixed utilisation (living and working) tends to lead to less traffic because it shortens commuting distances. ETH researcher Zhang puts it succinctly: "Traffic is created by what people do, not just by the existence of roads."

Impulses for urban and transport planning

The study focussed mainly on an international comparison rather than a detailed analysis of individual cities. The comparison shows major differences, for example between Singapore and Zurich: The Asian city is characterised by demarcated residential areas that face a centre with service jobs. Structural changes in residential areas have a direct impact on commuter flows. This link between urban development and transport is much less pronounced in Zurich, as flats are spread across the entire city.

The study by Yatao Zhang's team was supervised by Martin Raubal, Professor of Geoinformation Engineering at ETH Zurich. According to Raubal, the study holds great potential in store for urban and transport planning in the medium term: "The study provides an innovative method for predicting how the change in a specific urban feature - such as the construction of a large shopping centre - will impact on traffic in the medium term."

The study helps researchers to understand how transport policy measures actually work and what changes they can trigger in the urban fabric over the long term. Before the method can be used in Zurich or elsewhere for urban and transport planning, however, further detailed analyses are required.