The first warm days of spring still lure many of us out into the sun. However, hot summer temperatures drive us into the shade. Added to this are the increasingly high, record-breaking summer temperatures in Europe and around the world, which go hand in hand with the climate crisis. For older people, children, and those with pre-existing health conditions in particular, walking long distances in direct sunlight poses a health risk, making shady paths a precious commodity.
But how do we find sun-protected routes? And how can we draw attention to situations where they don’t exist at all? The HEAL research project at Heidelberg University brought together various people to answer exactly these questions. The results have led to a routing application that navigates people in Heidelberg and Worms through the city along shady paths. HeiGIT subsequently applied this approach to other cities and aims to encourage further climate adaptation measures.
Where is the shadiest path?
Which pedestrian route is best for avoiding particularly overheated areas, depending on the time of day and solar radiation? For Heidelberg and Worms, the web-based shaded.ors app has been providing exactly this information since the summer of 2024. For example, in the morning, the application recommends taking Wilhelm-Leuschner-Straße, followed by Martinsgasse and Judengasse, for the walk from Worms main station to the Rhine promenade. In the afternoon, the coolest route then leads via Goethestraße.
To achieve this, the app identifies heat stress along a route and suggests alternative paths that lead through parks and shaded areas.
A wealth of data leads to the destination
The calculations for the HEAL apps for Heidelberg and Worms incorporate current weather and solar radiation data. In addition, they provide information on the path conditions, gradient, and surface of the respective route.
However, the application underlying the apps, developed at HeiGIT, is primarily an analytical tool for assessing walkability in the heat, rather than a traditional navigation app, as Julian Psotta, Project Manager for Technical Innovations at HeiGIT, explained to RESET. It runs on openrouteservice, HeiGIT’s open-source routing service, and uses OpenStreetMap maps as its basis.
Additional data sources used to calculate the movement of shade are digital surface models (DSM), which record the height of all objects – such as buildings and trees – above the ground, and digital terrain models (DTM), which map the actual ground surface. “A solar index is calculated for each street section from this input data. The value indicates how heavily a section is exposed to the sun at a specific time of day and month,” Psotta explains.
In theory, the application can be used for any city for which DSM, DTM and OSM data are available. “The code is open source, and we are working on making the basic data generally available, as well as offering the routing itself via our free live API,” says Julian Psotta. This means that heat routing—navigating through the shade—can also be integrated into other navigation apps based on OpenStreetMap (OSM).
Additionally, cities can integrate the application into their own services, such as a master portal or a map of cool places that highlights shaded areas, rest areas and cooling infrastructure. “We can support cities that don’t have their own data by developing a shared methodology and using data directly from OpenStreetMap,” says Julian Psotta.
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At present, the routing is designed for pedestrians. However, the team is open to further developing the routing application for cyclists and wheelchair users, for example, should groups or cities express a specific need. According to Psotta, this would not be a technical problem.
However, in tackling heat stress, the project did not rely solely on data. It began by drawing on the experiences of local residents.
Bringing people together through ‘Heat Workshops’
As part of “heat workshops”, the HEAL research project brought together older people, individuals with pre-existing health conditions and families with toddlers to share their experiences of heat alongside senior citizens’ clubs, family networks and Heidelberg city council. Participants also had the opportunity to share their experiences in their own words and at their own pace through mobile interviews via instant messaging.
“This collaboratively generated knowledge brought challenges and needs to light that would not have been visible from data alone,” reports Julian Psotta. A key insight, he noted, was “that people in areas heavily affected by heat already know where the problem lies.” City councils, on the other hand, are usually unaware of these issues.
This routing application aims to contribute to climate adaptation measures
Consequently, the routing application that emerged from the project serves a dual purpose. On the one hand, it provides a basis for guiding people along cooler paths. On the other hand, citizens and communities can use it to highlight areas of high heat stress and streets lacking shaded alternatives in their neighbourhoods. If few or no shaded routes are found when calculating paths, this provides clear evidence that sufficient protection for vulnerable groups does not exist.
These visualised insights can thus become a powerful tool for alerting city councils to locations where investments in trees and other shade structures are urgently needed. In this way, the application can establish the foundation for evidence-based climate adaptation planning and serve as a planning tool for local authorities.
