Paper & Viz: Analysis of Denmark’s whole bicycle network

Today we published the paper which marks the Grand Finale of Ane’s PhD, which she will defend this thursday, May 22nd, in Auditorium 2 at ITU: https://en.itu.dk/Research/PhD-Programme/PhD-Defences/PhD-Defences-2025/May/Ane-Rahbek-Vier_

Her thesis is “Network Analysis of Denmark’s Bicycle Infrastructure: Data & Infrastructure for All?”, of which an important part is the paper:

Network Analysis of the Danish Bicycle Infrastructure: Bikeability Across Urban–Rural Divides, by A.R. Vierø and M. Szell, published in Geographical Analysis

To bridge the gap between urban and rural cycling research, we analyze the bicycle network of Denmark, covering around 43,000 km2 and nearly 6 million inhabitants. We divide the network into four levels of traffic stress and quantify the spatial patterns of bikeability based on network density, fragmentation, and reach. We find that the country has a high share of low-stress infrastructure, but with a very uneven distribution. The widespread fragmentation of low-stress infrastructure results in low mobility for cyclists who do not tolerate high traffic stress. Finally, we partition the network into bikeability clusters and conclude that both high and low bikeability are strongly spatially clustered. Our research confirms that in Denmark, bikeability tends to be high in urban areas. The latent potential for cycling in rural areas is mostly unmet, although some rural areas benefit from previous infrastructure investments. To mitigate the lack of low-stress cycling infrastructure outside urban centers, we suggest prioritizing investments in urban–rural cycling connections and encourage further research in improving rural cycling conditions.

With the paper comes an interactive visualization of the whole bicycle network of Denmark! Check it out at: bikenetwork.dk

This achievement also marks the conclusion of all our “deliverables” for the Vejdirektoratet grant by the Danish Ministry of Transport which financed Ane’s Phd.

If you are around ITU, don’t miss Ane’s defense on thursday!

Two new NERDS papers: Bias in LLM populations, recommending routes

We have two new publications out!

  1. Emergent social conventions and collective bias in LLM populations, by Ariel Flint Ashery, Luca Maria Aiello, and Andrea Baronchelli, published in Science Advances. Barplots of estimation of individual LLM bias vs. the collective bias they exhibit when playing the naming game
    We explore the collective behavior of LLMs starting from social conventions, the fundamental building blocks of coordinated societies. We used the naming game, a well-established framework that has been applied for decades to study conventions in humans. We found that LLM populations can spontaneously develop shared conventions through local interactions. These interactions can produce collective biases, invisible at the individual level, raising important considerations for AI alignment. Small committed minorities can trigger tipping points, steering the entire group toward new conventions—a dynamic well known in human societies
  2. The experience of running: Recommending routes using sensory mapping in
    urban environments, by Katrin Hänsel, Luca Maria Aiello, Daniele Quercia, Rossano Schifanella, Krisztian Zsolt Varga, Linus W. Dietz, and Marios Constantinides, published in the International Journal of Human-Computer Studies.Map of London with several pairs of alternative running trajectories (urban routes plotted in red, scenic in blue)
    We set out to build running routes not around distance, but around how people feel: before, during, and after a run. We surveyed 387 runners and found that not everyone wants the same kind of run. Some seek quiet and greenery; others thrive on the buzz of people and traffic. Their preferences often match their personality. Runners who prefer scenic paths (quiet, green, and natural) tended to score higher in neuroticism. Those who preferred urban paths (lively and full of people) were more likely to be extroverted. Then, we built a routing engine. Using millions of geotagged Flickr photos and open London data, we scored streets for beauty, noise, safety, and surface quality. We tested the engine on hundreds of 5-km London loops. Most runners preferred the scenic routes.

Two new NERDS papers published at CHIIR 2025

We have two new publications out at CHIIR 2025 in Melbourne (Proceedings of the 2025 ACM SIGIR Conference on Human Information Interaction and Retrieval):

  1. Exploring the Zero-Shot Known-Item Retrieval Capabilities of LLMs for Casual Leisure Information Needs, by T. Bogers, M. Gäde, M. Hall, M. Koolen, V. Petras, M. Skov, published in CHIIR 2025
    We compared four LLMs on their capability to answer a specific type of complex search task: known-item requests from casual leisure domains. We constructed a test collection by gathering known-item requests for books, games and movies from online forums along with verified answers by the original requester. We prompted four LLMs multiple times with the same prompt and analyzed the results with respect to accuracy and the degree to which answers were fabricated by the LLM. Our results show that LLMs are not particularly effective in fulfilling these complex casual leisure needs, but there are are big differences between LLMs and across domains.
  2. From Queries to Candidates: Exploring Search and Source Interaction Behavior of Recruiters, by T. Bogers, M. Kaya, M. Gäde, published in CHIIR 2025

    Recruitment is a professional search domain that has been largely overlooked in IR research, even though better support of recruiters could have a big impact on job seekers, companies and society as a whole. In this paper, we analyze the search formulation and source selection behavior of the recruiters at one of Scandinavia’s largest job portals and recruitment agencies using search logs for close to 18,000 recruitment search tasks. We provide an analysis of the search sessions of recruiters in terms search tactics, query operators, query length, term re-use and filter usage, and break down their behavior both by task type and task complexity. We also relate their short-term tactics to different learning stages in the search process and investigate their influence on search success. We find that identifying and assessing relevant candidates for a job posting is a complex task: recruiters usually submit multiple queries during sessions that can last for hours and that increase in complexity. Recruiters all spend more time per query as their session progresses. We also observed query reformulation strategies that indicate distinct patterns of knowledge gaining during sessions. Relating these tactics to positive responses from candidates we aim at predicting successful strategies.

Two new NERDS papers published: Triadic closure, CoolWalks

We have two new publications out, one on closing triads in social networks, and one on shaded walks (CoolWalks) in cities:

  1. Exploring Time-Ordered Triadic Closure in Online Social Networks, by A. Galdeman, C. Tidiane Ba, M. Zignani, S. Gaito, published in ACM Transactions on the Web

    By analyzing networks with timestamped links from diverse platforms based on different architectures, including communication, Web3-based, and trade networks, we developed a comprehensive analytical pipeline to support the study of triadic closure patterns. This pipeline includes an algorithm for the census of time-ordered triads, a vector-based model for representing growing networks (growth triadic profile), the identification of triadic closure rules (TERs), and the evaluation of the speed of the formation of closed triads. Our findings reveal significant variations in the impact of triadic closure across different OSNs, marked by diverse growth triadic profiles and varying formation speeds of closed triads as well as diversity in the predictability of evolutionary patterns based on triads. This study not only enhances the comprehension of triadic closure in the temporal evolution of OSNs but also provides valuable insights to be taken into account for the design and administration of online social platforms.
  2. CoolWalks for active mobility in urban street networks, by H. Wolf, A.R. Vierø & M. Szell, published in Scientific Reports

    Here we explore the potential for shaded walking, using building footprints and street networks from both synthetic and real cities. We introduce a route choice model with a sun avoidance parameter alpha and define the CoolWalkability metric to measure opportunities for walking in shade. We derive analytically that on a regular grid with constant building heights, CoolWalkability is independent of alpha, and that the grid provides no CoolWalkability benefit for shade-seeking individuals compared to the shortest path. However, variations in street geometry and building heights create such benefits. We further uncover that the potential for shaded routing differs between grid-like and irregular street networks, forms local clusters, and is sensitive to the mapped network geometry. Our research identifies the limitations and potential of shade for cool, active travel, and is a first step towards a rigorous understanding of shade provision for sustainable mobility in cities.

NERDS faculties move into new 4F11 office

Public service announcement: Given all the recent promotions, NERDS faculties also took the ascendance to a physical level to move upwards 3 meters, from our old 3F11 office to our new 4F11 office. The new office has exactly the same dimensions, but features one more work desk to accommodate our part-time professors, and it also comes with a coffee table and coffee seats for added hygge.

There was no particularly strong reason for this move – it’s mostly that we just could. It made the furniture update easier, and it now places our faculties on the floor with all the other research groups which could facilitate ITU-internal collaborations. Also, the view is now slightly improved:

before

after

Anastassia defends her PhD: Urban Data Science for Sustainable Mobility

Big congratulations to Dr. Anastassia Vybornova – she defended her thesis yesterday, on Urban Data Science for Sustainable Mobility! 🥳 🎓 This makes Anastassia the first PhD student who did their whole PhD journey at NERDS – about which we are especially proud, besides her brilliant scientific accomplishments.

Supervisor: Michael Szell
Committee: Luca Rossi, Malene Freudendal-Pedersen, Robin Lovelace

Read her thesis here: https://en.itu.dk/Research/PhD-Programme/PhD-Defences/PhD-Defences-2025/March/Anastassia-Vybornova

The thesis spans 8 papers that advance Urban Data Science, exploring how this emerging field can ethically support human mobility that is both environmentally and socially sustainable, through the two lenses of Data and Networks.

In her research she develops algorithms and pipelines for multi-purpose spatial network simplification, for data quality assessment and planning of bicycle networks and low-traffic neighborhoods, showcasing applications to various cities. Further, she outlines pathways for incorporating data ethics into computational approaches to spatial manifestations of social inequalities. Lastly, she investigates the impact of transportation infrastructure on social connections in cities, quantitatively corroborating that urban highways are barriers to social ties. Stemming from various interdisciplinary collaborations, the results of this thesis cover multiple conceptual levels of Urban Data Science, from open source software development and data quality assessment to transportation network planning and the intersection of social and spatial networks. Through these efforts, this thesis advances the emerging field of Urban Data Science, showcasing the field’s potential to make human mobility more sustainable.

With her PhD completed, Anastassia starts a postdoc position at SODAS with Roberta Sinatra, retaining a visiting affiliation at NERDS. Anastassia has been both a professional and social cornerstone of NERDS, so we are lucky we will keep seeing her around. 💜

Update April 4th 2025: The defense and time in Copenhagen made a lasting impact on our external examiner: https://www.robinlovelace.net/post/copenhagenize/

Call for PhD and Postdocs: Using data science to improve epidemic preparedness

We are looking for PhDs and Postdocs to join Jonas Juul improving inference, forecasting, and mitigation in future pandemics. The application deadline is May 1st and you can apply here.

The project (funded by the Novo Nordisk Foundation) will improve epidemic preparedness in several ways, for example:

  • By studying a unique data set of hundreds of thousands of COVID-19 genomes. Studying these nation-scale data in combination with the Danish collection of registers, we hope to understand the details of COVID-19 propagation and mitigation impact in Denmark.
  • By quantifying the effectiveness and efficiency of mitigation strategies analytically and numerically, we will get a better understanding of how mitigation strategies should adapt as an epidemic unfolds,
  • By improving the statistical foundation for the projection of epidemic case numbers we hope to save lives and money when the next pandemic hits.

The positions come with attractive opportunities for conference participation, travel, etc., and Copenhagen offers a world-class research community in network science, epidemiology, and data science.

Interested in large-scale data analysis, applied statistics, and/or analytical methods for studying epidemics in networks? Read more and apply here: https://candidate.hr-manager.net/ApplicationInit.aspx?cid=119&ProjectId=181783&DepartmentId=3439&MediaId=1282  and feel free to reach out to Jonas Juul if you have any questions (deadline May 1st 2025)

The NERDS group is a down-to-earth and fun place to be. Copenhagen is often named as the best city in the world to live in, and for good reasons. It’s world-renowned for food, beer, art, music, architecture, the Scandinavian “hygge”, and much more. In Denmark, parental leave is generous, and child-care is excellent and cheap.

New NERDS paper on highway barriers to social ties

This week we are on fire and have yet another big paper out, long time in the making, led by Luca Maria Aiello with multi-NERDS involvement, just published in PNAS: Urban highways are barriers to social ties, by L.M. Aiello, A. Vybornova, S. Juhász, M. Szell, and E. Bokányi.

Highways are physical barriers that cut opportunities for social connections, but the magnitude of this effect has not been quantified. Such quantitative evidence would enable policy-makers to prioritize interventions that reconnect urban communities—an urgent need in many US cities. We relate urban highways in the 50 largest US cities with massive, geolocated online social network data to quantify the decrease in social connectivity associated with highways. We find that this barrier effect is strong in all 50 cities, and particularly prominent over shorter distances. We also confirm this effect for highways that are historically associated with racial segregation. Our research demonstrates with high granularity the long-lasting impact of decades-old infrastructure on society and provides tools for evidence-based remedies.

New NERDS paper on academic mentorship

We have a big paper out today, long time in the making, led by Yanmeng Xin, our long-term PhD student visitor in 2021-2023, co-authored by Roberta Sinatra, just published in Nature Human Behavior: Academic mentees thrive in big groups, but survive in small groups, by Y. Xing, Y Ma, Y. Fan, R. Sinatra, and A. Zeng.

The main message of the paper is intriguing: If you “grow up” in a big research group, and if you survive, you will have high success. At the same time, in a big group it is also harder to survive, especially if your mentor is very productive. So what is then good mentorship, and what is a good group to be?

Interestingly, at NERDS we are a fairly big research group, but with several mentors who are by themselves not too busy, so we combine the best of both worlds 😁 In fact, this paper itself is another one in a long series of success stories where a visitor accomplished something great while staying at our ✨🦄 ~enchanted NERDS grounds~ 🧚‍♂️✨ in Copenhagen. (“NERDS is one of the best places I have ever stayed.”)

Mentoring is a key component of scientific achievements, contributing to overall measures of career success for mentees and mentors. Within the scientific community, possessing a large research group is often perceived as an indicator of exceptional mentorship and high-quality research. However, such large, competitive groups may also escalate dropout rates, particularly among early-career researchers. Overly high dropout rates of young researchers may lead to severe postdoc shortage and loss of top-tier academics in contemporary academia. In this context, we collect longitudinal genealogical data on mentor-mentee relations and their publication, and analyze the influence of a mentor’s group size on the future academic longevity and performance of their mentees. Our findings indicate that mentees trained in larger groups tend to exhibit superior academic performance compared to those from smaller groups, provided they remain in academia post-graduation. However, we also observe two surprising patterns: Academic survival rate is significantly lower for (1) mentees from larger groups, and for (2) mentees with more productive mentors. The trend is verified in institutions of different prestige. These findings highlight a negative correlation between a mentor’s success and the academic survival rate of their mentees, prompting a rethinking of effective mentorship and offering actionable insights for career advancement.