Benjamin Legros: “Increasing a system’s capacity can sometimes create more congestion”

Your two recent papers published in Production and Operations Management examine issues related to congestion, waiting systems and individual behaviour. What are the main questions addressed in this research?

Both papers start from a fairly simple idea: in many service systems, congestion depends not only on available capacity but also on how individuals respond to that capacity.
We can think of a hospital, a transport network, a digital platform or a call centre. Each person makes an individual decision, whether to enter the system, wait, come back later or use the service. Yet when these decisions are aggregated, they can produce collective outcomes that are sometimes unexpected.
In the first paper, we examine what happens when customers take into account the impact of their own decisions on others. In other words: do I consider the waiting time I impose on the people behind me? In the second paper, we focus on risk aversion: how individuals react when faced with uncertain waiting times, and how this can alter the effects of increased capacity.
When more capacity leads to more congestion
 

One of your papers shows that increasing a system’s capacity can sometimes generate more congestion. How can this paradox be explained?

The natural intuition is that adding capacity should reduce waiting times. In many cases, that is true. What our paper shows, however, is that this reasoning overlooks an important part of the problem: user behaviour. 

When capacity increases, a system becomes more attractive. It may appear more reliable, less risky and less uncertain. People who might previously have decided not to enter the system may now choose to use it. As a result, demand rises.
When customers are risk-averse, this additional demand can be strong enough to offset the initial benefits of the extra capacity. Larger systems are often perceived as less uncertain: even if average waiting times are longer, they may also become more predictable. Customers may therefore accept a longer average wait in exchange for greater reliability and less uncertainty.
This can lead to a paradoxical situation: capacity has increased, yet the system ends up with more users, longer queues and higher levels of congestion.
The key message is that service managers cannot focus solely on the supply side. They must also anticipate how users will react to changes in capacity.

Two UTD24 publications in a matter of weeks

Publishing two papers in a UTD24-ranked journal within a month remains extremely rare. What does this double achievement mean to you?
It is a tremendous source of satisfaction. Publishing in a journal such as Production and Operations Management is already a significant achievement, so having two papers appear only a few weeks apart is naturally something very special.

It is also the result of a long-term effort. These projects required several years of research, extensive discussions with my co-authors, the development and analysis of models, and multiple rounds of revision.
I also see it as recognition for a type of research that combines rigorous analytical modelling with questions about human behaviour: why systems become congested, how individuals react to waiting, and why certain managerial decisions can produce unintended consequences.

Traditionally, these behavioural questions are often approached through empirical studies. I enjoyed being able to demonstrate how mathematics and analytical models can help us understand and predict these behaviours.

What makes a UTD24 journal so special?

For non-specialists, what distinguishes a UTD24-ranked journal?
The UTD24 list brings together 24 academic journals that are widely regarded as leading international references in management research, covering fields such as finance, marketing, information systems, strategy, management and operations. These journals are highly selective and closely monitored by top business schools around the world, including SKEMA.

What makes them particularly demanding is not simply the difficulty of getting published. It is the level of contribution that is expected. Researchers must put forward a genuinely new idea, demonstrate why it matters, support it with a robust methodology and respond to often highly detailed reviews from leading specialists in the field.

The objective is not simply to extend an existing stream of research, but to provide a more substantial and original contribution to the resolution of an important question.

The process can be long and uncertain, often taking several years, but that is also what gives these publications their value. When a paper is accepted in a UTD24 (UTD journal list, a prestigious ranking established by the Naveen Jindal School of Management at the University of Texas at Dallas) journal, it means it has passed an exceptionally rigorous level of scientific scrutiny.