Many of us know the feeling of standing in front of a subway map in a strange city, baffled by the multi-coloured web staring back at us and seemingly unable to plot a route from point A to point B.

According to a new study published in the journal Science Advances, navigating routes through a complex urban transport system can often exceed the human brain's cognitive limits.

After analysing the world’s 15 largest metropolitan transport networks, the researchers at Oxford University estimated that the information limit for planning a trip is around 8 bits. (A ‘bit’ is a binary digit — the most basic unit of information.)

Additionally, similar to the "Dunbar number," which estimates a limit to the size of an individual’s friendship circle, this cognitive limit for transportation suggests that maps should not consist of more than 250 connection points to be easily readable.

Using journeys with exactly two connections as their basis (that is, visiting four stations in total), the researchers found that navigating transport networks in major cities — including London — can come perilously close to exceeding humans’ cognitive powers.

And when further interchanges or other modes of transport — such as buses or trams — are added to the mix, the complexity of networks can rise well above the 8-bit threshold. The researchers demonstrated this using the multimodal transportation networks from New York City, Tokyo, and Paris.

"Human cognitive capacity is limited, and cities and their transportation networks have grown to the point where they have reached a level of complexity that is beyond human processing capability to navigate around them," said Mason Porter, Professor of Nonlinear and Complex Systems in the Mathematical Institute at the University of Oxford. "In particular, the search for a simplest path becomes inefficient when multiple modes of transport are involved and when a transportation system has too many interconnections."

He adds that, in many cases, the maps currently in use need to be rethought and redesigned. Journey-planner apps of course help, but the maps themselves need to be redesigned.

The research – a collaboration between the University of Oxford, Institut de Physique Théorique at CEA-Saclay, and Centre d’Analyse et de Mathématique Sociales at EHESS Paris – is published in the journal Science Advances.