SUBSCRIBE TO OUR BLOG
The revelation comes from recent research by theoretical physicists Riccardo Gallotti and Marc Barthelemy and applied mathematician Mason Porter. Their paper, Lost in transportation: Information measures and cognitive limits in multilayer navigation, examines the world’s 15 largest metropolitan transportation networks, including New York, Beijing, Paris and London.
The researchers began by considering a journey between two stations on different subway lines with no more than two changes. This means four stations to think about: the start and destination, plus the two places where you make connections. It corresponds to the maximum of four pieces of information you can hold in your visual working memory at the same time.
When you read a map, you have to deal with more information besides the route you actually want to take, explains Mason Porter, Professor of Nonlinear and Complex Systems at the University of Oxford’s Mathematical Institute. “Where you change routes, you have so-called 'distractors' – places where you could have changed routes but don't actually change,” he explains.
Porter and his colleagues came up with a formula to measure the amount of information present in the metro networks of each of the 15 cities. By this measure, the most complex metro system in the world turned out to be New York, with 8.1 bits of information, followed by Paris and Tokyo. Hong Kong was the least complex.
The scientists then translated this measure of information into the maximum number of connections a map can display and still be easily navigable. They calculated that it was around 250. Previous studies using eye-tracking technology have shown that when an information limit is exceeded, our usual method of reading maps falls down. Instead of methodically following subway lines from A to B, peoples’ eyes fall on different areas of the map.
But while our cognitive abilities can cope with one mode of transport in all 15 cities, things become altogether trickier when bus routes are added. When the researchers performed the same calculation for the multimodal transport maps of New York, Paris and Tokyo, the information limit was exceeded, explains Porter.
“The amount of information required for most paths goes way beyond that for a single mode of transport. It's not just a little bit more than you need - it's quite a bit more. When we asked ourselves 'How many trips are actually under this amount?’, it was about 15% for New York and Tokyo and 10% for Paris,” he says.
Porter stresses that the calculations involve finding a “simplest” path through a network - that is, a path with the fewest route changes. “Some of the reaction to our paper has been people saying: ‘I'm able to find good paths – what are you guys doing?' Actually, it's not a contradiction to say that while it’s easy to find a good path, it's hard to find the best path.”
One outcome of the research is that adding more connections to a transport network doesn’t make it easier to navigate, even though it minimizes the number of transfers you need to make. Porter says more connections are only better if you’re perfect at finding your way around. “It’s analogous to putting more information on the web. I’m only making things better if I can navigate that information.”
So what’s the solution? Porter believes that the ‘spider maps’ seen in parts of London are useful tools because they allow you to remove any distractors. “A spider map doesn’t merely show where you are; it shows how the routes you can take are centred on you – it’s egocentric,” he says.
It’s similar to navigating your way around a city that you already know pretty well, says Porter. “Londoners will say ‘My 10-year-old kid can navigate the Tube!’ But if somebody has learned their way around, there are certain paths they're no longer considering. You can imagine apps that would do that in real time by allowing you to take away routes you don’t want to consider. Then you’d have a smaller amount to think about.”
Porter says there’s plenty of scope for future research, including future eye-tracking studies to see how easy transport maps are to read in practice. “The London Tube comes to mind. When people are waiting and looking at the maps, they’re standing in other people's way. Others can't even start looking at the maps because people are taking so long staring at them. Whether it's more than just annoying is open to debate,” he says.
Potential future studies involving neuroscientists would make for an interesting follow-up, ponders Porter. The cognitive limit of 250 connections is analogous to the maximum number of social relationships we can maintain, as proposed by the anthropologist Robin Dunbar. It’s believed that the human memory has the capacity to keep us acquainted with between 100 and 200 friends.
“People have mentioned Dunbar's number in relation to social networks. We're saying that we've done a measurement empirically for transport networks and it's suggestive,” says Porter.
The need for transport maps to be clear enough to read without causing us problems is only likely to become more acute in the future. According to a UN report, the number of ‘megacities’ – each home to more than 10 million people - has tripled since 1990. In 2014, there were 28 megacities around the world.
The good news is that this is one urban challenge we have the power to solve, says Porter. “Information overload is now part of our world and the number of cities in which we need to address it will go up. But the timescale is one we should be able to handle. After all, it doesn't take that long to install new maps.”