  Cities and their transportation systems become increasingly complex and
multimodal as they grow, and it is natural to wonder if it is possible to
quantitatively characterize our difficulty to navigate in them and whether such
navigation exceeds our cognitive limits. A transition between different
searching strategies for navigating in metropolitan maps has been observed for
large, complex metropolitan networks. This evidence suggests the existence of
another limit associated to the cognitive overload and caused by large amounts
of information to process. In this light, we analyzed the world's 15 largest
metropolitan networks and estimated the information limit for determining a
trip in a transportation system to be on the order of 8 bits. Similar to the
"Dunbar number," which represents a limit to the size of an individual's
friendship circle, our cognitive limit suggests that maps should not consist of
more than about $250$ connections points to be easily readable. We also show
that including connections with other transportation modes dramatically
increases the information needed to navigate in multilayer transportation
networks: in large cities such as New York, Paris, and Tokyo, more than $80\%$
of trips are above the 8-bit limit. Multimodal transportation systems in large
cities have thus already exceeded human cognitive limits and consequently the
traditional view of navigation in cities has to be revised substantially.
