  Multiplex networks are representations of multilayer interconnected complex
networks where the nodes are the same at every layer. They turn out to be good
abstractions of the intricate connectivity of multimodal transportation
networks, among other types of complex systems. One of the most important
critical phenomena arising in such networks is the emergence of congestion in
transportation flows. Here we prove analytically that the structure of
multiplex networks can induce congestion for flows that otherwise will be
decongested if the individual layers were not interconnected. We provide
explicit equations for the onset of congestion and approximations that allow to
compute this onset from individual descriptors of the individual layers. The
observed cooperative phenomenon reminds the Braess' paradox in which adding
extra capacity to a network when the moving entities selfishly choose their
route can in some cases reduce overall performance. Similarly, in the multiplex
structure, the efficiency in transportation can unbalance the transportation
loads resulting in unexpected congestion.
