  Traffic congestion is one of the major challenges faced by the transportation
industry. While this problem carries a high economical and environmental cost,
the need for an efficient design of optimal paths for passengers in multilayer
network infrastructures is imperative. We consider an approach based on optimal
transport theory to route passengers preferably along layers that are more
carbon efficient than the road, e.g. rails. By analyzing the impact of this
choice on performance, we find that this approach reduces carbon emissions
considerably, compared to shortest-path minimization. Similarly, we find that
this approach distributes traffic more homogeneously thus alleviating the risk
of traffic congestions. Our results shed light on the impact of distributing
traffic flexibly across layers guided by optimal transport theory.
