We analyze the statistics of the shortest and fastest paths on the road network between randomly sampled end points. We find that, to a good approximation, the optimal paths can be described as directed polymers in a disordered medium, which belong to the Kardar-Parisi-Zhang universality class of interface roughening. Comparing the scaling behavior of our data with simulations of directed polymers and previous theoretical results, we are able to point out the few characteristics of the road network that are relevant to the large-scale statistics of optimal paths. Indeed, we show that the local structure is akin to a disordered environment with a power-law distribution which become less important at large scales where long-ranged correlations in the network control the scaling behavior of the optimal paths. DOI: 10.1103/PhysRevE.96.050301 Complex networks of nodes and links can be used to model a wide array of systems. Examples range from biological networks such as those formed by neurons and synapses in the brain or chemical reactions inside a cell, to social or transportation networks and the World Wide Web. Their topology in the abstract space of edges and vertices has been much studied, allowing one to identify widespread properties such as "small-world" effects, scale-free connectivity, and a high degree of clustering, which can be captured by simple physical models [1][2][3][4][5]. Comparatively, less is understood about the spatial organization of complex networks embedded in a Euclidean space, a very active subject of research (see Ref.[6] for a review). The effect of geometry becomes especially relevant when the network is strongly constrained by the environment or when the "cost" to maintain edges increases significantly with their length (e.g., rivers [7], railways [8], or vascular networks [9]). The spatial structure of streets is another example that has been particularly studied to gain insight into the structure of cities and their development [10][11][12].Much information about the geometry of a network can be obtained by studying the shortest paths between the nodes of the network. In many cases, it is also a problem of practical importance to characterize the paths that optimize a given cost function. For example, in transportation networks, one would like to understand the properties of the paths that minimize the travel time, the distance, or the monetary cost to travel between two points. An obvious application is in the development of efficient global positioning system (GPS) routing algorithms which could use prior information on optimal paths to perform better [13]. The shortest paths between two generating nodes on the power grid are also important to predict the overloading of electric lines [14]. Understanding the properties of these optimal paths appears challenging since they are expected to depend strongly on the geometry of the network which can be shaped by various factors, from natural obstacles to historical development or differences in policy.The theory of directed polymers...