Universal Properties for Urban Street Networks

Urban street networks are relevant examples for transportation networks in the field of complex systems. In terms of network theory they can be analysed in the so called primal space, where they display fractal properties and different allometries and where their size distribution follows the Zipf's law. Moreover, they can be analysed in the so called information space, where they display the typical properties of small world networks, such as fat tail distribution and high hierarchical and modular organization. In this paper we present results about the city boundary definition problem, that arise from universal properties emerging by applying percolation theory to the primal space. We discuss the results in respect to relevant allometries in the primal space.