  Road networks are characterised by several structural and geometric
properties. Their topological structure determines partially its hierarchical
arrangement, but since these are networks that are spatially situated and,
therefore, spatially constrained, to fully understand the role that each road
plays in the system it is fundamental to characterize the influence that
geometrical properties have over the network's behaviour. In this work, we
percolate the UK's road network using the relative angle between street
segments as the occupation probability. We argue that road networks undergo a
non-equilibrium first-order phase transition at the moment the main roads start
to interconnect forming the spanning percolation cluster. The percolation
process uncovers the hierarchical structure of the roads in the network, and as
such, its classification. Furthermore, this technique serves to extract the set
of most important roads of the network and to create a hierarchical index for
each road in the system.
