Road networks comprise links and nodes. The nodes are the points at which links (highways, roads or streets) cross. The three general types of crossings are at-grade, grade separations without ramps and grade separations with ramps – otherwise interchanges. They are referred to in most countries as intersections, differentiating between three-legged, four-legged and multi-legged intersections although, in the United Kingdom, reference is to three-, four-and multi-armed junctions. In this book, reference is to ‘intersections’ and ‘legs’.