Geometric design relates the layout of a road in its terrain with driver and vehicle requirements. Its main features are horizontal and vertical curvature, and the visible features of the road cross-section. Road designers use design speed as a guide to determining radii, sight distances, superelevations, and transition lengths. Its selection is influenced primarily by the nature of the terrain and by drivers' expectations of the free speed at which it is safe or legal to drive. Sight distance, the length of carriageway visible to a driver in both horizontal and vertical planes, is the most important feature of a safe and efficient road; stopping and overtaking sight distances are especially important. Horizontal alignment is a series of intersecting tangents and circular arcs, with transition curves, which affects a road's safety, efficiency, and cost. Vertical alignment design aims mainly to ensure that motorists perceive a continuously unfolding ribbon of road, so that they can instantly and correctly anticipate directional change and future action. Cross-section elements include carriageway width, number of lanes, central reservations, shoulders, lay-bys, camber, side-slopes of cuttings and embankments, and safety fences, noise barriers, and anti-dazzle screens. Safety audits are also important. For the covering abstract, see IRRD 892228.
Samenvatting