The length along a roadway over which a driver has uninterrupted visibility – this is known as available sight distance. Different minimum sight distance design criteria exist for various operations and maneuvers, including stopping sight distance, passing sight distance and intersection sight distance. Sight distance may be obstructed by the road surface (hills or dips) or some feature beyond the shoulder (trees or vegetation). Also, the length of roadway ahead over which an object of a specific height is continuously visible to the driver.