A footbridge (also a pedestrian bridge, pedestrian overpass, or pedestrian overcrossing) is a bridge designed solely for pedestrians. While the primary meaning for a bridge is a structure which links "two points at a height above the ground", a footbridge can also be a lower structure, such as a boardwalk, that enables pedestrians to cross wet, fragile, or marshy land. Bridges range from stepping stones–possibly the earliest man-made structure to "bridge" water–to elaborate steel structures. Another early bridge would have been simply a fallen tree. In some cases a footbridge can be both functional and artistic. For rural communities in the developing world, a footbridge may be a community's only access to medical clinics, schools, businesses and markets. Simple suspension bridge designs have been developed to be sustainable and easily constructed in such areas using only local materials and labor. An enclosed footbridge between two buildings is sometimes known as a skyway. Bridges providing for both pedestrians and cyclists are often referred to as greenbridges and form an important part of a sustainable transport system. Footbridges are often situated to allow pedestrians to cross water or railways in areas where there are no nearby roads. They are also located across roads to let pedestrians cross safely without slowing traffic. The latter is a type of pedestrian separation structure, examples of which are particularly found near schools. Early history The simplest type of a bridge is stepping stones, so this may have been one of the earliest types of footbridge. Neolithic people also built a form of a boardwalk across marshes, of which the Sweet Track, and the Post Track are examples from England, that are around 6000 years old. Undoubtedly ancient peoples would also have used log bridges; that is a timber bridge that fall naturally or are intentionally felled or placed across streams. Some of the first man-made bridges with significant span were probably intentionally felled trees. Among the oldest timber bridges is the Holzbrücke Rapperswil-Hurden crossing upper Lake Zürich in Switzerland; the prehistoric timber piles discovered to the west of the Seedamm date back to 1523 B.C. The first wooden footbridge led across Lake Zürich, followed by several reconstructions at least until the late 2nd century AD, when the Roman Empire built a wooden bridge. Between 1358 and 1360, Rudolf IV, Duke of Austria, built a 'new' wooden bridge across the lake that has been used to 1878