more at http://sports.quickfound.net "Learn how to plan, lay out, construct, and maintain switchbacks." From the US Forest Service Missoula Technology Development Center. Public domain film from the US National Archives, slightly cropped to remove uneven edges, with the aspect ratio corrected, and mild video noise reduction applied. The soundtrack was also processed with volume normalization, noise reduction, clipping reduction, and/or equalization (the resulting sound, though not perfect, is far less noisy than the original). http://creativecommons.org/licenses/by-sa/3.0/ http://en.wikipedia.org/wiki/Trail#Trail_construction While most trails have arisen through common usage, the design and construction of good quality new paths is a complex process that requires certain skills. When a trail passes across a flat area that is not wet, brush, tree limbs and undergrowth are removed to create clear, walkable trail. A bridge is built when a stream or river is sufficiently deep to make it necessary. Other options are culverts, stepping stones, and shallow fords. For equestrian use, shallow fords may be preferred. In wet areas an elevated trailway with fill or a boardwalk are often used, though boardwalks require frequent maintenance and replacement, because boards in poor condition can become slippery and hazardous. Trails on slopes Trail gradient are determined based on a site specific assessment of soils and geology, drainage patterns of the slope, surrounding vegetation types, position on the slope of a given trail segment (bottom, mid-slope, ridgeline), average precipitation, storm intensities, types of use, volume and intensity of use, and a host of other factors affecting the ability of the trail substrate to resist erosion and provide a navigable surface. Trails that ascend steep slopes may use switchbacks, but switchback design and construction is a specialized topic. Trails accessible to off-road wheelchairs, have a grade no more than one in ten. Paved trail that are accessible to all wheelchairs, have a grade of be no more than one in twelve, with periodic level pull-offs. On a well constructed trail the slope of the trail from side to side is never be more than one in twelve, because side-sloped trails are prone to gullying. The ideal path is built almost, but not quite, level in cross-section. To achieve a proper slope in hilly terrain a sidehill trail is excavated. This type of trailway is created with the establishing a line of suitable slope across a hillside, which is then dug out by means of a mattock or similar tool. This may be a full-bench trail, where the treadway is only on the firm ground surface after the overlying soil is removed and sidecast (thrown to the side as waste), or a half-bench trail, where soil is removed and packed to the side so that the treadway is half on firm old ground and half on new packed fill. In areas near drainages, creeks and other waterways, excavation spoils are taken away in bulk and deposited in an environmentally benign area. In problem areas trails are established entirely on fill. In such cases the soil is packed down firmly and the site is periodically checked to maintain the stability of the trail. Cycle trails built for commuting may be built to a different set of standards than pedestrian-only trails and, in some cases, may require a harder surface, fewer changes in grade and slope, increased sight visibility, and fewer sharp changes in direction. On the other hand, the cross-slope of a bicycle trail may be significantly greater than a foot trail, and the path may be narrower in some cases. The American Association of State Highway and Transportation Officials recommends different widths for different types of bicycle facilities.[63] For example, a shared use path has a recommended one directional width of 8 feet (2.44 m), while a bidirectional path should be significantly wider (10 to 12 feet or 3.05 to 3.66 metres) to accommodate bidirectional traffic and users. The U.S. Department of Transportation provides additional guidance on recreational bicycle and pedestrian trail planning and design standards. A well designed recreational mountain bike path for the exclusive use of bicycles has an average grade of less than 10%, and generally follow a contour line, rather than straight downhill... http://en.wikipedia.org/wiki/Hairpin_turn A hairpin turn (also hairpin bend, hairpin corner, etc.), named for its resemblance to a hairpin/bobby pin, is a bend in a road with a very acute inner angle, making it necessary for an oncoming vehicle to turn almost 180° to continue on the road. Such turns in ramps and trails may be called switchbacks in American English, by analogy with switchback railways. In British English 'switchback' is more likely to refer to a heavily undulating road—a use extended from the rollercoaster and the other type of switchback railway...

• published: 22 Mar 2015
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