Designing the highway 2


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Designing the highway 2

  1. 1. DESIGNING THEHIGHWAY•The Cross Slope•Highway Median•The Grade Line•Right-of-Way•Road Alignment
  2. 2. The Cross SlopeThe cross slope is provided in all tangent sections of the roadway. Slopes usually fall in both directions from the centre line of the two lane highway except where super elevation of curves direct all water towards the inside.For a high type pavement, the crown or slope is often 1% to 2%. However, steeper slopes are strongly recommended because rainwater flow away more rapidly and thereby reducing the water thickness on the road pavement. A cross slope in one direction of a multi-lane highways makes driving comfortable, but with heavy rainfall, the water depth increases on the roadway.
  3. 3. Highway MedianA median is a portion of a divided highway separating the traveled way for traffic in the opposing direction. The median width is the dimension between the through-lane edges and includes left shoulders, if any. A median provides freedom from the interference of opposing traffic, a recovery area for out-of-control vehicles, a stopping area in case of emergencies, room for speed changes and storage of left- and U-turning vehicles and for less headlight glare, and space for additional future lanes.
  4. 4. Highway MedianAlthough medians should be as wide as possible, economic factors often limit this width because of cost of land and general maintenance. Medians are classified as traversable, deterring, or barrier. Traversable medians are merely paint stripes or buttons and are easily traversable. When a mountable curb or corrugation is provided, it is known as deterring median. Barrier median is usually in the form of a guardrail or a concrete wall that prevents traffic from crossing over.
  5. 5. The Grade LineGrade line is a line or slope used as a longitudinal reference for a railroad or highway. Inclinations with the horizontal of a road, railroad, etc., usually expressed by stating the vertical rise or fall as a percentage of the horizontal distance; slope.In vehicular engineering, various land-based designs (cars, SUVs, trucks, trains, etc.) are rated for their ability to ascend terrain. (Trains typically rate much lower than cars.) The highest grade a vehicle can ascend while maintaining a particular speed is sometimes termed that vehicles "gradeability" (or, less often, "grade ability"). The lateral slopes of highway geometry are sometimes called fills or cuts where these techniques have been used to
  6. 6. The Grade LineOne factor that significantly influences the selection of a highway location is the terrain the land, which in turn affects the laying of the grade line. The primary factor that the designer considers on laying the grade line is the amount of earthwork that will be necessary for the selected grade line. The height of the grade line is usually dictated by expected floodwater level. Grade lines should also be set such that the minimum sight distance requirements are obtained.Maximum grade - Maximum grade is determined by a table, with up to 6% allowed in mountainous areas and hilly urban areas.
  7. 7. The Grade Line Profile grade line (PGL) - This is a single line, straight or curved, along the length of the highway, sometimes but not always on the center of the highway. Grade separation - is the process of aligning a junction of two or more transport axes at different heights (grades) so that they will not disrupt the traffic flow on other transit routes when they cross each other. The composition of such transport axes does not have to be uniform; it can consist of a mixture of roads, footpaths, railways, canals, or airport runways. Bridges, tunnels, or a combination of both can be built at a junction to achieve the needed grade separation.
  8. 8. Right-of-WayA right-of-way (R/W) is a strip of land that is granted, through an easement or other mechanism, for transportation purposes, such as for a trail, driveway, rail line or highway. A right-of-way is reserved for the purposes of maintenance or expansion of existing services with the right-of-way. In the case of an easement, it may revert to its original owners if the facility is abandoned.The right-of-way has three basic categories distinguished by the degree of separation from other traffic:
  9. 9. Right-of-Way CATEGORY A: “a grade-separated” or “exclusive”. It is a fully controlled R/W without grade crossings or any legal access by other vehicles. In some ways, this category resembles a freeway system. CATEGORY B: includes R/W types that are longitudinally physically separated from other traffic, but with grade crossing for vehicles and pedestrians, including regular street intersections. A light-rail system that crosses a few streets at the surface falls into this category. CATEGORY C: surface streets with mixed traffic. Most bus systems and streetcar systems fall into this category.
  10. 10. Road AlignmentRoad alignment should be consistent. An abrupt change from flat to sharp curve and long tangents followed by sharp curve should be avoided because it will only create hazard and invite accident. Similarly, designing circular curves of different radius from end to end or compounded curve is not a good practice, unless suitable transitions between them are provided.To have a short tangent between two curves is also poor practice. Along flat curve is acceptable at all times. It is pleasuring to look at, with less probability of future obsolescence. Alignment should be provided with tangent because there are drivers who hesitate to pass on curves. A short