道路工程畢業(yè)設(shè)計(jì)外文翻譯---公路線形設(shè)計(jì)-展示頁
2024-08-21 14:46本頁面
【正文】 grade lengths vary from 1700 ft for a 3 percent grade to 500 ft for an 8 percent grade. Long sustained grades should be less than the maximum grade used on any particular section of a highway. It is of preferred to break the long sustained uniform grade by placing steeper grades at the bottom and lightening the gr4ade near the top of the ascent. Dips in the profile grade in which vehicles may be hidden from view should also be avoided. Maximum grade for highway is 9 percent. Standards setting minimum grades are of importance only when surface drainage is a problem as when water must be carried away in a gutter or roadside ditch. In such instances the AASHTO suggests a minimum of %.C. Sight Distance For safe vehicle operation, highway must be designed to give drivers a sufficient distance of clear vision ahead so that they can avoid unexpected obstacles and can pass slower vehicles without danger. Sight distance is the length of highway visible ahead to the driver of a vehicle. The concept of safe sight distance has two facts: “stopping” (or “nonpassing”) and “passing”. At times large objects may drop onto a roadway and will do serious damage to a motor vehicle that strikes them. Again a car or truck may be forced to stop in the traffic lane in the path of following vehicles. In either instance, proper design requires that such hazards bee visible at distances great enough that drivers can stop before hitting them. Furthermore, it is unsafe to assume that one oning vehicle may avoid trouble by leaving the lane in which it is traveling, for this might result in loss of control or collision with another vehicle. Stopping sight distance is made up of two elements. The first is the distance traveled after the obstruction es into view but before the driver applies his brakes. During this period of perception and reaction, the vehicle travels at its initial velocity. The second distance is consumed while the driver brakes the vehicle to a stop. The first of these two distances is dependent on the speed of the vehicle and the perception time and brakereaction time of the operator. The second distance depends on the speed of the vehicle。 otherwise, accident hazards will be created. Likewise, placing circular curves of different radii end to end (pound curves) or having a short tangent between two curves is poor practice unless suitable transitions between them are provided. Long, flat curves are preferable at all times, as they are pleasing in appearance and decrease possibility of future obsolescence. However, alignment without tangents is undesirable on twolane roads because some drivers hesitate to pass on curve. Long, flat curves should be used for small changes in direction, as short curves appear as “kink?. Also horizontal and vertical alignment must be considered together, not separately. For example, a sharp horizontal curve beginning near a crest can create a serious accident hazard.”A vehicle traveling in a curved path is subject to centrifugal force. This is balanced by an equal and opposite force developed through superelevation and side friction. Form a highway design standpoint, both superelevation and side friction cannot exceed certain maximums, and these controls place limits on the sharpness of
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