建筑工程設(shè)計外文翻譯外文文獻(xiàn)英文文獻(xiàn)公路線形設(shè)計-閱讀頁
2024-11-24 14:31本頁面
【正文】 nexpected 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 facets: “stopping” (or “no passing”) and “passing”. At times large objects may drop into 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 dither instance, proper design requires that such hazards bee visible at distances great enough that drivers can stop before hitting them. Further more, 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。 and the alignment and grade of the highway. On twolane highways, opportunity to pass slowmoving vehicles must be provided at intervals. Otherwise capacity decreases and accidents increase as impatient drivers risk headon collisions by passing when it is unsafe to do so. The minimum distance ahead that must be clear to permit safe passing is called the passing sight distance. In deciding whether or not to pass another vehicle, the driver must weigh the clear distance available to him against the distance required to carry out the sequence of events that make up the passing maneuver. Among the factors that will influence his decision are the degree of caution that he exercises and the accelerating ability of his vehicle. Because humans differ markedly, passing practices, which depend largely on human judgment and behavior rather than on the laws of mechanics, vary considerably among drivers. To establish design values for passing sight distances, engineers observed the passing practices of many drivers. Basic observations on which passing sight distance standards are based were made during the period 19381941. Assumed operating conditions are as follows: 1. The overtaken vehicle travels at a uniform speed. 2. The passing vehicle has reduced speed and trails the overtaken one as it enters the passing section. 3. When the passing section is reached, the driver requires a short period of time to perceive the clear passing section and to react to start his maneuver. 4. Passing is acplished under what may be termed a delayed start and a hurried return in the face of opposing traffic. The passing vehicle accelerates during the maneuver and its average speed during occupancy of the left lane is 10 mph higher than that of the overtaken vehicle. 5. When the passing vehicle returns to its lane there is a suitable clearance length between it and an oning vehicle in the other lane. The five distances, in sum, make up passing sight distance.
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