公路线形设计外文翻译

1、公路线形设计a 平面设计道路的线形反映在平面图上是由一系列的直线和与直线相连的圆曲线构成的。现代设计时常在直线与圆曲线之间插入缓和曲线。线形应是连续的，应避免平缓线形到小半径曲线的突变或者长直线末端与小半径曲线相连接的突然变化，否则会发生交通事故。同样，不同半径的圆弧首尾相接（曲线）或在两半径不同的圆弧之间插入短直线都是不良的线形。除非在圆弧之间插入缓和曲线。长而平缓的曲线总是良好的线形，因为这种曲线线形优美，将来也不会废弃。然而，双向道路线形全由曲线构成也是不理想的，因为一些驾驶员通过曲线路段时总是犹豫。长而缓的曲线应用在拐角较小的地方。如果采用短曲线，则会出现“扭结”。另外，线路的平、纵断

2、面设计应综合考虑，而不应只顾其一，不顾其二，例如，当平曲线的起点位于竖曲线的顶点附近时将会产生严重的交通事故。行驶在曲线路段上的车辆受到离心力和作用，就需要一个大小相同方向相反的由超高和侧向磨擦提供的力抵消它，从公路设计的角度看，超高或横向摩擦力都不能超过某一最大值，这些控制值对于某一规定设计车速可能采用曲线的曲率作了限制。通常情况下，某一圆曲线的曲率是由其半径来体现的。而对于线形设计而言，曲率常常通过曲度来描述，即100ft长的曲线所对应的中心角，曲度与曲线的半径成反比。公路的直线地段设置正常的路拱，而曲线地段则设置超高，在正常断面与超高断面之间必须设置过渡渐变路须。通常的做法是维持道路每一

3、条中线设计标高不变，通过抬高外侧边缘，降低内侧边缘以形成超高，对于直线与圆曲线直接相连的线形，超高应从未到达曲线之前的直线上开始，在曲线顶点另一端一定距离以外达到全部超高。如果车辆以高速度行驶在一段受限制的路段，如直线与小半径的圆曲线相连，行车会极不舒服。汽车驶近曲线路段时，超高开始，车辆向内侧倾斜，但乘客须维持身体的垂直状态，因为此时未受到离心力的作用。当汽车到达曲线路段时，离心力突然产生，迫使乘客向外倾斜，为了维持平衡，乘客必须迫使自己的身体向内侧倾斜。由于剩余超高发挥作用，乘客须作进一步的姿势高干中。当汽车离开曲线时，上述过程刚好相反。插入缓和曲线后，半径从无穷大逐渐过渡到圆曲线上的某一

4、固定值，离心力逐渐增大，沿缓和曲线心设置超高，离心力平稳逐渐增加，避免了行车颠簸。缓和曲线在铁路上已经使用多年，但在公路上最近才得以应用，这可以理解。火车必须遵循精确的运行轨道，采用缓和曲线后，上述那种不舒服的感觉才能消除。然而，汽车司机在公路上可以随意改变侧向位置，通过迂迴进入圆曲线来为自己提供缓和曲线。但是在一个车道上（有时在其他车道上）做这种迂迴行驶是非常危险的。设计合理的缓和曲线使得上述迂迴没有必要。多从安全为计，公路部门广泛采用了缓和曲线。对于半径相同的圆曲线来说，在未端加上缓和曲线就会改变曲线与直线的相关位置，因此应在最终定线勘测之前应决定是否采用缓和曲线。一般曲线的起点标为pc或

5、bc，终点标为pt或ec。对含有缓和曲线的曲线，通常的标记配置增为：tc、sc、cs和st。对于双向道路，急弯处应增加路面宽度，这主要基于以下因素：（1）驾驶员害怕驶出路面边缘；（2）由于车辆前轮和后轮的行驶轨迹不同，车辆有效横向宽度加大；（3）车辆前方相对于公路中线倾斜而增加的宽度。对于宽度为24ft的道路，增加的宽度很小，可以忽略。只有当设计车速为30mile/h，且曲度大于22时，加宽可达2ft。然而，对于较窄的路面，即使是在较平缓的曲线路段上，加宽也是很重要推荐加宽值及加宽设计见公路线形设计b纵坡线公路的竖向线形及其对车辆运行的安全性和经济性的影响构成了公路设计中最重要的要素之一。竖向

6、线形由直线和竖向抛物线或圆曲线组成，称为纵坡线。纵坡线从水平线逐渐上升时称为坡度变化的影响。在确定坡度时最理想的情况是挖方与填方平衡，没有大量的借方或弃方。所有运土都尽可能下坡运并且距离不长，坡度应随地形而变，并且与既有排水系统的升、降方向一致。在山区，坡度要使得挖填平衡以使总成本最低。在平原或草原地区，坡度与地表近似平行，介是高于地表足够的高度，以利于路面排水，苦有必要，可利用风力来清除表面积雪。如公路接近或沿河流走行，纵坡线的高度由预期洪水位来决定。无论在何种情况下，平缓连续的坡度线要比由短直线段连接短竖曲线构成的不断变向的坡度线好得多。由上坡向下坡变化的路段应设在挖方路段，而由下坡向上坡

7、变化的路段应设在填方路段，这样的线形设计较好，往往可以避免形成与现状地貌相反的圭堆或是凹地。与挖填方平衡相比，在确定纵坡线时，其他考虑则重要得多。城市项目往往比农村项目要求对控制要素进行更详尽的研究，对高程进行更细致地调整。一般来说，设计与现有条件相符的坡度较好，这样可避免一些不必要的花费。在坡度的分析和控制中，坡度对机动车运行费用的影响是最重要的考虑因素之一。坡度增大油耗显然增大，车速就要减慢。一个较为经济的方案则可使坡度减小而增加的年度成本与坡度不减而增加的车辆运行年度成本之间相平衡。这个问题的准确方法取决于对交通流量和交通类型的了解，这只有通过交通调查方能获知。在不同的州，最大纵坡也相差

8、悬殊，aashto建议由设计车速和地形来选择最大纵坡。现行设计以设计车速为70mile/h时最大纵坡为5%，设计车速30mile/h时，根据地形不同，最大纵坡一般为712。当采用较长的待续爬坡时，在没有为慢行车辆提供爬坡道时，坡长不能够超过临界坡长。临界坡长可从3纵坡的1700ft变化至8%纵坡的500ft。持续长坡的坡度必须小于公路任何一个断面的最大坡度，通常将长的持续单一纵坡断开，设计成底部为一陡坡，而接近坡顶则让坡度减小。同时还要避免由于断面倾斜而造成的视野受阻。调整公路的最大纵坡为9%只有当路面排水成问题时，如水必须排至边沟或排水沟，最小坡度标准才显示其重要性。这种情况下，aashto

9、建议最小坡度为0.35%。c视距为保证行车安全，公路设计必须使得驾驶员视线前方有足够的一段距离，使他们能够避让意外的障碍物，或者安全地超车。视距就是车辆驾驶员前方可见的公路长度。安全视距具有两方面含义：“停车视距”或“不超车视距”或“超车视距”。有时，大件物体也许会掉到路上，会对撞上去的车辆造成严重的危害。同样，轿车或卡车也可能会被一溜车辆阻在车道上。无论是哪种情况发生，合理设计要求驾驶员在一段距离以外就能看见这种险情，并在撞上去之前把车刹住。此外，认为车辆通过离开所行驶的车道就可以躲避危险的想法是不安全的，因为这会导致车辆失控或与另一辆车相撞。停车视距由峡谷部分组成：第一部分是当驾驶员发现障

10、碍物面作出制动之前驶过的一段距离，在这一察觉与反应阶段，车辆以其初始速度行驶；第二部分是驾驶员刹车后车辆所驶过的一段距离。第一部分停车视距取决于车速及驾驶员的察觉时间和制动时间。第二部分停车视距取决于车速、刹车、轮胎、路面的条件以及公路的线形的坡度。在双车道公路上，每间隔一定距离，就应该提供超越慢行车辆的机会。否则，公路容量将降低，事故将增多，因为急燥的驾驶员在不能安全超车时冒着撞车危险强行超车，能被看清的允许安全超车的前方最小距离叫做超车视距。驾驶员在做出是否超车的决定时，必须将前方的能见距离与完成超车动作所需的距离对比考虑。影响他做出决定的因素是开车的小心程度和车辆加速性能。由于人与人的显

11、著差别，主要是人的判断和动作而不是力学定理决定的超车行为随着驾驶员的不同而大不相同。为了确立超车视距值，工程人员观察了许多驾驶员的超车行为。在19381941年间，进行了建立超车视距标准的基本调查。假设操作条件如下：1. 被超车辆匀速行驶2. 超车在进入超车区时减速行驶在被超车后。3. 当到过超车区时，驾驶员需一短时间来观察超车区，并开始超车。4. 面对相向车辆，在一个延迟的启动和一个匆忙的拐弯的动作中，完成超车。在超车过程中，超车在超车道上加速，其平均速度比被超车快10mile/h。5. 当超车返回到它原来的车道上时，在它与另一车道上的相向车辆之间必须有一定的安全距离。以上五项之和就是超车视

12、距。geometric design of highwaysa alignment designthe alignment of a road is shown on the plane view and is a series of straight lines called tangents connected by circular. in modern practice it is common to interpose transition or spiral curves between tangents and circular curves.alignment must be

13、consistent. sudden changes from flat to sharp curves and long tangents followed by sharp curves must be avoided; otherwise, accident hazards will be created. likewise, placing circular curves of different radii end to end (compound curves) or having a short tangent between two curves is poor practic

14、e 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 two-lane roads because some drivers hesitate to pass on cu

15、rves. 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 tra

16、veling in a curved path is subject to centrifugal force. this is balanced by an equal and opposite force developed through cannot exceed certain maximums, and these controls place limits on the sharpness of curves that can be used with a design speed.usually the sharpness of a given circular curve i

17、s indicated by its radius. however, for alignment design, sharpness is commonly expressed in terms of degree of curve, which is the central angle subtended by a 100-ft length of curve. degree of curve is inversely proportional to the radius.tangent sections of highways carry normal cross slope; curv

18、ed sections are superelevated. provision must be made for gradual change from one to the other. this usually involves maintaining the center line of each individual roadway at profile grade while raising the outer edge and lowering the inner edge to produce the desired superelevation is attained som

19、e distance beyond the point of curve. if a vehicle travels at high speed on a carefully restricted path made up of tangents connected by sharp circular curve, riding is extremely uncomfortable. as the car approaches a curve, superelevation begins and the vehicle is tilted inward, but the passenger m

20、ust remain vertical since there is on centrifugal force requiring compensation. when the vehicle reaches the curve, full centrifugal force develops at once, and pulls the rider outward from his vertical position. to achieve a position of equilibrium he must force his body far inward. as the remainin

21、g superelevation takes effect, further adjustment in position is required. this process is repeated in reverse order as the vehicle leaves the curve. when easement curves are introduced, the change in radius from infinity on the tangent to that of the circular curve is effected gradually so that cen

22、trifugal force also develops gradually. by careful application of superelevation along the spiral, a smooth and gradual application of centrifugal force can be had and the roughness avoided.easement curves have been used by the railroads for many years, but their adoption by highway agencies has com

23、e only recently. this is understandable. railroad trains must follow the precise alignment of the tracks, and the discomfort described here can be avoided only by adopting easement curves. on the other hand, the motor-vehicle operator is free to alter his lateral position on the road and can provide

24、 his own easement curves by steering into circular curves gradually. however, this weaving within a traffic lane (but sometimes into other lanes) is dangerous. properly designed easement curves make weaving unnecessary. it is largely for safety reasons, then, that easement curves have been widely ad

25、opted by highway agencies.for the same radius circular curve, the addition of easement curves at the ends changes the location of the curve with relation to its tangents; hence the decision regarding their use should be made before the final location survey. they point of beginning of an ordinary ci

26、rcular curve is usually labeled the pc (point of curve) or bc (beginning of curve). its end is marked the pt (point of tangent) or ec (end of curve). for curves that include easements, the common notation is, as stationing increases: ts (tangent to spiral), sc (spiral to circular curve), cs (circula

27、r curve to spiral), and st (spiral go tangent).on two-lane pavements provision of a wilder roadway is advisable on sharp curves. this will allow for such factors as (1) the tendency for drivers to shy away from the pavement edge, (2) increased effective transverse vehicle width because the front and

28、 rear wheels do not track, and (3) added width because of the slanted position of the front of the vehicle to the roadway centerline. for 24-ft roadways, the added width is so small that it can be neglected. only for 30mph design speeds and curves sharper than 22does the added width reach 2 ft. for

29、narrower pavements, however, widening assumes importance even on fairly flat curves. recommended amounts of and procedures for curve widening are given in geometric design for highways.b gradesthe vertical alignment of the roadway and its effect on the safe and economical operation of the motor vehi

30、cle constitute one of the most important features of road design. the vertical alignment, which consists of a series of straight lines connected by vertical parabolic or circular curves, is known as the “grade line.” when the grade line is increasing from the horizontal it is known as a “plus grade,

31、” and when it is decreasing from the horizontal it is known as a “minus grade.” in analyzing grade and grade controls, the designer usually studies the effect of change in grade on the centerline profile.in the establishment of a grade, an ideal situation is one in which the cut is balanced against

32、the fill without a great deal of borrow or an excess of cut to be wasted. all hauls should be downhill if possible and not too long. the grade should follow the general terrain and rise and fall in the direction of the existing drainage. in mountainous country the grade may be set to balance excavat

33、ion against embankment as a clue toward least overall cost. in flat or prairie country it will be approximately parallel to the ground surface but sufficiently above it to allow surface drainage and, where necessary, to permit the wind to clear drifting snow. where the road approaches or follows alo

34、ng streams, the height of the grade line may be dictated by the expected level of flood water. under all conditions, smooth, flowing grade lines are preferable to choppy ones of many short straight sections connected with short vertical curves.changes of grade from plus to minus should be placed in

35、cuts, and changes from a minus grade to a plus grade should be placed in fills. this will generally give a good design, and many times it will avoid the appearance of building hills and producing depressions contrary to the general existing contours of the land. other considerations for determining

36、the grade line may be of more importance than the balancing of cuts and fills.urban projects usually require a more detailed study of the controls and finer adjustment of elevations than do rural projects. it is often best to adjust the grade to meet existing conditions because of the additional exp

37、ense of doing otherwise.in the analysis of grade and grade control, one of the most important considerations is the effect of grades on the operating costs of the motor vehicle. an increase in gasoline consumption and a reduction in speed are apparent when grades are increase in gasoline consumption

38、 and a reduction in speed is apparent when grades are increased. an economical approach would be to balance the added annual cost of grade reduction against the added annual cost of vehicle operation without grade reduction. an accurate solution to the problem depends on the knowledge of traffic vol

39、ume and type, which can be obtained only by means of a traffic survey.while maximum grades vary a great deal in various states, aashto recommendations make maximum grades dependent on design speed and topography. present practice limits grades to 5 percent of a design speed of 70 mph. for a design s

40、peed of 30 mph, maximum grades typically range from 7 to 12 percent, depending on topography.wherever long sustained grades are used, the designer should not substantially exceed the critical length of grade without the provision of climbing lanes for slow-moving vehicles. critical grade lengths var

41、y 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 on any particular section of a highway. it is often preferred to break the long sustained uniform grade by placing steeper grades at the bottom and lightening the grade n

42、ear 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 r

43、oadside ditch. in such instances the aashto suggests a minimum of 0.35%.c sight distancefor safe vehicle operation, highway must be designed to give drivers a sufficient distance or clear version ahead so that they can avoid unexpected obstacles and can pass slower vehicles without danger. sight dis

44、tance 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

45、may be forced to stop in the traffic lane in the path of following vehicles. in dither instance, proper design requires that such hazards become visible at distances great enough that drivers can stop before hitting them. further more, it is unsafe to assume that one oncoming vehicle may avoid troub

46、le 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 comes into view but before the driver applies his brakes. during th

47、is 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 brake-reaction time of the operator.

48、 the second distance depends on the speed of the vehicle; the condition of brakes, times, and roadway surface; and the alignment and grade of the highway.on two-lane highways, opportunity to pass slow-moving vehicles must be provided at intervals. otherwise capacity decreases and accidents increase as impatient drivers risk head-on collisions by passing when it is unsafe to do so. the minimum distance ahead that must be clear to permit safe passing is