公路线形设计-外文翻译.pdf

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1、Geometric Design of Highways A Alignment Design 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 consistent.Sudd

2、en 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 practice unless suitable tran

3、sitions 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 curves.Long,flat curves shoul

4、d 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

5、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 is indicated by its radius.However,for

6、 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;curved sections are super Provision must be mad

7、e 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 super is attained some distance beyond the point of curve.If a vehicle travels at h

8、igh 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,super begins and the vehicle is tilted inward,but the passenger must remain vertical since there is on centrifugal force requiring compensati

9、on.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 remaining super takes effect,further adjustment in position is required.This process is r

10、epeated 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 centrifugal force also develops gradually.By careful application of super along the spiral,a smoo

11、th 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 come only recently.This is understandable.Railroad trains must follow the precise alignment of the tracks,and

12、 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 his own easement curves by steering into circular curves gradually.However,this weaving within a traffic lane(

13、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 adopted by highway agencies.For the same radius circular curve,the addition of easement curves at the ends changes the lo

14、cation 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 circular curve is usually labeled the PC(point of curve)or BC(beginning of curve).Its end is marked the PT(point of tangent)

15、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(circular curve to spiral),and ST(spiral go tangent).On two-lane pavements provision of a wilder roadway is advisable on sharp curves.This will al

16、low 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 rear wheels do not track,and(3)added width because of the slanted position of the front of the vehicle to the roadway center.For 24-ft roadways,

17、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 narrower pavements,however,widening assumes importance even on fairly flat curves.Recommended amounts of and procedures for curve widening are given in Geom

18、etric Design for Highways.B Grades The vertical alignment of the roadway and its effect on the safe and economical operation of the motor vehicle constitute one of the most important features of road design.The vertical alignment,which consists of a series of straight lines connected by vertical par

19、abolic 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,”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

20、 in grade on the center profile.In the establishment of a grade,an ideal situation is one in which the cut is balanced against 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 terrai

21、n and rise and fall in the direction of the existing drainage.In mountainous country the grade may be set to balance excavation 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

22、 surface drainage and,where necessary,to permit the wind to clear drifting snow.Where the road approaches or follows along 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 s

23、hort straight sections connected with short vertical curves.Changes of grade from plus to minus should be placed in 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 p

24、roducing depressions contrary to the general existing contours of the land.Other considerations for determining 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 d

25、o rural projects.It is often best to adjust the grade to meet existing conditions because of the additional expense 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 i

26、n gasoline consumption and a reduction in speed are apparent when grades are increase in gasoline consumption 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 op

27、eration without grade reduction.An accurate solution to the problem depends on the knowledge of traffic volume 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 spee

28、d and topography.Present practice limits grades to 5 percent of a design speed of 70 mph.For a design speed 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 o

29、f grade without the provision of climbing lanes for slow-moving vehicles.Critical 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 on any particular section of a highway.It is often preferred to break t

30、he long sustained uniform grade by placing steeper grades at the bottom and lightening the grade 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 import

31、ance 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 0.35%.C Sight Distance For safe vehicle operation,highway must be designed to give drivers a sufficient distance or clear version ahead so

32、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 facets:“stopping”(or“no passing”)and“passing”.At times large objects may drop into a roadwa

33、y 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 become visible at distances great enough that drivers can stop before hitting

34、 them.Further more,it is unsafe to assume that one oncoming 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 th

35、e obstruction comes 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

36、 the vehicle and the perception time and brake-reaction time of the operator.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 prov

37、ided 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 called the passing sight distance.In deciding whether or not to pass anothe

38、r 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 v

39、ehicle.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 obser

40、vations on which passing sight distance standards are based were made during the period 1938-1941.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.Whe

41、n 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 accomplished under what may be termed a delayed start and a hurried return in the face of opposing traffic.The passing vehicle accelerates

42、 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 oncoming vehicle in the other lane.The five distances,in sum,make up

43、passing sight distance.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

44、采用短曲线，则会出现“扭结”。另外，线路的平、纵断面设计应综合考虑，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

45、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。

46、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

47、之前应决定是否采用缓和曲线。一般曲线的起点标为 PC 或 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

48、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

49、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

50、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