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本文档由会员 wanli1988go 发布
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腀@骋蛔畲笾担庑┛刂浦刀杂谀骋还娑ㄉ杓瞥邓倏赡懿捎们叩那首髁讼拗啤ＭǔＧ榭鱿拢骋辉睬叩那适怯善浒刖独刺逑值摹６杂谙咝紊杓贫裕食３Ｍü壤疵枋觯?00ft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
The 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 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 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 two-lane roads because some drivers hesitate to pass on curves. 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 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 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 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 some distance beyond the point of curve.
