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早期的公路的平面线形主要是由直线和圆弧构成,圆弧和直线相切。这样的线形满足了车辆的直行和转向要求,但在直线和圆弧相切处却出现了曲率的不连续。在公路建设中,直线是最常用的线形要素之一。直线对于规划和设计人员,往往比对公路的使用者更具有有利的特殊性质(人们普遍认为直线易于精确地进行设计、计算和中线敷设)。平时用肉眼或望远镜能通视的地方,很容易自然而然地确定为直线。
Early linear plane of the road is mainly composed of straight lines and arcs, arcs and straight lines tangent. Such a line to meet the vehicle straight and steering requirements, but the straight line and arc tangent at the curvature of the discontinuity. In highway construction, straight lines are one of the most commonly used linear elements. Straight lines tend to be more advantageous and special for planners and designers than road users (it is generally accepted that straight lines are easy to precisely design, calculate, and center lay). Usually with the naked eye or telescope can pass the place, it is easy to naturally determine a straight line.