Sight Distance
For safe vehicle operation, highway must be designed to give drivers a sufficient distance of clear vision ahead so 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 "nonpassing ") and“passing".
At times large objects may drop onto 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 cither instance, proper design requires that such hazards become visible at distances great enough that drivers can stop before hitting them. Furthermore, it is unsafe to unsafe to assume that one oncoming vehicle may avoid trouble by leaving the in is traveling 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 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 brake-reaction time of the operator. The second distance depends on the speed of the vehicle; the condition of brakes, tires, 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 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 drivers. To establish design values for passing sight distances observed the passing practices of many drivers. Basic observations on which passing sight distance standards are based were made during the period 1938-1941. assumed operating conditions are as follows:
视距
为车辆安全运行，公路的设计必须让司机有足够的距离，清晰的未来远景，使他们能避免意想不到的障碍和可以通过较慢的车辆没有危险。视线距离的长短，公路有形提前给司机的车辆。的概念，安全视距有两个层面： “停车” （或“ 禁止通行 ” ）和“过关” 。
在时代的大型物体可能下降到一个巷道，并会做严重损害
汽车罢工。再次汽车或卡车可能被迫停止在该车道，在道路下面的车辆。再次举例来说，妥善的设计要求，这种危险变成可见的距灶枝离足够大的司机，可以停止之前，打他们。此外孙毁，这是不安全的不安全假设一迎面驶来的车辆可避免麻烦，离开是在旅行的结果，在失去控制或碰撞与另一辆车。
停车视距是成立的两个要素。首先是距离后，阻塞生效的看法，但前司机适用于他的刹车。在此期间，知觉和反应，汽车
旅游在其初始速度。第二个是消费的距离，而司机刹车的车辆停止。第一，这两个距离是依赖于速度的车辆和看法，时间和制动隐凯敏反应时间的经营者。第二次的距离依赖于速度的车辆;条件的制动器，轮胎，和巷道表面;路线和等级公路。
对双车道公路，有机会通过缓慢移动的车辆必须在规定的时间间隔。否则的能力降低，意外增加的风险，急不可待的司机头部碰撞就通过时，这是不安全的这样做。最小距离，未来必须明确允许安全通过，是所谓通过视距。
在决定是否或不通过的另一辆车，司机必须权衡清楚的距离，向他提供对距离须进行事件的顺序弥补通过演习。其中的因素会影响他的决定是谨慎的程度，他演习和加速的能力，他的汽车，因为人类明显不同，通过做法，这取决于司机。建立设计值，通过视线的距离，观察通过的做法，很多司机。基本的意见，其中通过视距标准是根据发了言期间， 1938年至1941年。假设操作条件如下：