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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 either 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 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 ① .
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 highway， 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 ② . 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. Passing sight distance standards are established based on basic observations which 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）When 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 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 passing sight distance.
sight distance 视距
operation n. 操作，作用
facet n. 面，某一方面
stopping n. 停车
nonpassing 禁止超车，停车
passing a. 超车
strike v. 撞击，冲击
hazard n. 危险
oncoming a. 迎面而来，接近
collision n. 碰撞，冲突
obstruction n. 阻挡，阻碍
apply v. 应用，使用，适用
brake n. 制动器，刹车； v. 阻碍
perception n. 感觉
initial a.＆ v.＆ n. 最初，开始
velocity n. 速度
consume v. 消耗
alignment n. （道路）线形
grade n. 坡度
carry out 进行，实现，执行
sequence n. 连续
event n. 事件，事情
make up 组成
maneuver n.＆ v. （调动车辆的）机动动作
accelerate v.＆ n. 加速，促进
markedly ad. 显著地
mechanics n. ［复］力学
establish v. 建立，规定，安置
overtake v. 追上，超过
trail v. ＆ n. 跟踪，痕迹
perceive v. 察觉，看见
accomplish v. 完成
delay v. 延误，推迟
hurry n.＆ v. 匆忙
in the face of 面对
occupancy n. 占有，占用
clearance n. 净空
①Furthermore， it is unsafe to assume that the 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.而且，认为迎面（对着障碍）开来的车辆以离开它所行驶的车道来避开障碍是不安全的，因为这可能会失去控制或与其他车辆相撞。
②Among the factors that will influence his decision are the degree of caution that he exercises and the accelerating ability of his vehicle.影响驾驶员决策的因素有二：一是他开车的小心程度；二是车辆的加速性能。
本句主语是degree和ability，are是谓语动词，that will influence his decision是定语从句，其先行词是factors，that he exercises也是定语从句，其先行词是degree。