【正文】 ength of grade without the provision of climbing lanes for slowmoving 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 the 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 importance 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 %. C Sight Distance For safe vehicle operation, highway must be designed to give drivers a sufficient distance or clear version 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 “no passing”) and “passing”. At times large objects may drop into 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 dither instance, proper design requires that such hazards bee visible at distances great enough that drivers can stop before hitting them. Further more, it is unsafe to assume that one oning 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 the obstruction es 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 brakereaction time of the operator. The second distance depends on the speed of the vehicle。 Geometric Design of Highways A Alignment Design 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 mon 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。為了確立超車視距值，工程人員觀察了許多駕駛員的超車行為。 停車視距由峽谷部分組成：第一部分是當駕駛員發(fā)現(xiàn)障礙物面作出制動之前駛過的一段距離，在這一察覺與反應階段，車輛以其初始速度行駛；第二部分是駕駛員剎車后車輛所駛過的一段距離。這種情況下， AASHTO 建議最小坡度為%。這個問題的準確方法取決于對交通流量和交通類型的了解，這只有通過交通調(diào)查方能獲知。無論在何種情況下，平緩連續(xù)的坡度線要比由短直線段連接短豎曲線構(gòu)成的不斷變向的坡度線好得多。然而，對于較窄的路面，即使是在較平緩的曲線路段上，加寬也是很重要推薦加寬值及加寬設計見《公路線形設計》 B縱坡線 公路的豎向線形及其對車輛運行的安全性和經(jīng)濟性的影響構(gòu)成了公路設計中最重要的要素之一。設計合理的緩和曲線使得上述迂迴沒有必要。當汽車到達曲線路段時，離心力突然產(chǎn)生，迫使乘客向外傾斜，為了維持平衡，乘客必須迫使自己的身體向內(nèi)側(cè)傾斜。另外，線路的平、縱斷面設計應綜合考慮，而不應只顧其一，不顧其二，例如，當平曲線的起點位于豎曲線的頂點附近時將會產(chǎn)生嚴重的交通事故?，F(xiàn)代設計時常在直線與圓曲線之間插入緩和曲線。然而，雙向道路線形全由曲線構(gòu)成也是不理想的，因為一些駕駛員通過曲線路段時 總是猶豫。通常的做法是維持道路每一條中線設計標高不變，通過抬高