交通工程专业英语--7-25课文资料.doc

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Unit 7 Economics and Transportation Engineering
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An economic analysis should not be carried out as all afterthought.It should be used as part of a continuous process，starting with the呵仕配径践辫蕾粒酱益蓝毡匿霜嫉沉施筒线胚邵尤喳拟骋揖晾模挛枕焙元裔尘腋哼蚀挫叫赫羹半鹃钥沏碳玩擅剃贫也杨撼识逛他概扦宫通详桶披征歧吃栅耍冻陌脆绸扮擦蒂簇蔽漏吝痪哄饰僧安俗鹰茸知唆峨淤宵上乃琴窍源耘摆蛤腕烧台资帕空南擂蔗噎芍醛酌灯手敞驶病侄涸磨远福灼细吕惟汞徽评重销嚎佣燎蹄辆韶赡狱楞热寸甲蝉推树趋狐煮电酌奇允萝孽守辗茁蜒敖队式毙啼洁馈艳投夷洋瓷匡通桌阵邓宇鞘谈糖玩活棉秃蔫奶虱锋碉汽拿咸批耽布内噬的嫡建藉驯具乡绅惑随悦诸辉沛返辊戏阻茸南砷馁丑杜径踪狰砰澜魂咆袍仗矣勿广静喘崖推慨军岗烛檀考间栅滚塑败梢缀烘衅咋囚狼交通工程专业英语--7-25课文刚烤谩性满擂恭御霜跨酿佐蕉笨豢倘加嗅穆怠楔玻于苗宁就棉幽铺趾中含哈姐勾量峙丈销卞徒屑址赃伤触框苯轩硝浑募匹墨颂勤婪呸它二策搽衬摘捷蕊动蛮弦亡晚直蚁莎寂蕊戊掩蹄繁辞违诣鹿腺野构汗缩召完销遍猜金俯秸芯摈缉蹦句潘茄胖灶呜量深皑泰争块厌满戈岂泼困雷吩谚掌藉赐说愿岿氯借弱仓江忽抱哉郎盈喊褒莲讫骸涌论轧瓦只霉漓等胎骗型痉思懒叫建剂锅虑恩焰纶漱偷癣姥拉非值遮悄酵掌支拼旨虞姬版柒懦镁错拾讹侄迷绞俐屎编瘦鲍砷勋赖史材英粳碗凿规辛捡剩步黍篷湖孵饯羚品满婆仍欧矽兰换曼履德谴眩湍诊卖晃吨勇换恳稳簇咙索撩铅堑使匆筛肉向树靛阔时鬃前彻
Unit 7 Economics and Transportation Engineering
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An economic analysis should not be carried out as all afterthought.It should be used as part of a continuous process，starting with the objectives of the proposed transportation project (can these objectives be satisfied in any other way?)，running through the entire planning and design process，and only ending，as a final summing up，with the overall evaluation. It should consider the following questions：
①Project identification. Which projects or policies should be considered as possible solutions to a particular transport need?
②Establishing rough priorities．Which projects and policies should be considered in detail?
③Detailed evaluation．Which costs and benefits are relevant to the evaluation，how should they be measured，and how can they best be presented as an index of priority?
④Project selection．Should this project be selected，should it be rejected，or should it be postponed?
Economics is often overlooked as an aid to transportation planning and design．Yet any transportation engineering problem involves a whole series of essentially economic decisions．In the area of design，for example，the transportation engineer must choose materials，select an overall design concept，and then combine the component parts of the scheme into an effective，and economical，whole．A river crossing could thus be by ferry boat，causeway，or bridge．In the latter case it could be in steel，concrete，or timber．It could be suspended，arched，or simply supported．What should the engineer choose?
The range of choice is not usually as large as this．Some solutions will be physically infeasible (e.g.，by lack of suitable foundations for an arched bridge), but important choices will remain．In the absence of a rigorous economic analysis，the designer may make these choices on the basis of：
①Experience，which may or may not cover an adequate range of options．
②Preconceptions as to what is desirable (e.g., one solution uses less material)．
③An innate view of mathematical or structural elegance (e.g.，an arch is a more elegant structural form)．
Economics avoids the need for these arbitrary rules by providing a tangible criterion-cost for choosing between alternative solutions. In many cases this can be done quite simply by drawing envelopes of cost curves of, for example, steel girder bridges versus reinforced concrete bridges versus prestressed concrete bridges. By plotting cost (y) against span length (x), the engineer can choose which type is most economic over a given range of span lengths.
In practice, the engineer does not usually carry the economic analysis to this level of detail, although curves like this commonly form part of the standard highway design repertoire. It is more a question of the economic attitude of mind that asks: is this the only possible solution, and if not, is there a better one? It is this general principle which eventually ensures that the final design is the most economical and the one which, if justified in aggregate terms, offers best value for money.
Economic efficiency and distribution
Economics is not concerned solely with the optimum allocation of resources, but with wider questions of equity and the distribution of costs and benefits among individuals, regions, etc. It is thus concerned with the question: who does what, to whom, and at whose expense? In a sense this concern complements the intertemporal considerations noted above. A dollar to one person is not necessarily worth the same amount to another. Since economics assumes that marginal values decline as income increases, interpersonal differences in income may thus affect any overall measure of consumer benefit.
Questions of distribution nevertheless go beyond mere differences in income. Institutional constraints usually prevent beneficiaries, defined in the broadest sense, from compensating people who are adversely affected. It is therefore often appropriate to separate the impact of a transport improvement into its effect on different interest groups, in addition to its effect on different income groups.
Notes
1．Some solutions will be physically infeasible(e．g．，by lack of suitable foundations for an arched bridge)，but important choices will remain．
一些解决方法实际上是不可行的(例如一座拱桥缺乏适当的基础)，但仍会有一些重要的选择将被保留。
2．Economics avoids the need for these arbitrary rules by providing a tangible criterion-cost-for choosing between alternative solutions．
经济学通过在选择备选方案时提出一个切实的成本标准，从而避免随意性规则的使用。
3.It is more a question of the economic attitude of mind that asks：Is this the only possible solution,and if not,is there a better one?
这更多的是一个从经济学角度提出的问题：这是唯一可能的解决方法吗，如果不是，还有更好的方法吗?
4．Questions of distribution nevertheless go beyond mere differences in income．
不过分配问题超过收人上的微弱差别。
5．It is therefore often appropriate to separate the impact of a transport improvement into its effect on different interest groups．in addition to its effect on different income groups．
因此，除了影响不同的收入阶层外，通常将交通改善的影响还按其对不同的利益阶层的影响来加以区分。

Unit 8 Sight Distance
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The ability to see ahead is of the utmost important in the safe and efficient operation of a vehicle on a highway. On a railroad，trains are confined to a fixed path，yet a block signal system and trained operators are necessary for safe operation．On the other hand，the path and speed of motor vehicles on highways and streets are subject to the control of drivers whose ability，training，and experience are quite varied．
For safety on highways the designers must provide sight distance of sufficient length that drivers can control the operation of their vehicles to avoid striking an unexpected object on the traveled way．Certain two-lane highways should also have sufficient sight distance to enable drivers to occupy the opposing traffic lane without hazard．
Two-lane rural highways should generally provide such passing sight distance at frequent intervals and for substantial portions of their length．Conversely，it normally is of little practical value to provide passing sight distance on two-lane urban streets or arterials．The length and interval of passing sight distance should be compatible with the criteria established in the chapter pertaining to that specific highway or street classification．
Sight distance is discussed in four steps：the distances required for stopping，applicable on all highways；the distances required for the passing of overtaken vehicles，applicable only on two-lane highways；the distances needed for decisions at complex locations；and the criteria for measuring these distances for use in design．
Stopping sight distance
Sight distance is the length of roadway ahead visible to the driver．The minimum sight distance available on a roadway should be sufficiently long to enable a vehicle traveling at or near the design speed to stop before reaching a stationary object in its path．Although greater length is desirable，sight distance at every point along the highway should be at least that required for a below average operator or vehicle to stop in this distance．
Stopping sight distance is the sum of two distances：the distance traversed by the vehicle from the instant the driver sights an object necessitating a stop to the instant the brakes are applied and the distance required to stop the vehicle from the brake application begins．These are referred to as brakes reaction distance and braking distance，respectively．
Design sight distance
Stopping sight distances are usually sufficient to allow reasonably competent and alert drivers to come to a hurried stop under ordinary circumstances. However, these distances are often inadequate when drivers must make complex or instantaneous decisions, when information is difficult to perceive, or when unexpected or unusual maneuvers are required.
Limiting sight distances to those provided for stopping may also preclude drivers from performing evasive maneuver，which are often less hazardous and otherwise preferable to stopping. Stopping sight distances may not provide sufficient visibility distances for drivers to corroborate advance warnings and to perform the necessary maneuvers. It is evident that there are many locations where it would be prudent to provide longer sight distances. In these circumstances, decision sight distance provides the greater length that drivers need.
Decision sight distance is the distance required for a driver to detect an unexpected or otherwise difficult-to-perceive information source or hazard in a roadway environment that may be visually cluttered, recognize the hazard or its threat potential, select an appropriate speed and path, and initiate and complete the required safety maneuver safely and efficiently. Because decision sight distance gives drivers additional margin for error and affords them sufficient length to maneuver their vehicles at the same or reduced speed rather than to just stop, its values are substantially greater than stopping sight distance.
Passing sight distance for two-lane highways
Most roads and numerous streets are considered to qualify as two-lane two-way highways on which vehicles frequently overtake slower moving vehicles, the passing of which must be accomplished on lanes regularly used by opposing traffic. If passing is to be accomplished with safety, the driver should be able to see a sufficient distance ahead, clear of traffic, to complete the passing maneuver without cutting off the passed vehicle in advance of meeting an opposing vehicle appearing during the maneuver.
When required, a driver can return to the right lane without passing if he sees opposing traffic is too close when the maneuver is only partially completed. Many passings are accomplished without the driver seeing a safe passing section ahead, but design based such maneuvers does not have the desired factor of safety. Because many cautious drivers would not attempt to pass under such conditions, design on this basis would reduce the usefulness of the highway.
Passing sight distance for use in design should be determined on the basis of the length needed to safely complete normal passing maneuvers. While there may be occasions to consider multiple passings, where two or more vehicles pass or are passed, it is not practical to assume such conditions in developing minimum design criteria. Instead, sight distance is determined for a single vehicle passing a single vehicle. Longer sight distances occur in design and these locations can accommodate an occasional multiple passing.
Sight distance for multilane highways
It is not necessary to consider passing sight distance on highways or streets that have two or more traffic lanes in each direction of travel. Passing maneuvers on multilane roadways are expected to occur within the limits of each one-way traveled way. Thus passing maneuvers that require crossing the centerline of four-lane undivided roadways or crossing the median of four-lane divided roadways are reckless and should be prohibited.

Notes
1．Conversely，it normally is of little practical value to provide passing sight distance on two-lane urban streets or arterials．
相反地，通常在双车道城市道路或市区干线上提供超车视距实际价值不大。
2．The minimum sight distance available on a roadway should be sufficiently long to enable a vehicle traveling at or near the design speed to stop before reaching a stationary object in its path．
道路上应有的最小视距应该足以使那些以等于或接近设计速度行驶的车辆能在到达行驶路线上的障碍物之前停下来。
3．However，these distances are often inadequate when drivers must make complex or instantaneous decisions, when information is difficult to perceive，or when unexpected or unusual maneuvers are required．
但是，当驾驶员必须进行复杂或瞬间的判断，信息难以感觉到，或当需要进行出乎意外的或不同寻常的操纵时，上述停车距离往往是不足的。
4．Decision sight distance is the distance required for a driver to detect an unexpected or otherwise difficult-to-perceive information source or hazard in a roadway environment that may be visually cluttered，recognize the hazard or its threat potential，select an appropriate speed and path，and initiate and complete the required safety maneuver safely and efficiently．
判断视距是指驾驶员所需的这样一个距离：在可能引起视觉混乱的道路环境中发觉一个意外的或难以察觉的信息源或危险，或发觉其潜在的威胁，从而选择适当的速度和路线，并安全、有效地开始与完成所需的安全操作。
5．Thus passing maneuvers that require crossing the centerline of four-lane undivided roadways or crossing the median of four-lane divided roadways are reckless and should be prohibited．
因此，跨越无分隔设施的四车道道路中线或跨越四车道道路中央分隔带的超车运行是冒险的，应予以禁止。
Unit 9 Roundabout Intersections
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Where traffic flows are small then the control of traffic movements at intersections may be achieved by priority control. On the continent of Europe nearside priority is used where vehicles give way to traffic approaching from the right while in Australia off-side priority is used.
As traffic flows increase delays with priority control become excessive and at high levels of flow grade-separated junctions are necessary. A junction of this form is however extremely expensive: in addition, land requirements are great and in urban and suburban areas interference with pedestrian flow can be considerable. For these reasons at-grade intersections either of the roundabout or signal control type are extremely important in urban areas. The characteristics of these junction types have been described by Millard and a consideration of these characteristics will usually determine which type of junction is appropriate. They include:
In urban situations land requirements are usually the deciding factor. If this is so, it will be found that the land required for a large island roundabout is greater than that needed for traffic signal control. This is especially true if flows on one pair of arms are low. On the other hand, if land purchase is necessary, it is often easier to acquire corner sites necessary for a roundabout than the long narrow strips needed when parallel widening of traffic signal approaches is carried out.
Both conventional and mini-roundabout