城市交通规划外文翻译文献

城市交通规划外文翻译文献
(文档含中英文对照即英文原文和中文翻译)
Urban transportation Planning
An urban transportation system is basic component of an urban area's social，economic，and physical structure. Not only does the design and performance of a transportation system provide opportunities for mobility，but over the long term，it influences patterns of growth and the level of economic activity through the accessibility it provides to land. Planning for the development or maintenance of the urban transportation system is thus an important activity，both for promoting the efficient movement of people and
goods in an urban area and for maintaining the strong supportive role that transportation can play in attaining other community objectives.
There are several basic concepts about an urban transportation system that should be kept in mind. Most important，a transportation system in an urban area is defined as consisting of the facilities and services that allow travel throughout the region，providing opportunities for:(I)mobility to residents of an urban area and movement of goods and (2) accessibility to land .Given this definition，an urban transportation system can be further characterized by three major components: the spatial configuration that permits travel from one location to another; the transportation technologies that provide the means of moving over these distances; and the institutional framework that provides for the planning, construction, operation, and maintenance of system facilities.
The Spatial Configuration of a Transportation System
One way to describe the spatial dimension of an urban transportation system is to consider the characteristics of individual trips from an origin to a destination. For example, a trip can consist of several types of movement undertaken to achieve different objectives. Travelers leaving home might use a local bus system to reach a suburban subway station(a trip collection process)，proceed through the station to the subway platform (a transfer process)，ride the subway to a downtown station (a line-haul process)，and walk to a place of employment (a distribution process). Similarly，one can view a home-to-work
trip by car as consisting of similar segments，with the local street system providing the trip collection process, a freeway providing the line-haul capability，a parking lot in the central business district serving as a transfer point，and walking，as before，serving the distribution function.
The facilities and services that provide these opportunities for travel，when interconnected to permit movement from one location to another，form a network. Thus，another way of representing the spatial dimension of an urban transportation system is as a set of road and transit networks. Even in the smallest urban areas，where mass transit is not available，the local street network provides the basic spatial characteristic of the transportation system.
The transportation system of a city can influence the way in which the city's social and economic structure, often called the urban activity system，develops. At the same time，changes in this structure can affect the ability of the transportation system to provide mobility and accessibility. Thus , the transportation system is closely related to the urban activity system and; historically, has been an important determinant of urban form.
Because of the relation between transportation and urban activities，many of the methods used by transportation planners depend on estimates of trips generated by specific land uses. The relation also suggests that the options available to public officials dealing with transportation problems should include not only those related directly to the transportation system, but also actions such as zoning that affect the distribution of land use, and thus influence the
performance of the transportation system.
The foregoing considerations point to two important principles for transportation planning: The transportation system should be
Considered as an integral part of the social and economic system in an urban area.
Viewed as a set of interconnected facilities and services designed to provide opportunities for travel from one location to another.
The Technology of Urban Transportation
The technology of urban transportation is closely related to the spatial configuration of the transportation system in that the design transportation networks reflects the speed, operating , and cost characteristics of the vehicle or mode of transportation being used. Technology includes the means of propulsion, type of support，means of guidance，and control technique.
The development and widespread use of electric streetcars in urban areas during the late nineteenth century was a technological innovation that initiated the transformation of most North American cities. The advent of the electric streetcar permitted urban areas to expand beyond the boundaries that had been dictated by previous transportation technologies (e. g.，walking，horse，horsecar)，spawning `streetcar suburbs' with dramatically lower residential densities along streetcar lines radiating from the central city. Whereas many industries had decentralized along railroad lines leading from the central city，
and workers initially had to live near these factories, the introduction of streetcars now permitted more distant living.
The success of the streetcar in providing access from selected suburban areas to central business districts was followed by public acceptance of a second major technological innovation-the automobile，powered by the internal combustion engine. Increasing consumer preferences for lower-density living and for an ability to travel beyond established urban boundaries sparked a phenomenal growth in automobile ownership and usage，beginning in the 1920s .
④The automobile continues and accelerated the evolution of urban structure started by the electric streetcar. Its availability permitted further expansion of urban areas and, more important, provided access to land between the radial streetcar and railroad lines leading into the central city.
The technology of the internal-combustion engine，however, also led to the decline of other transportation modes used in urban areas by providing a less expensive and more flexible replacement for rail-based modes. While the automobile provided new opportunities for personal mobility and urban growth, motor buses rapidly replaced electric streetcars, to the extent that only five North American cities today still operate large-scale streetcar systems-Boston, Philadelphia, Pittsburgh, Toronto, and San Francisco (although this trend has reversed somewhat in recent years with new `light rail' systems in operation in Edmonton, Calgary, San Diego, and Buffalo). At the same time, the growth of private automobile use has dramatically reduced the use of public transportation
in general, particularly since the end of World War II. According to the latest census figures, in 1980, 62. 3 million Americans normally drove alone to work each day, another 19 million car-pooled, and 6 million used public transportation.
The technologies and the resulting modes available today for urban transportation are common to most cities but are often applied in different ways to serve different purposes. It should be noted that certain types of modes are appropriate than others in serving different types of urban trips.
The technological dimension of the urban transportation system suggests a third principle for urban transportation planning:
Transportation planners must consider the transportation system as consisting of different modes , each having different operational and cost characteristics.
From; Michael D. Meyer and Eric J. Miller "Urban Transportation Planning", 1984
城市交通规划
城市交通系统是市区的社会、经济、和物质结构的一个基本组成部分。

一个交通系统的设计和实施不仅为流动性提供机会，并且从长远观点来看，
通过它能对土地提供良好使用价值也使经济活动和发展受到益处。

这样，为了发展和维护城市交通系统而制定的规划是一项重要的活动，既是为了促进人和货物在市区的有效运转，同时也是为了保持交通在实现其他社团目标方而所能起到的强有力的支援作用。

对于城市交通系统有几个基本概念是应该记住的。

最重要的是,一个城市的交通系统被认为是包括交通设施和服务,这两者有助于贯穿全区的出行，并且为以下两方面提供机会:(1)居民的流通量和商品的运转，(2)对于土地的可达性。

鉴于这种认识,城市交通系统可以进一步分解为以下三个部分:空间布置,可使一点到另一点的出行成为可能；交通技术,提供两地区运转的手段；机构的机制，提供交通系统设施的规划、建设、运营和维护。

交通系统的空间布置
描述一个城市交通系统的空间尺度的方式是考虑一个人由起点到目的地的出行特点。

例如，出行可以包括为达到不同目的的几种类型的流动。

离开家的出行者可以乘坐当地的公共汽车而到达另一个郊区地铁车站（出行集合过程），经车站转到地铁站台（换乘过程），乘地铁到达一个商业车站（沿线运行过程），然后步行到工作地点（分散过程）。

相似的，人们可以把乘汽车由家到工作地点的出行看作是包括相似的过程，利用当地的街道系统实现出行集合过程，高速公路提供线路的出行能力，在商业中心区的停车场起到换乘点的作用，而步行与前面说的一样，起到分散作用。

当提供这些出行机会的公共设施和服务集合起来,使从一个地点到另一个地点的运动成为可能时就会形成网络。

这样，表示一个城市交通系统
的空间尺度的另一种方式是一组道路和公共交通的网络系统。

甚至在不能利用公共交通的很小的市区内，当地的街道网络也会提供交通系统的基本空间特征。

城市交通系统将会影响到城市的社会和经济结构（通常被称为城市活动系统）的发展方式。

同时,这种结构的变化会影响交通系统提供流动性和可达性的能力。

因此,交通系统是与城市活动系统密切相关的;从历史上看,城市交通系统曾经是一个决定城市形态的重要因素。

由于交通和城市活动之间的关系，许多交通规划人员所使用的方法取决于特定的土地利用所产生的出行评估。

这个关系还暗示可供与交通问题打交道的政府官员多采用的选择方案。

而且也应该包括，例如，影响土地利用分布的区域划分，并因此影响到交通系统特性的一些措施。

上述的考虑指出用于交通规划的两个重要原则：
被认为是市区社会和经济系统的一个完整部分。

被视为目的在于提供从一个地点到另一个地点出行机会的一套互相结合的交通设施和服务。

城市交通的技术
城市交通技术是同交通系统的空间布局紧密相关的，其中交通网设计反映车辆的速度、运行和费用特征所采用的交通模式。

技术上包括推进的手段。

支撑的类型引导的手段，以及控制技术。

十九世纪晚期，在市区发展和广泛使用有轨电车是一项技术创新，启动了北美大多数城市的转型。

有轨电车的出现使城市地区超出了之前运输
技术（例如走路、骑马、马车）所限定的界限进行了扩展。

产生了大量的居住密度明显很低的位于由市中心辐射出去的电车电车路线沿线的“电车郊区”。

与此同时，许多工业也从由市中心延伸出去的提路沿线疏散了，工人们起初需要在这些工厂附近居住，现在引进了电车，住的远也没有什么关系了。

在电车提供由经过挑选的郊区到中心商业区的道路取得成功后，接着是公众对于第二个重大技术革新，即对内燃机为动力的汽车的欢迎。

消费者越来越多的对于低密度的居住生活的热爱，以及对于跨越已确定的城市边界出行能力的偏爱，促进了从二十年代开始的汽车购置和使用方便急剧增长。

汽车的可用性促使市区进一步扩展，并且更重要的是，汽车为通向市中心的放射状电车线和铁路路线之间地区的可能性提供条件。

然而，由于内燃机技术提供了比有轨交通方式更为廉价的和更为灵活的汽车，从而导致了市区内其他交通方式的衰落。

在汽车为个人流通和和城市发展提供了新机会的同时，公共汽车很快取代了电车，以致目前只有五个北美城市还在使用大型电车系统，即波士顿、费城、匹茨堡、多伦多和旧金山（不过近年来这种趋势又有点逆转，新型“轻轨”系统正在埃德蒙顿、卡尔加里、圣地亚哥和布法罗运营）。

于此同时，特别是自第二次世界大战结束以来，私人汽车使用的增长，一般来说已显著的使公共交通的使用缩减。

根据最近的统计数字，在1980年有六千二百三十万美国人每天是自己驾驶汽车上班，另外有一千九百万人合伙使用汽车，而有六百万人乘用公共交通。

当前可以应用的城市交通技术和交通模式，对于大多数城市来说是通
用的，但也常常以不同的方式应用，为不同的目的服务。

应当注意到，某些交通方式较其他方式更适合于承担不同类型的城市出行。

城市交通系统的技术方面提出一个用于城市交通规划的第三个原则：交通规划人员应当认为交通系统包括不同的交通方式，每一个方式具有不同的运营和费用特征。

摘自：迈克尔 D.梅耶和艾瑞克J.米勒《城市交通规划》，1984
Traffic signals
In the United States alone ,some 250,000 intersections have traffic signals , which are defined as all power-operated traffic-control devices except flashers，signs，and markings for directing or warning motorists, cyclists，or pedestrians.
Signals for vehicular，bicycle，and pedestrian control are ‘pretimed’where specific times intervals are allocated to the various traffic movements and as 'traffic actuated' where time intervals are controlled in whole or in part by traffic demand.
Pretimed Traffic Signals
'Pretimed' traffic signals are set to repeat regularly a given sequence of signal indications for stipulated time intervals through the 24-hr day. They have the advantages of having controllors of lower first cost and that they can be interconnected and coordinated to vehicles to move through a series of intersections with a minimum of stops and other delays. Also, their operation is unaffected by conditions brought on by unusual vehicle behavior such as forced stops，which，with some traffic-actuated signal installations may bring a traffic jam. Their disadvantage is that they cannot adjust to short-time variations in traffic flow and often hold vehicles from one direction when there is no traffic in the other. This results in inconvenience, and sometimes a decrease in capacity.
‘Cycle length’the time required for a complete sequence of indications, ordinarily falls between 30 and 120s. Short cycle lengths are to be preferred, as the delay to standing vehicles is reduced. With short cycles, however a relatively high percentage of the total time is consumed in clearing the intersection and starting each succeeding movement. As cycle length increases, the percentage of time lost from these causes decreases. With high volumes of traffic, it may be necessary to increase the cycle length to gain added capacity.
Each traffic lane of a normal signalized intersection can pass roughly one vehicle each 2.1s of green light. The yellow (caution) interval following each green period is usually between 3 and 6s，depending on street width，the needs of pedestrians, and vehicle approach
speed. To determine an approximate cycle division, it is common practice to make short traffic counts during the peak period. Simple computations give the number of vehicles to be accommodated during each signal indication and the minimum green time required to pass them. With modern control equipment, it is possible to change the cycle length and division several times a day, or go to flashing indications to fit the traffic pattern better.
At many intersections，signals must be timed to accommodate pedestrian movements. The Manual recommends that the minimum total time allowed be an initial interval of 4 to 7s for pedestrians to start plus walking time computed at 4 ft/s (1. 2m/s). With separate pedestrian indicators，the WALK indication(lunar white) covers the first of these intervals, and flashing DON'T WALK (Portland orange ) the remainder. The WALK signal flashes when there are possible conflicts with vehicles and is steady when there are none. Steady DON'T WALK tells the pedestrian not to proceed.
If pedestrian control is solely by the vehicle signals，problems develop if the intersection is wide, since the yellow clearance interval will have to be considerably longer than the 3 to 5s needed by vehicles. This will reduce intersection capacity and may call for a longer cycle time. On wide streets having a median at least 6 ft (1. 8m)wide，pedestrians may be stopped there. A separate pedestrian signal activator must be placed on this median if pedestrian push buttons are incorporated into the overall control system.
Coordinated Movement
Fixed-time traffic signals along a street or within an area usually are coordinated to permit compact groups of vehicles called `platoons’to move along together without stopping. Under normal traffic volumes，properly coordinated signals at intervals variously estimated from 2500 ft (0. 76km)to more than a mile (1. 6km) are very effective in producing a smooth flow of traffic. On the other hand，when a street is loaded to capacity，coordination of signals is generally ineffective in producing smooth traffic flow.
Four systems of coordination-simultaneous, alternate，limited progressive, and flexible progressive-have developed over time. The simultaneous system made all color indications on a given street alike at the same time .It produced high vehicle speeds between stops but low overall speed. Because of this and other faults，it is seldom used today.
The alternate system has all signals change their indication at the same time，but adjacent
signals or adjacent groups of signals on a given street show opposite colors. The alternate system works fairly well on a single street that has approximately equal block spacing. It also has been effective for controlling traffic in business districts several blocks on a said, but only when block lengths are approximately equal in both directions. With an areawide alternate system，green and red indications must be of approximately equal length. This cycle division is satisfactory where two major streets intersect but gives too much green time to minor streets crossing major arteries. Other criticisms are that at heavy traffic volumes the later section of the platoon of vehicles is forced to make additional stops，and that adjustments to changing traffic conditions are difficult.
The simple progressive system retains a common cycle length but provides 'go' indications separately at each intersection to match traffic progression. This permits continuous or nearly continuous flow of vehicle groups at a planned speed in at least one direction and discourages speeding between signals. Flashing lights may be substituted for normal signal indications when traffic becomes light.
The flexible progressive system has a master controller mechanism that directs the controllers for the individual signals. This arrangement not only gives positive coordination between signals，but also makes predetermined changes in cycle length，cycle split，and offsets at intervals during the day. For example，the cycle length of the entire system can be lengthened at peak hours to increase capacity and shortened at other times to decrease delays.
Flashing indications can be substituted when normal signal control is not needed. Also the offsets in the timing of successive signals can be adjusted to favor heavy traffic movements, such as inbound in the morning and outbound in the evening. Again，changes in cycle division at particular intersections can be made. The traffic responsive system is an advanced flexible progressive system with the capacity to adjust signal settings to measured traffic volumes.
Where traffic on heavy-volume or high-speed arteries must be interrupted for relatively light cross traffic，semi-traffic-actuated signals are sometimes used. For them，detectors are placed only on the minor street. The signal indication normally is green on the main road and red on the cross street. On actuation, the indications are reversed for an appropriate interval after which they return to the original colors.
交通信号
仅在美国，约250000十字路口有交通信号,这被定义为所有除了闪光灯、标志、和标记的用于指导或警告驾驶员、骑自行车的人和行人的电动交通控制装置,。

控制车辆，骑自行车的人和行人的信号是在特定时间间隔分配到各种交通活动的预定时间和在交通运转中控制全部或部分交通需求的的时间间隔。

定时交通信号
定时交通信号是设定为在一天二十四小时内规定的时间定期重复一个规定的信号指示。

它们有这些优势，包括有低生产成本的控制器，可以联系和协调使车辆以最少的停靠和其他的延迟通过一系列的交叉口。

而且，它们的操作不受异常车辆行为引起的状况的的影响，如一些交通信号控制装置可能带来的交通堵塞引起的强迫性停车。

它们的劣势是它们无法适应交通流量的变化和在没有其他交通的情况下约束一个方向的车辆。

结果是导致交通不便和有时的通行能力下降。

一套完整顺序的“周期时间”通常在30秒到120秒之间。

当车辆延迟减少，短循环时间是优先考虑的。

然而对于短循环，清理路口和开始每一次顺利的运转消耗的时间相对较高。

当循环周期增加，从这些原因失去的时间减少。

对于高流量的交通，可能需要增加循环周期来获得更大的通行能力。

每一个行车道信号正常的路口，一辆车在绿灯下大约2.1秒通过。

根据街道宽度、行人需求和车辆靠近速度，每一个绿灯周期后的黄灯（警示）间隔在3秒到6秒之间。

要确定一个大致的周期划分，常见的做法是确定高峰期间的短期交通数目。

简单的计算给出每一个适应信号指示下车辆数目和车辆通过时最小的绿灯时间。

随着现代控制设备，改变循环周期和每天时间次数的划分，或者使闪光信号更好的适应交通模式变的可能。

在许多路口，信号必须被定好来调整行人流动。

手册建议行人开始加上步行时间以4英尺每秒（1.2米每秒）计算的最初的间隔允许的最小的总时间是4到7秒。

对于单独的行人指示，步行指示（浅白色）包括最开始的时间间隔，禁止通行闪光灯（橘黄色）包括了剩余的时间间隔。

当和车辆有可能的冲突步行信号灯闪烁，当没有时步行信
号灯不变。

禁止通行信号稳定不变时是告诉行人不要继续前行。

如果行人完全由车辆信号控制，如果路口宽问题会变大，因为黄灯间隔将大大高于车辆需要的3到5秒。

这将减少交叉路口的通行能力，可能需要一个较长的周期时间。

有中线的宽阔街道至少要6英尺（1.8米）宽，行人可能停在那儿。

如果行人命令按钮被合并到整体的控制系统，一个单独的行人信号必须放在这个街道中间。

联动移动
一个车道或一个区域的定时交通信号通常联动调来允许一排紧凑的车辆一起移动而不停止。

在正常的交通量下，适当的调整从2500英尺（0.76千米）到超过一英里（1.6千米）的各种信号间隔对于形成流畅的交通是非常有效的。

另一方面，当一个街道超过了通行能力，调整信号来形成流畅的交通一般是无效的。

四个联动的，同步的，交互的，单一推进式的和弹性连续前进的信号系统随着时间推移已经开发出来。

同步的系统使给定的街道在相同的时间内所有的颜色指示相同。

它出现在两站之间车速高而整体速度低的街道上。

因为这些和其他的错误，今天已经很少使用。

交互式系统在相同的时间有所有的信号转换。

但是在一个给定的街道附近的信号灯或者附近的信号群显示互补的颜色。

交互式系统在大约相等的街区间隔的信号街上能相当好的工作。

它也能有效的在上述说明的商业区控制交通，但是只能当长度在两个方向大约相等时才行。

对于全区的交互式系统，绿色指示灯和红色指示灯必须有大约相等的长度。

当两条主要街道相交，但是穿过主要街道的次要街道太多绿灯，这种周期划分是令人满意的。

批评是在重交通量下后面区域的一排汽车必须强迫做出额外的停止，而改变交通状况的调整是非常困难的。

简易连续式系统保持一个一般的周期时间，但是在每个交叉路口分别提供“前进”指示来匹配交通进展。

这允许在设计速度中的至少一个方向的一组车辆速度连续或接近连续流动，并阻碍了信号之间的超速驾驶。

当交通变的轻的时候，闪烁的灯光可以取代正常的信号指示。

弹性连续前进信号系统有一种主控制器装置来指导个别信号控制器。

这种布置不仅在信号之间做出了积极协调，而且使预定的周期长度，周期分配，白天的时间间隔偏移的变化。

例如，全部系统的周期长度在高峰时段可以延长来提升通行能力或者在其他时间缩短来减少延迟。

当正常的信号控制不需要，闪烁的信号指示可以被代替。

而且连续信号偏移的计
时可以进行调整来适应重交通，比如早上入境和晚上出境。

此外，在特定交叉路口的分配周期可以被计算出来。

交通感应系统是一种具有调整交通信号设置来测量交通量的能力的先进弹性连续性系统。

在车流量大或车速高的道路动脉一定被流量和车速相对较轻的交叉路所中断处，半触动式交通信号有时被使用。

对于它们，探测器仅被布置在次要街道。

正常的信号指示是在主路的绿灯和十字街道的红灯。

关于此装置，当它们变回原来的颜色，指示经过一个适当的时间间隔变得相反。

摘自：克拉克森H.奥格尔斯比和R.加里.希克斯《公路工程》，1982
Highway Capacity And Levels of Service
Capacity Defined
A generalized definition of capacity is: The capacity of any element of the highway system is the maximum number of vehicles which has a reasonable expectation of passing over that section (in either one or both directions) during a given time period under prevailing roadway and traffic conditions. A sampling of capacities for modern highway elements is as follows:
In treating capacity，TRB Circular 212 divides freeways into components: basic freeway segments and those in the zone of influence of weaving areas and ramp junctions. Capacities of expressways，multilane highways，and two- and three-lane facilities also have the two components: basic and those in the zone of influence of intersections. Each of these is treated separately below.
Speed-Volume-Capacity Relationships for Basic
Freeway and Multilane Highway Segments
A knowledge of the relationships among speed，volume，and capacity is basic to understanding the place of capacity in highway design and operation. Figurel3.1，which gives such a relationship for a single freeway or expressway lane, is used for illustrative purposes.
If a lone vehicle travels along a traffic lane，the driver is free to proceed at the design speed. This situation is represented at the beginning of the appropriate curve at the upper left of Fig. 13.1. But as the number of vehicles in the lane increases, the driver's freedom to select speed is restricted. This restriction brings a progressive reduction in speed. For example，many observations have shown that，for a highway designed for 70 mph (113km/h)，when volume reaches 1900 passenger cars per hour，traffic is slowed to about 43 mph (69km/h). If volume increases further, the relatively stable normal-flow condition usually found at lower volumes is subject to breakdown. This zone of instability is shown by the shaded area on the right side of Fig. 13. 1. One possible consequence is that traffic flow will stabilize at about 2000 vehicles per hour at a velocity of 30 to 40 mph (48 to 64km/h) as shown by the curved solid line on Fig. 13. 1. Often，however , the quality of flow deteriorates and a substantial drop in velocity occurs; in extreme cases vehicles may come to a full stop. In this case the volume of flow quickly decreases as traffic proceeds under a condition known as ‘forced flow.’ V olumes under forced flow are shown by the dashed curve at the bottom of Fig.
13. 1. Reading from that curve，it can be seen that if the speed falls to 20 mph (32km/h)，the rate of flow will drop to 1700 vehicles per hour; at 10 mph （16km/h) the flow rate is only 1000;and,of course，if vehicles stop，the rate of flow is 0. The result of this reduction in flow rate is that following vehicles all must slow or stop，and the rate of flow falls to the levels shown. Even in those cases where the congestion lasts but a few seconds, additional vehicles are affected after the congestion at the original location has disappeared. A ‘shock wave’develops which moves along the traffic lane in the direction opposite to that of vehicle travel. Such waves have been observed several miles from the scene of the original point of congestion，with vehicles slowing or stopping and then resuming speed for no apparent reason whatsoever.
Effects of the imposition of speed limits of 60, 50, and 40 mph are suggested by the dotted lines on Fig. 13. 1. A 55-mph (88km/h) curve could also be drawn midway between the 60 and 50 mph dotted curves to reflect the effects of the federally imposed 55-mph limit, but this is conjectural since the level of enforcement varies so widely.
Vehicle spacing，or its reciprocal, traffic density, probably have the greatest effect on capacity since it generates the driver's feeling of freedom or constraint more than any other factor. Studies of drivers as they follow other vehicles indicate that the time required to reach
a potential collision point，rather than vehicle separation，seems to control behavior. However，this time varies widely among drivers and situations. Field observations have recorded headways (time between vehicles) ranging from 0. 5 to 2 sec, with an average of about 1. 5s.Thus，the calculated capacity of a traffic lane based on this 1. 5 s average, regardless of speed，will be 2400 vehicles per hour. But even under the best of conditions, occasional gaps in the traffic stream can be expected，so that such high flows are not common. Rather, as noted，they are nearer to 2000 passenger cars per hour.
The ‘Level of Service’ Concept
As indicated in the discussion of the relationships of speed, volume or density, and vehicle spacing, operating speed goes down and driver restrictions become greater as traffic volume in crease. ‘Level of service’ is commonly accepted as a measure of the restrictive effects of increased volume. Each segment of roadway can be rated at an appropriate level，A to F inclusive，to reflect its condition at the given demand or service volume. Level A represents almost ideal conditions; Level E is at capacity; Level F indicates forced flow.
The two best measures for level of service for uninterrupted flow conditions are operating or travel speed and the radio of volume to capacity达到最大限度的广播，called the v/c ratio. For two- and three-lane roads sight distance is also important.
Abbreviated descriptions of operating conditions for the various levels of service are as follows:
Level A—Free flow; speed controlled by driver's desire，speed limits, or physical roadway conditions.
Level B—Stable flow; operating speeds beginning to be restricted; little or no restrictions on maneuverability from other vehicles.
Level C—Stable flow; speeds and maneuverability more closely restricted.
Level D—Approaches unstable flow; tolerable speeds can be maintained but temporary restrictions to flow cause substantial drops in speed. Little freedom to maneuver，comfort and convenience low.
Level E—V olumes near capacity; speed typically in neighborhood of 30 mph (48km/h); flow unstable; stoppages of momentary duration. Ability to maneuver severely limited.
Level F—Forced flow，low-operating speeds，volumes below capacity; queues formed.
A third measure of level of service suggested in TR
B Circular 212 is traffic density. This is，for a traffic lane，the average number of vehicles occupying a mile (1. 6km) of lane at a given instant. To illustrate，if the average speed is 50 mph，a vehicle is in a given mile for 72 s. If the lane carrying 800 vehicles per hour，average density is then 16 vehicles per mile ;spacing is 330 ft (100m)，center to center. The advantage of the density approach is that the various levels of service can be measured or portrayed in photographs.
From: Clarkson H. Oglesby and R. Gary Hicks “Highway engineering”, 1982
公路通行能力和服务水平
通行能力的定义
道路通行能力的广义定义是：在繁忙的道路和交通条件下公路系统任何元素的通行能力是对在指定的时间通过一断面（一个或两个方向）的最大数量的车辆有一个合理的预期。

一个现代公路通行能力的的抽样情况如下：
关于通行能力处理量，运输交通委员会发布的公路通行能力手册将高速公路划分为以下部分：基本高速公路路段，这些区域有影响的交织地区和砸道连接处，高速公路，多车道公路。

两车道和三车道的通行能力同样有两部分组成：基本路段和这些区域有影响作用的交叉路口。

基本高速公路和多车道公路路段
速度，车流量和通行能力的关系
速度，车流量和通行能力之间关系是了解某一地方公路设计和运行能力的基础。

图3.1说明了高速公路中速度、车流量和通行能力之间的关系。

如果司机驾驶一辆汽车一直自由的以设计时速独自行驶在一个行车道上，这种情形在左上角的图13.1中以适当的曲线表示出来。

但随着车道上车辆数目的增加，司机自由选择速度受到限制。

例如，许多研究表明，一个高速公路的设计速度为70英里每小时（113km/h），当车辆容量达到1900辆每小时时，交通速度下降到43英里每小时（69。