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1、Highway Capacity And Levels of ServiceCapacity 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 p
2、eriod under prevailing roadway and traffic conditions. A sampling of capacities for modern highway elements is as follows:FacilityCapacity in Passenger CarsFreeways and expressways away from ramps and weaving sections, per lane per hour2000Two-Lane highways, total in both directions, per hour2000Thr
3、ee-lane highways, total in each direction, per hour2000Twelve-foot lane at signalized intersection, per hour of green signal time(no interference and ideal progression)1800 In treating capacity，TRB Circular 212 divides freeways into components: basic freeway segments and those in the zone of influen
4、ce 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 BasicFre
5、eway and Multilane Highway SegmentsA 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.
6、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 drivers freedom to select speed is restri
7、cted. 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 no
8、rmal-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)
9、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
10、. Volumes 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，
11、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 lo
12、cation has disappeared. A shock wavedevelops 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 apparen
13、t 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 c
14、onjectural 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 drivers feeling of freedom or constraint more than any other factor. Studies of drivers as they follow other vehicles i
15、ndicate 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
16、 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
17、 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 increase. Level of service is commonly accepte
18、d 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 fl
19、ow. 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 v
20、arious levels of service are as follows: Level AFree flow; speed controlled by drivers desire，speed limits, or physical roadway conditions.Level BStable flow; operating speeds beginning to be restricted; little or no restrictions on maneuverability from other vehicles.Level CStable flow; speeds and
21、maneuverability more closely restricted.Level DApproaches 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 EVolumes near capacity; speed typically in neighborhood of 30
22、mph (48km/h); flow unstable; stoppages of momentary duration. Ability to maneuver severely limited.Level FForced flow，low-operating speeds，volumes below capacity; queues formed. A third measure of level of service suggested in TRB Circular 212 is traffic density. This is，for a traffic lane，the avera
23、ge 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 adva
24、ntage 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 待添加的隐藏文字内容1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
25、力的的抽样情况如下： 设施小客车通行能力远离斜坡和交织路段的高速公路和每小时每个车道的车流量2000两车道公路，每小时两个方向的车流量2000三车道公路，每小时一个方向的车流量2000有信号的交叉路口的十二英寸车道，在绿灯条件下每小时的车流量（没有干扰的理想通行条件下）1800 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
26、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 km /h）。如果车辆数进一步增加，则建立在低车辆数目的相关稳定和正常的流动条件将会被打破。这种不稳定的区域如右侧图13.1阴影区域所示。一个可能的结果是交通流量将如图13.
27、1的实曲线所示以30到40英里每小时（48到64km/h）的速度下稳定在大约2000车辆每小时。然而在通常情况下，车流量的质量恶化，车速大幅度下降；在极端情况下车辆可能完全停止。在这种情况下，车流量迅速下降，这种情况下的交通受益被称为“强制性流动”。强制性流动下的车流量如图13.1下部的虚曲线所示。从曲线上可以看出，如果速度下降到20应力每小时（32km/h），车流量将下降到1700辆每小时，以10英里每小时（16km/h）的流量算只有1000辆车；当然如果车辆停止，车辆流速为0。流速减少的结果是以后的车辆都必须减速或者停止，车辆流速下降到显示的水平。即使在这种情况下，交通拥挤的情况人在持续。
28、短暂时间后，拥挤处原来的车辆离开后，其他的车辆又会受到影响。一个沿着相反车道行驶的车辆的冲击波逐渐形成。这样的冲击波已经在视野里从原来的拥挤点达到几英里。伴随着车辆减速或者停止，而且不会明显恢复原来的速度。速度限制在60、50和40英里每小时的影响在图13.1中的虚线表示出来。55英里每小时（88km/h）的曲线也可以画在60和50英里每小时的虚线中来反映联邦政府限制的55英里每小时的速度的影响。但是因为执法水平的宽泛和多样性，这只是推测而已。车辆间距，或者它的倒数，交通密度可能对通行能力有最大的影响，因为它对于司机形成自由或者约束的感觉比其他任何因素都要多。对于司机的研究显示，他们跟随其他车
29、辆以到达一个潜在的碰撞点所需要的时间来控制自己的行为，而不是以汽车分开所需要的时间。不过这个时间点对于不同的司机来说差别非常大。实地观察记录车辆的间隔时间（车与车之间的时间）是从0.5秒到2秒，平均大约是1.5秒。因此，根据这1.5秒平均时间，而不是车速，计算的道路通行能力将会达到2400辆车每小时。但是，即使在最好的交通条件下，车流中的差距也是可能会出现的，所以如此高的车流是不常见的。相反，正如文中所指出的，每小时可以接近2000辆小客车通过。服务水平的概念正如讨论中所说明的当交通量增加时速度、流通量、车流密度和行车间距、运行速度下降、延迟增加之间的关系。“服务水平”通常被认为是流通量增加时
30、的延迟性效应的程度。每一个路段可以列出一个合理的水平，包括A级到F级，在给定的要求和服务量下来反映道路的运行质量。A级表示几乎理想的条件，E级表示最大限度的状态，F级表示车辆强制性的流动。对于达到车辆不间断流动的服务水平的最好措施是行驶速度，到达最大限度时的播报，v/c率。对于两车道和三车道，道路视距也是非常重要的。对于各种服务水平的运行条件的简单说明如下：A级车辆流通顺畅，车速受司机控制，速度受到限制，或者适应实际道路条件。B级车辆流通稳定，运行速度是开始稍有延迟，对于特殊车辆的机动性很少或没有受到限制。C级车辆流通稳定，速度和机动性更容易受到限制。D级车辆流通不太稳定；可以维持在容忍速度，
31、但是临时性的限制可以引起车流速度大量下降；车辆运行不自由，舒适和方便性低。E级交通量接近道路通行能力，车流量不太稳定，车速维持在30英里每小时（48km/h）左右；出现短暂时间的停滞，运行能力受到限制。F级车辆流通受到限制，运行速度低，通行能力下降，出现阻塞的长队。运输研究委员会手册说明的另一种表明服务水平的方法是交通密度。就是对于一个行车道，在一定的时间内一英里车道上平均的车道数。比如，如果平均速度是50英里每小时，车辆在在一确定的一英里车道上是72s。如果车道上每小时通过800辆汽车每小时，平均车辆密度是16车每英里；车辆中心之间的间隔是330英尺。车辆密度方法的优势是不同的服务水平可以在照片中判断和描述出来。摘自：克拉克森H.奥格尔斯比和R.加里.希克斯公路工程，1982
