公路通行能力和服务水平 毕业论文外文翻译.doc

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 period 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 hour2000three-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 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 basicfreeway 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. 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 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. 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，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 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 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 drivers 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 increase. 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 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 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 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 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： 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13.1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。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，他们跟随其他车辆以到达一个潜在的碰撞