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達到最大限度的廣播，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 待添加的隱藏文字內容1公路通行能力和服務水平通行能力的定義 道路通行能力的廣義定義是：在繁忙的道路和交通條件下公路系統任何元素的通行能力是對在指定的時間通過一斷面（一個或兩個方向）的最大數量的車輛有一個合理的預期。一個現代公路通行能

25、力的的抽樣情況如下： 設施小客車通行能力遠離斜坡和交織路段的高速公路和每小時每個車道的車流量2000兩車道公路，每小時兩個方向的車流量2000三車道公路，每小時一個方向的車流量2000有信號的交叉路口的十二英寸車道，在綠燈條件下每小時的車流量（沒有干擾的理想通行條件下）1800 關于通行能力處理量，運輸交通委員會發布的公路通行能力手冊將高速公路劃分為以下部分：基本高速公路路段，這些區域有影響的交織地區和砸道連接處，高速公路，多車道公路。兩車道和三車道的通行能力同樣有兩部分組成：基本路段和這些區域有影響作用的交叉路口?；靖咚俟泛投嘬嚨拦仿范嗡俣龋嚵髁亢屯ㄐ心芰Φ年P系 速度，車流量和通行能

26、力之間關系是了解某一地方公路設計和運行能力的基礎。圖3.1說明了高速公路中速度、車流量和通行能力之間的關系。如果司機駕駛一輛汽車一直自由的以設計時速獨自行駛在一個行車道上，這種情形在左上角的圖13.1中以適當的曲線表示出來。但隨著車道上車輛數目的增加，司機自由選擇速度受到限制。例如，許多研究表明，一個高速公路的設計速度為70英里每小時（113km/h），當車輛容量達到1900輛每小時時，交通速度下降到43英里每小時（69 km /h）。如果車輛數進一步增加，則建立在低車輛數目的相關穩定和正常的流動條件將會被打破。這種不穩定的區域如右側圖13.1陰影區域所示。一個可能的結果是交通流量將如圖13.

27、1的實曲線所示以30到40英里每小時（48到64km/h）的速度下穩定在大約2000車輛每小時。然而在通常情況下，車流量的質量惡化，車速大幅度下降；在極端情況下車輛可能完全停止。在這種情況下，車流量迅速下降，這種情況下的交通受益被稱為“強制性流動”。強制性流動下的車流量如圖13.1下部的虛曲線所示。從曲線上可以看出，如果速度下降到20應力每小時（32km/h），車流量將下降到1700輛每小時，以10英里每小時（16km/h）的流量算只有1000輛車；當然如果車輛停止，車輛流速為0。流速減少的結果是以后的車輛都必須減速或者停止，車輛流速下降到顯示的水平。即使在這種情況下，交通擁擠的情況人在持續。

28、短暫時間后，擁擠處原來的車輛離開后，其他的車輛又會受到影響。一個沿著相反車道行駛的車輛的沖擊波逐漸形成。這樣的沖擊波已經在視野里從原來的擁擠點達到幾英里。伴隨著車輛減速或者停止，而且不會明顯恢復原來的速度。速度限制在60、50和40英里每小時的影響在圖13.1中的虛線表示出來。55英里每小時（88km/h）的曲線也可以畫在60和50英里每小時的虛線中來反映聯邦政府限制的55英里每小時的速度的影響。但是因為執法水平的寬泛和多樣性，這只是推測而已。車輛間距，或者它的倒數，交通密度可能對通行能力有最大的影響，因為它對于司機形成自由或者約束的感覺比其他任何因素都要多。對于司機的研究顯示，他們跟隨其他車輛以到達一個潛在的