【導(dǎo)讀】本科畢業(yè)設(shè)計（論文）。河南理工大學(xué)土木工程學(xué)院

　　

【正文】 elocity 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 e 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 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 55mph (88km/h) curve could also be drawn midway between the 60 and 50 mph dotted curves to reflect the effects of the federally imposed 55mph 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 driver39。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 河南理工大學(xué)本科畢業(yè)設(shè)計（論文） 外文翻譯 16 about 1. ， 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 mon. Rather, as noted， they are nearer to 2020 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 bee greater as traffic volume increase. ?Level of service? is monly 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 達到最大限度的廣播， called the v/c ratio. For two and threelane 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 driver39。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，fort and convenience low. Level E— Volumes 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， lowoperating speeds， volumes below capacity。 queues formed. 河南理工大學(xué)本科畢業(yè)設(shè)計（論文） 外文翻譯 17 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 河南理工大學(xué)本科畢業(yè)設(shè)計（論文） 外文翻譯 18 公路通行能力和服務(wù)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(tǒng)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(shù)量的車輛有一個合理的預(yù)期。一個現(xiàn)代公路通行能力的的抽樣 情況 如下： 設(shè)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 2020 兩車道公路，每小時兩個方向的車流量 2020 三車道公路，每小時一個方向的車流量 2020 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(guān)于通行能力處理量，運輸交通委員會發(fā)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(qū)域有影響的交織地區(qū)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(qū)域有影響作用的交叉路口。 基本高速公路和多車道公路路段 速度，車流量和通行能力的關(guān)系 速度，車流量和通行能力之間關(guān)系是了解某一地方公路設(shè)計和運行能力的基礎(chǔ)。圖 說 明了高速公路中速度、車流量和通行能力之間的關(guān)系。 如果司機駕駛一輛汽車一直自由的以設(shè)計時速獨自行駛在一個行車道上，這種情形在左上角的圖 中以適當(dāng)?shù)那€表示出來。但隨著車道上車輛數(shù)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(shè)計速度為 70 英里每小