路桥工程专业英语第五课

1、第五课Traffic Engineering Introduction交通工程学概论A. Text: Traffic Engineering BasicsB. Reading Material: Highway CapacityTextTraffic Engineering Basics1 Prediction of Traffic Volume GrowthTraffic volume growth may be predicted using a number of methods but the two primary ones are the use of the ARTS long-

2、range plan and manual methods. The ARTS plan may be used to establish average daily traffic volumes for streets within a study area for a future year as predicted by the ARTS model. This data can be used to develop a factor to apply to existing traffic counts to predict future-year traffic volumes.M

3、anual extrapolation of existing data using growth factors may be available from the City, or the consultant may need to develop them. Growth factors may be and can be representative of a number of algebraic trends. Among these are the straight-line, exponential, and decaying-rate exponential trends.

4、 For a good discussion of these trend methods, see the Trip Forecasting Manual published by the Florida Department of Transportation. Generally, manual extrapolation of turning movement counts will not be accepted for periods of time exceeding five years.2 Trip Generation EstimationTrip generation m

5、ay be estimated using the Institute of Transportation Engineers (ITE Trip Generation. An accompanying publication, the Trip Generation Handbook provides additional information on how to use Trip Generation and how to apply adjustments such as pass-by trips. Generally, the City of Aiken accepts the m

6、ethodology provided in Trip Generation and the Trip Generation Handbook. In some cases, trip generation data may not be available from ITE for certain land uses. In these cases the applicant s consultant will be required to develop estimated trip generation data for use in the study after consultati

7、on with the City of Aiken staff.The Trip Generation provides estimated trip generation data for various land use categories, known as land use codes in the Manual. Average rates for weekdays, weekends, and morning and afternoon peak hours are given for most land use codes. In addition, many of the l

8、and use codes provide equations that may be used to determine estimated traffic generation. An example of the data for a land use, Code 210, single-family residential, is given on the following page. As seen in this example, there are average rates as well as an equation given for this particular la

9、nd use. The Trip Generation Handbook gives information that recommends when to use the average rate data and when to use the equations. A summary decision chart from the Handbook takes into accountthe size of the development and the statistics indicating the reliability of the data in Trip Generatio

10、n. For example, for a single-family residential development, the average daily trip generation rate given is 9.57 trips per dwelling unit. The equation given is Ln(T = 0.92×Ln(X + 2.71. If a proposed single-family residential project has 300 dwelling units, then the equation would give the foll

11、owing estimate of daily trips:Ln(T = 0.92×Ln(300 + 2.71Solving for T gives 2857 trips. Using the average rate of 9.57 trips per dwelling unit would give 300×9.57 = 2671. So, there is a discrepancy between the two methods and a decision must be made as to which method to use.For a second ex

12、ample consider a proposed mini-warehouse facility with 125 units, land use code 151. In this case there is an equation given, but the formula is based upon only 14 studies. As per Figure 4, we see that the number of data points is less than 20, so we check the standard deviation and see that it is 1

13、.78 versus the average of 2.50 for a percentage of 72%. Since the R2 value is 0.73, which is less than 0.75, we would use the average of 2.50 per unit. This would result in 2.50×125 = 313 trips. The equation would have given:Ln(T = 1.01×Ln(125 + 0.82Solving for T gives 298 trips. Again, th

14、is is a different result than we would get with the average rate, but in this case the ITE Trip Generation Handbook recommends use of the average rate.As seen above, care must be used when developing trip generation estimates from the ITE Trip Generation and the guidelines of the ITE Trip Generation

15、 Handbook must be used to make the correct selection between the use of average rates and equations provided.3 Traffic AssignmentPreviously the estimation of generated trip ends has been discussed together with the distribution of trips between the traffic zones. Modal split methods also have been r

16、eviewed in which the proportion of trips by the varying travel modes are determined. At this stage the number of trips and their origins and destinations are known but the actual route through the transportation system is unknown. This process of determining the links of the transportation system on

17、 which trips will be loaded is known as traffic assignment.Apart from the largest transportation survey traffic assignment tends to deal with highway traffic. This is because it is usually not difficult to estimate the route taken by public transport users and also because the loading trips on the p

18、ublic transport network dose not materially affect the journey time. Trip ends where there is no choise of travel mode, which is from non-car-owing households, are accumulated as public transport trip ends. Choice trips where a car is available are separated by the modal choice procedure into car tr

19、ips and public transport trips.Usually it will then be found that proposed road network is overloaded and some car trips will need to be restrained. If a car cannot be used then some trips will not be made at all, while other trips will be transferred to public transport and accumulated.As the basis

20、 of assignment is usually travel time, the travel times on the network links will vary the imposed loading. In addition as travel time is used in the trip distribution process, it is necessary to carry out an iterative procedure between distribution, modal choice and assignment. The change in speed

21、with volume on a highway link is carried out using speed flow relationships for the varying highway types and it is interesting to consider just what are the effects of a speed change. Firstly it affects the choice of route because assignment is made on thebasis of travel time through the network. S

22、econdly it affects the destinations of trips because trips are distributed to varying destinations on the basis of travel time when a gravity model is used. Finally it may affect the choice of travel mode because modal choice is often made a comparison of travel times.There are many problems associa

23、ted with speed flow relationships , considerable variation being observed between differing highways even of the same type. There is also the additional problem that most transportation studies are based on 24-hour flows so that it is necessary to know the hourly variation and the directional distri

24、bution of flows.In assignment it is first necessary to describe the transport network to which trips are being assigned. The network is described as a series of nodes and connecting links; in a highway network the nodes would be the junctions and the links the connecting highways. Centroids traffic

25、zones, at which it is assumed that all zonal trips are generated and to which they are attracted, are either at nodes or connected to them by additional links. The cost of using a link and a junction, usually in the form of travel times and delays, is given on the basis of the review of transport fa

26、cilities carried out during the initial stages of the transportation survey.There are three methods by which the assignment may be made. These are:(1All-or-nothing assignment(2Capacity restrained assignment(3Multipath proportional assignmentIn all-or-nothing assignment method, an algorithm is used t

27、o compute the route of least cost, usually based on travel time between all the zone centroids. For each zone centroids selected as origin, a set of shortest routes from the origin to all the other centroids is referred to as a minimum tree. When the trips between two zones are assigned to the minim

28、um path between the two zones, then the assignment is said to take place on all-or-nothing basis.There are obvious difficulties with such a simplified approach, some of which are inherent in the other assignment methods. It is obviously incorrect to assume that all trips commence and terminate at a

29、zone centroid. K the length of the links within the zones is small compared with the length of remainder of the minimum link path, then the errors may not be so serious. Because of its simplicity, travel time is usually employed as a measure of link impedance, but travel times may not be precisely e

30、stimated by the traveler. The use of a cost function which reflects the perceived cost of travel is desirable. The loading on a link in this method is extremelysensitive to estimated link and node costs, if these have been incorrectly estimated, then the resulting assignment is open to question. The

31、re is also the problem that links with small travel cost will attract trips without any adjustment in link cost.4 Traffic Flow AnalysisPrior to development of traffic flow analysis for both existing and future conditions, a review will be made by the consultant and a proposed plan will be submitted

32、to and discussed with the Planning Director. This plan will address the methods to be used in data collection and analysis. Generally , traffic flow analysis will be by the current version of the Highway Capacity Manual (HCM. Computerized software packages such as HCS+, Synchro, SimTraffic, Corsim,

33、and other programs may be used with prior approval of the Planning Director.Any deviations from typical values for parameters used in the HCM or any software packages will require prior approval of the Planning Director. Among these parameters are standard normal ranges of saturation flow for travel

34、 lanes, general traffic signal timing settings,normal values for percentages of heavy vehicles, and various adjustment factors. The consultant should discuss the use of any extraordinary settings for these and any other parameters with the Planning Director before use. For example, the Highway Capac

35、ity Manual normally recommends the use of a saturation flow of 1900 vehicles per hour for arterial streets. Use of a saturation flow rate substantially different would require prior approval of the Planning Director.Words and ExpressionsPrediction :预测; a number of:若干、许多;Primary :原始的、初等的、基本的; long-ra

36、nge :远程的,长期的;manual :手册、指南; ARTS plan:ARTS 图 (计划 ;Factor :系数; Extrapolation :推断、外推;growth factors:增长系数; Available :有用的、可以得到的; Consultant :求教者、征求意见者; develop :发展、开发;Representative :表现的、代表性的,代表; Algebraic :代数的;Trend :走向、趋势; Among :在 之中,其中;Exponential :指数; Decay :衰减、衰败;Discussion :议论、讨论; Turning :转向、转弯

37、;Count :算、计数; Trip Generation:出行生成、出行产生; Institute :协会、学会; Accompanying :陪伴的;Handbook :手册; Adjustment :调整、校正;pass-by trips:过境出行; methodology :方法论;land use:土地利用; Applicant :申请人、请求人;Require :要求、命令、需要; Consultation :商量、专家会议;Staff :杖、杆,支柱,标尺,职员、参谋; Category :类型、种类;Code :法典,规则,代号、密码; Rate :比率;Peak :高峰; Single :单一的;Residential :住宅的、关于居住的; as well as:除 之外又 ,既