1、I. INTRODUCTIONHighway Capacity Manual 2000In this chapter, a brief overview of the analytical procedures in this manual, theirorganization into chapters, and guidance on their general application are provided.The analytical procedures in this manual can be used for a number of applicationscovering
2、a broad range of facility types. The facility types are distributed among fivecategories: urban streets, pedestrian and bicycle facilities, highways, freeways, andtransit.In Chapters 10 through 14, for each of the five categories, general concepts arepresented, required inputs for each methodology a
3、re identified, reasonableapproximations for specific parameters are suggested for use if local data are notavailable, and example service volume tables are provided. The Part II chapters alsocontain special procedures used to supplement the planning applications defined in thePart III chapters.II. O
4、VERVIEW OF ANALYTICAL PROCEDURESFor the analytical methods defined in Part III, the calculations of average speed,density, and delay will provide insight into the level of service for what is considered asteady-state condition. This means that the outputs provided by the computationalmethods are con
5、sidered representative for the length or area of the analysis and for theduration of the analysis period. Thus, the Highway Capacity Manual (HCM) methods aregenerally not appropriate (unless the analyst performs a special intervention) for theevaluation of inclement weather conditions, accidents or
6、construction activities, queuesthat are building over both time and space, or the possible effects of vehicle guidance ordriver guidance systems typical of intelligent transportation systems. However, someguidelines are identified in Chapter 22 to address these conditions.The Part III methods have b
7、een designed to be sensitive to roadway, traffic, andcontrol characteristics of the facility. However, the methods cannot predict the effects ofchanges in the posted speed limit, the level of police enforcement, safety features, drivereducation, or vehicle performance.A ground transportation system
8、is composed of six modal and facility typesubsystems located in a defined study area or corridor. The six subsystems are freeway,urban street, rural highway, transit, pedestrian, and bicycle. Each transportationsubsystem is composed of two or more individual facilities. The facilities within eachsub
9、system are all of a single type (freeway, urban street, rural highway) or mode (transit,pedestrian, bicycle). Each facility is in turn made up of segments and points. Forexample, a freeway contains basic, weaving, and ramp merge/diverge segments. Anurban street contains street segments and intersect
10、ions (points).A segment is a length of facility where demand and capacity are relatively constant.Each segment begins and ends at a point. Segments are generally directional; forexample, each stretch of two-way street is composed of two one-way segments. Theexception to this is two-lane highways, wh
11、ere each segment is bidirectional but can besplit into two directional segments for analysis. A point is a very short length of facilitywhere demand or capacity changes abruptly from conditions on the upstream ordownstream segment.Analysis of the transportation system proceeds from estimates of trav
12、el times anddelays at the segment and point levels using the methods described in Part III. Segmentand point delays and travel times are converted to total person hours of delay or traveltime and then summed to obtain facility estimates.Chapters 1014 of Part IIpresent general conceptsPart III, Chapt
13、ers 1527,presents methodologiesA facility is composed ofsegments and points9-1 Chapter 9 - Analytical Procedures OverviewIntroductionHighway Capacity Manual 2000Part IV, Chapters 2830,presents corridor andareawide analysesPart V containsinformation on simulationand other modelsFor analyses that comb
14、ine facility types or that address a corridor or expanded area,the analyst must consult Part IV. Part V contains useful information on applications ofsimulation and other models to complement the use of HCM 2000 methodologies.Exhibit 9-1 illustrates the content, by chapter, of the analytical section
15、s of thismanual. Outputs from computations based on the methodologies are also indicated.Most of the analytical processes require estimates of hourly demand in one direction.The section on equivalency of hourly and daily volumes provides guidance ondetermining directional hourly volumes from average
16、 daily traffic volumes. Theanalytical procedures in Part III (Chapters 15 through 27) require information on thegeometric design, control, and demand for the facility being analyzed. The followingsections provide some brief guidance on the development of local default values for inputdata that are d
17、ifficult to obtain. Generic default values that may be used for specificfacility analyses in the absence of local values are provided in Chapters 10 through 14.Some of the analytical procedures can be quite complex. Analysts may wish todevelop tables of maximum service volumes for typical highway fa
18、cilities in their area.The tables may be used in planning studies to roughly size a facility when resources donot permit more detailed analyses. Guidance on the development of local service volumetables is provided in Appendix B. Examples of service volume tables are given inChapters 10 through 14.I
19、II. PRECISION AND ACCURACY OF THE MANUALThe presentation of numerical values and calculations in this manual is based on along history of evolving methodologies for assessing capacity and quality of service. Thefirst HCM was produced in 1950. It was followed by a series of manuals, the last updatebe
20、ing the 1997 HCM. A large number of researchers and research projects in the past 50years have contributed to the methodologies presented in this, the 2000 edition. Toprovide a better understanding of the framework in which this edition was developed, theaccuracy and precision of numerical values ar
21、e discussed.The terms accuracy and precision are independent but complementary concepts.Accuracy relates to achieving a correct answer, while precision relates to the size of theestimation range of the parameter in question. As an example of accuracy, consider amethod that is applied to estimate a p
22、erformance measure. If the performance measure isdelay, an accurate method would provide an estimate closely approximating the actualdelay that occurs under field conditions. The precision of such an estimate is the rangethat would be acceptable from an analysts perspective in providing an accurate
23、estimate.Such a range might be expressed as the central value for the estimated delay plus orminus several seconds. In general, the inputs used for the methodologies in this manualare from field observations or estimates of future conditions. In either case, andparticularly for future conditions, th
24、e inputs can only be expected to be accurate to within5 or 10 percent of the true value. Thus, the computations performed cannot be expectedto be extremely accurate, and the final results must be considered as estimates that areaccurate and precise only within the limits of the input values used.To
25、provide numerical values and computational results that are relatively easy to useand that indicate the presumed accuracy and precision, a framework of guidelines wasestablished during preparation of this manual. In the following sections, an explanationof this framework is given.PRECISION AND ACCUR
26、ACY FRAMEWORKThe user of the HCM should be aware of the limitations of the accuracy andprecision of the methodologies in the manual. Such awareness will help the user toChapter 9 - Analytical Procedures OverviewOverview of Analytical Procedures9-2Highway Capacity Manual 2000interpret the results of
27、an analysis and to use the results to make a decision on design oroperation of a transportation facility.Many of the models in the HCM are based on theoretically derived relationships,which include assumptions and contain parameters that must be calibrated on the basis offield data. Other models in
28、the HCM are primarily statistical. Both types require datacollected at a sampling of sites. The degree to which the models reflect reality is oftenstated in terms of the accuracy and precision of the model. Accuracy and precision areterms used to express the probable error associated with an estimat
29、e.Frequently, after a model is developed, it is validated by comparing the estimatesfrom the model with values measured in the field from an independent set of sites. Aregression line fitted to the plot of points for field-measured versus model-estimatedvalues will result in a line with a slope diff
30、erent from 45 degrees. The difference can beconsidered the relative accuracy of the model. The dispersion of the points around theregression line can be considered the precision of the model. The measure of dispersionwith which many analysts are familiar is the R2 value. These statistics, based on f
31、ieldand predicted data, indicate the limitations of the models in predicting with greatprecision and accuracy.Few of the models in the HCM have well-documented measures of accuracy andprecision. Typically, when research is completed and statistical relationships arereported, the Committee on Highway
32、 Capacity and Quality of Service will exercise itsjudgment in modifying the results.Prediction error from other sources may also result when the user applies the HCM.For example, the accuracy of results may be reduced by the use of default values for oneor more of the parameters in the models. In ad
33、dition, there are limitations on theaccuracy and precision of traffic inputs used in these models. Traffic measurements andpredictions, including magnitude and mix of traffic, have inherent limitations onaccuracy.The limitations on the accuracy and validity of predictions of performance measuresshou
34、ld be recognized in applying the results of an analysis. For instance, smalldifferences between the values of performance measures for alternative designs shouldnot always be assumed to be real (statistically significant) differences. Furthermore, ifthe predicted value for a measure of effectiveness
35、 is near, but below, a critical threshold,there is some probability that it will in fact be higher than predicted and exceed thecritical threshold. The HCM user should recognize, therefore, that judgment is requiredin applying the results of analyses. One basis for that judgment is a good understand
36、ingof the structure and basis of the models used in this manual.Constraint of Prior Research ResultsThe methodologies in this manual have been developed by a number of researchersworking on many research projects. Few of these projects have presented results withaccompanying statements on precision
37、and accuracy. Rather, most of the methods haveinvolved the use of mean or average values for parameters. Results have been presentedin a variety of forms with regard to the use of tables, graphs, and interpolated values. Thenumber of digits to the right of the decimal point in factors, calculated va
38、lues ofperformance measures, and threshold values used to define level of service has alsovaried. In general, it was considered prudent to follow the presented results and thesignificant figures used in prior research rather than to change the recommended valuesarbitrarily. Whenever possible, the ta
39、bulated factors and adjustments and the finalcalculated values of performance measures used in the reported research were maintainedfor the methods in this manual.Several factors result inlimitation on the accuracy andprecision of HCM analysisResearch precision andaccuracy9-3 Chapter 9 - Analytical
40、Procedures OverviewPrecision and Accuracy of the ManualCalculation Precision Versus Display PrecisionPrecision in calculationdiffers from precision inpresenting final resultsConventions for displayof results in the HCMThe extensive use of personal computers has allowed calculations of capacity andle
41、vel of service to be carried to a large number of digits to the right of the decimal point.Because of this ease of calculation, there is a need to state clearly that the final result ofcalculations done manually and carried to the suggested number of significant figuresmight be slightly different fr
42、om the result of calculations performed on a computer. Thisdifference has been explicitly recognized in this manual. For example, lists of factors areoften displayed with three or four digits to the right of the decimal point to more closelyadhere to the calculation protocol inherent in computers.Im
43、plied Precision from Displayed ResultsThe typical interpretation given to a value such as 2.0 is that the value is in aprecision range of two significant figures and that results from calculations should berounded to this level of precision. Occasionally, particularly in the running text of themanua
44、l, editorial flexibility allows a zero to be dropped from the number of digits. Inmost cases, however, the number of the digits to the right of the decimal point does implythat a factor or numerical value has been calculated to that level of precision.Directives from TRB CommitteePrior to publicatio
45、n of this manual, the Committee on Highway Capacity and Qualityof Service (A3A10) developed guidelines for the presentation of results. The guidelineswere presented in mid-1997 in the form of advice to the preparers of this manual. Severalrecommendations were included and were particularly aimed at
46、the exhibits and valuesshown and used in Chapter 16, Signalized Intersections. This advice was considered,along with the factors mentioned above, in developing the HCM.Specific Components for Presentation GuidelineThe overall objective of the guideline is to present tabular values and calculatedresu
47、lts in a consistent manner throughout the manual. Another objective is to use anumber of significant digits that is reasonable and indicates to the analyst that the resultsare not extremely precise but take on the precision and accuracy associated with the inputvariables. As stated earlier, such acc
48、uracies for traffic volume counts and measurementof geometric conditions seldom are better than a central value plus or minus 5 percent.Prediction to a future time frame presents even greater differences between the assumedinput values and what will actually occur at that time horizon. The guideline
49、 for thismanual recognizes that rounding intermediate results in a series of calculations for agiven method is not appropriate and can be more confusing than worthwhile. The thirdobjective of the guideline is that prior research results, advice, and recommendations ofthe Committee on Highway Capacity and Quality of Service and the standard practice ofthe profession in these calculations are to be respected.Chapter 9 - Analytical Procedures OverviewPrecision and Accuracy of the Manual9-6
