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6 guidance is given, formulas for calculating measures are Although several of the recommended performance meas- provided, and references made to other well-accepted, pub- ures presented are derived from the perspective of the indi- lished sources of guidance. These steps include selection of vidual traveler (e.g., delay per traveler and several of the appropriate measures, data collection and processing, and travel-time-based indexes), the analytical methods defined specific fundamental or "building block" applications, such are not intended to drive traveler information (TI) systems or as deficiency analysis or alternatives analysis. These steps can programs. While travel-time measures are becoming more be applied in varying combinations to address a high common components of TI programs, the methods in this percentage of the planning applications and decisions an report are specifically designed to be applied using less com- analyst is likely to confront for which travel-time, delay, and prehensive, less real-time data than is typically used for TI. In reliability information will provide useful decision support. order for reports or estimates of travel time to be useful to Chapter 8 provides additional guidance on reporting system users en route or planning an imminent trip, they performance results and incorporating those results into plan- need to be based on near real-time and historic data. In con- ning processes. Six typical planning applications are illustrated, trast, planning applications will be more reliable and useful if covering a large spectrum of likely applications for travel-time they are based on trends and on predictive relationships and reliability measures in planning. The approach to each ap- between commonly available data and system performance. plication is described in terms of the building blocks contained in Chapters 2 through 7. 1.4 Measuring Mobility and Reliability 1.3 Limitations of the Guidebook The need for meaningful mobility and reliability informa- The focus of this guidebook and its procedures are plan- tion is best satisfied by travel-time measures. Travel-time ning applications. These applications generally involve the measures do not preclude the use of other data, procedures, assessment of current or future performance for a large surrogates, or models when appropriate. The key is that the regional system of facilities or significant individual compo- set of mobility and reliability measures should satisfy the nents of such a system. The emphasis is on procedures that needs of analysts and decision makers, and the presentation provide no more precision in the results than is commensu- of that information should be tailored to the range of rate with the precision with which current measurements or audiences. future forecasts can be made for large systems of facilities and The decision process used by travelers to select trip modes whose data needs and analytical requirements are similarly and routes, and by the transportation or land use professional consistent with planning-level applications. analyzing alternatives, is influenced by travel time, conven- These procedures are not intended to replace or be equal in ience, user cost, dependability, and access to alternative travel precision to those procedures commonly used for the evalua- choices. Travel time also is used to justify capital and operating tion of individual intersections or road segments or even improvements. individual facilities. Rather, these procedures are intended to A system of performance measurement techniques that support a higher-level screening process used to identify defi- uses travel-time-based measures to estimate the effect of im- ciencies in existing and future system performance, and to provements on person travel and freight movement offers a identify types of improvements that would be most cost- better chance of satisfying the full range of potential needs effective at correcting these deficiencies. When the decision is than conventional level of service (LOS) measures. Techni- made to proceed with a specific project to correct a deficiency, cal procedures and data used to create the LOS measures can the agency designing the project will want to use more specific be adapted to produce time-based measures. The proce- and precise procedures for assessing whether the improve- dures were developed in a time when construction was ments meet agency performance objectives, engineering stan- typically the selected option. Operational improvements dards, cost constraints, and other relevant considerations. generally were implemented on a smaller scale and cost Where results of a systems planning-level assessment level. The more complicated situation that transportation conflict with the results of a detailed facility-specific analysis, professionals face in the 21st century means that new tech- the analysis using more precise data is generally more accu- niques and data are available, but the analysis needs are rate and reliable. However, the analyst should recognize the broader, must address transportation system management possibility of procedural or technical errors, regardless of the and operations, and often cross traditional modal and fund- extent and detail of the data employed in the analysis. ing category boundaries. Professional judgment and experience should be applied to Measuring mobility and reliability is a task performed in a the interpretation and validation of the results, regardless of variety of ways, in several different types of analysis, and for the level of detail of the analysis. many purposes. While the measures often are dictated by
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7 legislative or regulatory mandates, it is useful to view the A similar line of thinking applies at the detailed level (e.g., selection of the measure or measures as an important task street, bus route, or demand management program). While the to be accomplished before the data are collected and the improvement options may not be as broad, and the financial estimation or calculation procedures begin. This section investment may not be as great, it is always instructive to think identifies key elements necessary for a complete analysis that about desirable outcomes or adverse impacts before beginning includes travel time, speed, and reliability measures. As with the analysis. Not only will this ensure proper consideration of any process, the continuous evaluation of assumptions, all options, it also will lead to selection of measures that can methods, and techniques will lead to improvement; it is fairly evaluate the range of alternatives. important to compare the measures with the uses throughout It is this step where the expectations of the public and the process and adjust the measures as necessary. It also is policy makers can be formulated into a set of statistics that important to recognize that there are many analytical tech- can be used at the project or program evaluation level. The niques that relate to mobility and reliability measurement. "agreed upon norms" of the stakeholders are used to identify The steps outlined in this section are part of many of those broad outcome goals to be considered by the engineer, procedures. Exhibit 1.1 provides an overview of the three- planner, economist, or other professionals who must evalu- stage process to measure mobility and reliability. Each stage ate the need for an improvement. contains one to three considerations that are described in It is essential, therefore, that performance measures be more detail in the following subsections. For additional consistent with the goals and objectives of the process in information on each of the sections described in this chapter, which they are being employed. Performance measures are the reader is encouraged to review NCHRP Report 398, key to controlling process outcome, whether the process is Quantifying Congestion (1). alternative selection, congestion management, growth management, or system management and operation. For example, within congestion management, performance 1.4.1 Identify the Vision and Goals measures are used for problem identification and assess- The long-range plan for an area or system ideally contains ment, evaluation and comparison of alternative strategies, a description of the situation the public wishes to create demonstration of effectiveness, and ongoing system through investment, operation, and maintenance. As an im- monitoring. Variations of the same measure may satisfy a portant element of that plan, existing transportation facilities range of uses. must be analyzed, and improvements (if any) identified. In order for the selected programs and projects to move the area 1.4.2 Identify the Uses and Audiences toward the vision, the measures must enable the selection of transportation improvements of the type and scale appropri- The analyses and potential targets of the measurement ate to the situation. process must be determined before the proper mobility and STAGE 2A: Identify the Measures STAGE 1: STAGE 3: Develop a Set of Define the Problem and Identify Perform Analysis and Mobility Measures Preliminary Scope of Solutions Evaluate Alternatives Identify the Vision Collect or Estimate and Goals Data Elements Identify the Uses Identify and Audiences STAGE 2B: Problem Areas Identify Analysis Procedures Consider Test Solutions Possible Solutions Develop Analysis Procedures Lomax, T., et al. (1). Exhibit 1.1. Illustration of mobility and reliability analysis process (2).
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8 reliability measures can be selected. The set of measures must 1.4.4 Develop a Set of Mobility be technically capable of illustrating the problems and the and Reliability Measures effect of the potential improvements. They also must be able to be composed into statistics useful for the variety of poten- Many analyses, especially multimodal alternatives or re- tial audiences. Increasing the flexibility of the measures also gional summaries, require more than one measure to de- may improve the ability to use the information beyond the scribe the problem. Analyses of corridor improvements particular analysis. Corridor statistics also may satisfy annual might require travel time and speed measures to be expressed reporting requirements, for example. in person and freight movement terms. Some analyses are rel- Cost and schedule also are key considerations in the per- atively simple, and it may be appropriate to use only one formance measure development process. Different uses and measure. Analyses of traffic signal timing, where carpool and audiences frequently have different timelines for the delivery bus treatments are not part of the improvement options, of performance measure results. The available budget is a might not require person movement statistics--vehicle vol- related consideration. For example, consider the situation ume and delay information may be sufficient. if your state legislature were to mandate the development and Poor selection of measures has a high probability of lead- implementation of a statewide performance measurement ing to poor outcomes. In contrast, goals and objectives that program for all state facilities within six months. Clearly, are measured appropriately can guide transportation profes- the timeline is established (and short). The quality of the sionals to the best project, program, or strategy; analysts and answer will depend upon the available budget for person- policy-makers can then check (using evaluation results) that nel to develop the measures and accompanying estimation the goals and objectives are best served by the solutions procedures. In contrast to such a legislative directive, con- offered (3). sider an MPO that would like to fund a regional congestion management program that will develop and implement 1.4.5 Develop Analysis Procedures performance measures over a three-year period. Assuming adequate funding and all else is equal, certainly this exam- While the set of mobility and reliability measures is deter- ple provides more opportunity for delving deeper into po- mined by what we want to know, the accompanying analysis tential estimation methods, working with the public to iden- procedures are determined by what data are available or can tify performance measures that work for both technical and be obtained. As shown in Exhibit 1.1, identifying the analysis nontechnical audiences, and perhaps even identifying and procedures is often done at about the same time as identify- improving data sources than a three-month time frame. ing the performance measures. Analysis procedures vary based upon several factors, including the use and/or audi- ences and how this affects the level of accuracy or precision 1.4.3 Consider Possible Solutions required; budget and schedule; data formats; and data types. Before measure selection and data collection begins, it is When continuous data sources are available, the estimation useful to reflect on the problem areas and consider possible procedures typically comprise software programs that solutions. Possible solutions include potential projects, oper- compute the performance measures from archived data. ational programs, and policies. Understanding the possible Alternatively, in the absence of continuous data, performance solutions will help ensure that key considerations are vetted measures can be estimated by post-processing the output and understood as measures and procedures are established from transportation models (e.g., travel demand models, in the next step. The following questions should be given ini- economic analysis models). tial consideration at this stage and should be fully evaluated All estimation methods include quality control and quality with prototype results of the analysis. assurance of the input data, as well as reasonableness checks of the output. Analysis procedures can be expected to improve · Can all the improvement types be accounted for with the over time as the performance measurement program receives typical measures? feedback from analysts and users of the results and as data · Will the measures be able to illustrate the effect of the im- collection and/or data elements improve. provements by mode? · Are there aspects of the projects, programs, or policies that 1.4.6 Collect or Estimate Data Elements will not be covered by the measures? · Are the measures understandable to all the potential Data collection can proceed after an analysis of potential audiences? sources of information. The level of precision and statistical · Are the uses of the measures appropriate, and will the reliability must be consistent with the uses of the information procedures yield reliable information? and with the data collection sources. Estimates or modeling
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9 processes may be appropriate additions to traffic count, travel similar projects are performed. Inconsistencies or irregulari- time, and speed data collection efforts. Statistical sampling ties in results are sometimes a signal that different procedures procedures may be useful for wide area analyses, as well as for or data are required to generate the needed products. validating models and adapting them to local conditions. Direct data collection may be available from a variety of 1.4.9 Summary of Implementing Mobility sources, including specific corridor studies, real-time data and Reliability Measures collection, and annual route surveys of travel times. An areawide travel monitoring program will consist of The use of a set of mobility and reliability measures may both travel speed data collection and estimated speed infor- mean more computer-based analyses, which might be per- mation obtained from equations or models. The directly ceived as a move away from direct measurement for some collected data may be more expensive to obtain; statistical levels of analysis. This does not mean that travel-time data will sampling techniques will decrease the cost and improve the be less useful or less cost-effective to collect. On the contrary, reliability of the information. It may be possible to focus the direct measurement of travel time can be used to not only data collection on a relatively small percentage of the road- quantify existing conditions, but also to calibrate wide-scale way system responsible for a large percentage of the travel models of traffic and transportation system operation and to delay. Such a program would be supplemented with travel- perform corridor and facility analyses. Incorporating the time studies on a few sections of road and estimation important process elements into a sequence of events leading procedures on the remainder of the system. up to a public discussion of alternative improvement plans might result in a series of steps like the following: 1.4.7 Identify Problem Areas · Existing traffic and route condition data are collected The collected data and estimates can be used to develop directly. measures that will illustrate the problem areas or situations. · Measures are calculated. These should be compared to observations about the system · Results are compared to target conditions determined to make a reasonableness check; the measures should identify from public comments during long-range plan discussion. well-known problem areas. The data will provide informa- · Trip patterns, areas, and modes that need improvement tion about the relative size of the mobility and reliability are identified. problems so that an initial prioritization for treatment can be · Solutions are proposed. Areawide strategies should guide made. the selection of the type and magnitude of specific solutions. · A range of the amount and type of improvements is tested. 1.4.8 Test Solutions · Mobility and reliability measures are estimated for each Testing the potential solutions against the mobility and strategy or alternative, including forecasts of future values reliability measures during the data collection process may of measures as appropriate to the application. improve the data collection effort and the ultimate results. · Measures are compared to corridor, subarea, and regional After data collection and estimation are complete, testing so- goals. lutions for effect will be another chance to determine the need · Individual mode or facility improvements that fit with the to modify mobility and reliability measures. Even after the areawide strategy are identified for possible inclusion in the analysis is complete, the measures should be evaluated before plan, subject to financial analyses.