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10 CHAPTER 2 Selecting Appropriate Performance Measures 2.1 Introduction Because of the wide range and diversity of available measures, it is important to have a clear basis for assessing and compar- This chapter helps the user to understand the range of ing mobility and reliability measures. Such an evaluation performance measures or metrics available to measure and makes it possible to identify and separate measures that are monitor travel time, delay, and reliability, and to identify useful for an analytical task from measures that are either less appropriate metrics for a given application, taking into ac- useful or inappropriate for certain analyses. count factors such as data availability and the intended use or audience for the results. We have adopted the terms "mobil- ity" and reliability, because these are the desirable outcomes 2.2.1 Choosing the Right Mobility sought for the transportation system user. "Travel time" and and Reliability Measures "delay" and the variability in those two quantities are key The ideal mobility and reliability measurement technique determinants of mobility and reliability. for any combination of uses and audiences will include the A system of mobility and reliability measures should be features summarized in Exhibit 2.1. These issues should be developed only after an examination of the uses and audiences examined before data are collected and the analysis begins, to be served, the consideration of program goals and objectives, but after the analyst has considered all reasonable responses and identification of the nature or range of likely solutions. to the problem or issue being studied. Having an idea of what This chapter illustrates a system of travel-time-based measures the possible solutions are will produce a more appropriate set to estimate mobility and reliability levels. These procedures of measures. are useful for roadway systems, person and freight move- ment modes, and transportation improvement policies and programs. The user should consider the way that measures 2.2.2 The Data Collection Issue might be used before selecting the appropriate set of mo- Concerns about the cost and feasibility of collecting travel- bility and reliability measures. time data are frequently the first issues mentioned in discus- The following sections describe techniques for measuring sions of mobility and reliability measures. There are many ways mobility and reliability on various portions of a transportation to collect or estimate the travel time and speed quantities; data network. Some of the material in this chapter has been collection should not be the determining factor about which excerpted from the Keys to Estimating Mobility in Urban Areas: measures are used. While it is not always possible to separate Applying Definitions and Measures That Everyone Understands, data collection issues from measure selection, this should be and the reader is encouraged to review that source for more the goal. Chapter 3 discusses data collection in more detail. detailed background information (2). 2.2.3 Aspects of Congestion, Mobility, 2.2 Measure Selection and Reliability Given a basic understanding of the performance measure- The selection of a proper set of mobility and reliability ment process as described in Chapter 1, this section provides measures includes an assessment of what traveler concerns several considerations that can be used to identify the most are most important. This assessment can be drawn from appropriate mobility and reliability measures for a situation. experiences with measuring congestion in roadway systems.

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11 Checklist Item Short Discussion Relate to goals and objectives The measures must indicate progress toward transportation and land use goals that the project or program attempts to satisfy. Measuring transportation and land use characteristics that are part of the desired future condition will provide a continual check on whether the area is moving toward the desired condition. Clearly communicate results to While the technical calculation of mobility and reliability information may require audiences complicated computer models or estimation techniques, the resulting information should be in terms the audience can understand and find relevant. Include urban travel modes Mobility and reliability are often a function of more than one travel mode or system. At least some of the measures should contain information that can be calculated for each element of the transportation system. The ability to analyze the system, as well as individual elements, is useful in the selection of alternatives. Have consistency and accuracy Similar levels of mobility and reliability, as perceived by travelers, should have similar mobility and reliability measures. This is important for analytical precision and also to maintain the perception of relevancy with the audiences. There also should be consistency between levels of analysis detail; results from relatively simple procedures should be similar to those obtained from complex models. One method for ensuring this is to use default factors for unknown data items. Another method is to frequently check expected results with field conditions after an improvement to ensure that simple procedures those that use one to three input factors produce reasonable values. Illustrate the effect of The improvements that may be analyzed should be consistent with the measures improvements that are used. In relatively small areas of analysis, smaller urbanized areas, or portions of urban areas without modal options, this may mean that vehicle-based performance measures are useful. Using a broader set of measures will, however, ensure that the analysis is transferable to other uses. Be applicable to existing and Examining the need for improvements to current operations is a typical use of future conditions mobility and reliability measures that can be satisfied with data collection and analysis techniques. The ability to relate future conditions (e.g., design elements, demand level, and operating systems) to mobility and reliability levels also is required in most analyses. Be applicable at several A set of mobility and reliability measures should include statistics that can illustrate geographic levels conditions for a range of situations, from individual travelers or locations to subregional and regional levels. Using quantities that can be aggregated and averaged is an important element of these criteria. Use person- and goods- A set of measures should include factors with units relating to the movement of movement terms people and freight. In the simplest terms, this means using units such as persons and tons. More complex assessments of benefits will examine the different travel patterns of personal travel, freight shipping, and the intermodal connections for each. Use cost-effective methods to Using readily available data or data collected for other purposes is a method of collect and/or estimate data maximizing the usefulness of any data collection activities. Focusing direct data collection on significant problem areas also may be a tactic to make efficient use of data collection funding. Models and data sampling procedures also can be used very effectively. Exhibit 2.1. Checklist of considerations for mobility and reliability measure selection (1). A set of four aspects of congestion was discussed at the Work- components that interact in a congested roadway or sys- shop on Urban Congestion Monitoring (4) in May 1990, as a tem (1). These components are duration, extent, intensity, and way to begin formulating an overall congestion index. These variation. They vary among and within urban areas. Smaller four components provide a useful framework for mobility urban areas, for example, usually have shorter duration than and reliability estimation procedures as well. larger areas, but many have locations with relatively intense congestion. The four components and measurement concepts that can be used to quantify them are discussed below. 2.2.4 Summarizing Congestion Effects Using Four General Components 1. Duration. This is the length of time during which conges- While it is difficult to conceive of a single value that will tion affects the travel system. The peak hour has expanded describe all travelers' concerns about congestion, there are four to a peak period in many corridors, and mobility and

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12 reliability studies have expanded accordingly. The meas- 2.2.5 Summarizing Mobility and urement concept that illustrates duration is the amount of Reliability Effects Using Four time during the day that the travel speed indicates con- General Components gested travel on a system element or the entire system. The travel speed might be obtained in several ways depending Developing a summary of mobility and reliability using on data sources or travel mode being studied. concepts similar to those used for congestion will ensure that 2. Extent. This is described by estimating the number of people the appropriate measures are used. A similar typology uses or vehicles affected by congestion and by the geographic dis- different terms; there is a positive tone in the phrasing of the tribution of congestion. The person congestion extent may definitions and a slightly different orientation from conges- be measured by person-miles of travel or person-trips that tion, but the aspects are basically the same. The image of a box occur during congested periods. The percent, route-miles, also is appropriate to the description of the amount of or lane-miles of the transportation system affected by con- mobility and reliability provided by a transportation and land gestion may be used to measure the geographic extent of use system. The axes are time, location, and level. Reliability mobility and reliability problems. is now the change in box volume. 3. Intensity. The severity of congestion that affects travel is a measure from an individual traveler's perspective. In con- Time. The time that mobility and/or reliability is provided cept, it is measured as the difference between the desired or available is an expression of the variation of mobility condition and the conditions being analyzed. and/or reliability through the day, week, or year. It can be 4. Variation. This key component describes the change in a function of the existence of congestion or the presence of the other three elements. Recurring delay (the regular, transit service, operational improvements, or priority daily delay that occurs due to high traffic volumes) is rel- treatments. It can be measured as the times when travelers atively stable. Delay that occurs due to incidents is more can get to their destinations in satisfactory travel times. difficult to predict. Location. The places or trips for which mobility and relia- bility are available is an important aspect of measurement The relationship among the four components may be for transportation and land use analyses, as well as for thought of as a three-dimensional box describing the magni- other issues such as economic development and social eq- tude of congestion. Exhibit 2.2 illustrates three dimensions-- uity. It can be described by accessibility maps and statistics duration, extent, and intensity--of congestion. These pre- and travel time contours that illustrate the areas that can sent information about three separate issues: 1) how long be traveled to in a certain period of time. Descriptions of the system is congested, 2) how much of the system is af- transit routes or special transportation services also can be fected, and 3) how bad the congestion problem is. The vari- used to identify locations where mobility and reliability are ation in the size of the box from day to day is a measure possible by more than private auto modes. or indicator of reliability, i.e., the more extreme and un- Level. The amount of mobility and reliability provided is predictable the variation from one time period to another, analogous to the intensity of congestion. The amount of the poorer the reliability of the facility or system being time it takes to travel to a destination and whether this is measured. satisfactory are the key elements of the level of mobility and Duration Extent Volume of the box is a measure of magnitude of congestion; smaller volume is better. Variation in volume of the box is an indication of reliability. Intensity Exhibit 2.2. Components of congestion (2).