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5 support for the management and stewardship of transporta- straight-forward to use and can be driven with existing and tion systems. This trend responds to calls for increased readily available data sources. To date, much of the work on accountability for expenditure of public funds, better consid- travel-time-based measures has focused on utilizing relatively eration of user and stakeholder priorities in selecting from comprehensive and deep data sets generated for traffic man- among competing project opportunities, and a rational desire agement systems via continuous, automatic data collection to improve the quality of information upon which such deci- processes. This guidebook strives to present methods for sions are based. At the same time, system users--the traveling generating similar measures using data that are more likely to public, as well as commercial operators--are increasingly be readily available to the typical transportation planning or sensitive to delay and unreliable conditions. By measuring operating agency. travel-time performance, and related system metrics based on Much work previously has been conducted to develop travel time, agencies will be better able to plan and operate effective measures of congestion, and to present the data their systems to achieve the best result for a given level of collection and analysis methods required to generate the meas- investment. At the same time, travelers, shippers, and other ures. More recently, measures of reliability have similarly been users of those systems will have better information for plan- studied and published, making better use of continuous data ning their use of the system. sources. References to these other excellent resource docu- Agencies are seeking to develop and employ system ments are made where additional detail and context would be performance measures that express congestion and mobility useful to some users. We find, however, that most of the exist- in terms that decision makers and system users can appreci- ing published work on congestion and reliability measurement ate and understand. Interest specifically in measures of travel focuses on monitoring and reporting existing values and time, delay, and reliability is increasing, as system users seek historical trends, and not on application of the measures to the to gain more control over their trip making decisions and "what-if" type of questions prevalent in system planning. This outcomes. Interest also is increasing in measurements that guidebook, and NCHRP Project 7-15 on which it is based, individuals can use to reduce the uncertainty and loss of strive to help fill the need for practical advice on use of relevant productivity that occur when system reliability is low. mobility and reliability measures in typical planning applica- This growing demand for available measures of mobility tions. The main objective of these applications is to inform a and congestion that are travel time-based and user-friendly planning process (e.g., to identify needs and suggest appropri- has pointed out the need for improved monitoring and ana- ate solutions) and support decision-making about some future lytical procedures to generate the measures. These methods action or investment in the transportation system. Thus, this need to be able to measure and predict how individual trav- guidebook places more emphasis on estimating and forecast- elers and goods movements will be affected by incidents and ing future values of performance measures and comparative other sources of nonrecurring delay, as well as by capital and analysis of hypothetical situations. operational improvements to different components of the transportation system. 1.2 How to Use the Guidebook Use of travel time, delay, and reliability as performance meas- ures is hampered by complex data requirements, data accuracy This guidebook is intended for use by analysts familiar issues, and inadequate procedures for incorporating these with various forms of quantitative analysis, including basic measures into the transportation planning process. One reason statistical analysis. The information presents the fundamen- these measures have not been more widely implemented is they tal steps necessary to conduct the most common planning can be expensive and difficult to generate. A relatively small per- analyses for which travel-time-based measures can be useful. centage of public transportation planning agencies have the The remainder of Chapter 1 presents an orientation to the data collection programs or analytical forecasting capabilities to process of measuring mobility and reliability. While the generate reliable estimates of these measures. In many states, material in Section 1.4 may be familiar to many readers, it is travel-time data are available for relatively few corridors. The useful to repeat the logical sequence of activities that describe high costs associated with more comprehensive data collection performance-based planning analysis. This process starts programs deter many states from investing in such programs. with the guiding vision or goals, and proceeds through such States and MPOs are using loop detector data and other data essential steps as identifying the audience; considering possi- collected by intelligent transportation systems (ITSs) or traffic ble solutions; selection and calculation of performance meas- management systems (TMCs) to develop travel time, delay, and ures; testing alternatives; and summarizing results. This reliability measures, but these efforts too are fairly sophisticated, discussion provides a point of departure for more detailed limited in extent, and at present, costly. material that follows. As a result, agencies are in need of methods for generating The common elements of typical planning applications are travel-time-based performance measures that are relatively explained in detail in Chapters 2 through 7, where specific