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23 Mobility and Reliability Measures Buffer Index Travel-Time Accessibility Travel Time Travel Rate Total Delay Congested Congested Congested Percent of Delay per Roadway Traveler Travel Travel Index Uses of Mobility and Reliability Measures Basis for government P P P P P P P P investment or policies Basis for national, state, or P P P P P P P P regional policies and programs Information for private-sector P P S P P S P P P decisions Measures of land development P P P P P P S S S P impact Input to zoning decisions P P P P P Inputs for transportation P P P models Inputs for air quality and P P P P energy models Identification of problems P P P P P S S S S Base case (for comparison with S P P P S S S P improvement alternatives) Measures of effectiveness for P P P P P S S S P alternatives evaluation Prioritization of improvements P P P P S Assessment of transit routing, scheduling, and stop P P P P P S placement Assessment of traffic controls, P P P geometrics, and regulations Basis for real-time route choice P P P P P decisions P = Primary measure; and S = Secondary measure. Exhibit 2.7. Recommended mobility and reliability measures for various types of analyses (1). the analysis (1). Travel time and speed measures, and data cally cover larger areas or multiple modes and those target- and estimating techniques used to create them, are flexible ing nontechnical audiences, use index measures and sum- analysis tools. When combined with person and freight mary statistics. movement quantities, they illustrate a range of mobility and reliability situations. Different values will be used for the tar- 2.12 Index Measure Considerations get travel rate or target travel time depending on the facility type or travel mode, but the calculation and application of the Following are a few additional considerations to take into measures are identical. account when using performance measures, particularly While it is difficult to cover every type of mobility and those dimensionless indexes such as the TTI or BI that are not reliability analysis, Exhibit 2.4 illustrates recommended expressed in familiar units such as minutes or miles per hour. measures for many common types of studies and informa- Setting targets or benchmarking to a regional or national tion requirements. As with Exhibit 2.6, the analyses where standard is one possible approach. Expressing targets and small areas are analyzed or quick answers are needed use performance results in a user-familiar context such as the simple measures. More complex analyses, those that typi- door-to-door trip time is another.
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24 2.12.1 How Much Congestion Is Too Much? determinations of service quality. Ten pairs of O-D trip pat- terns, for example, could be used to show the change in travel Analyses of system adequacy, the need for improvements, time. The information for these key travel patterns can be or time-series analyses conducted in a corridor or area can updated daily, monthly, or annually with system monitoring benefit from comparisons using target conditions. equipment. Every five years the key patterns could be reexam- Free-flow conditions will not be the goal of most large ined for relevance to the existing and future land use develop- urban transportation improvement programs, but using ment patterns and transportation system. them provides a consistent benchmark relevant for year-to- Using target conditions as the comparison standard pro- year and city-to-city comparisons. The attainment of goals vides the basis for a map or table showing system deficien- standard also might be used at the national or state level, but cies in a way readily understood and uniquely relevant to will be used more often during a discussion of planning and improvement analyses. A map showing the target travel project prioritization techniques. rates on the system links would accompany such a presen- The use of a target travel rate can improve the guidance pro- tation. This approach also could be easily used in a multi- vided to system planners and engineers. If the target travel rates modal analysis, with a target TTI for the corridor. Future are a product of public discussion, they will illustrate the bal- travel rates for the corridor can be changed by improving ance the public wishes to have between road space, social ef- a facility or service, or by shifting travel to other modes/ fects, environmental impacts, economic issues, and quality of facilities. The target comparison standard would be broader life concerns. Areas or system elements where the performance than simply a mobility or reliability measure since it would is worse than the target can be the focus of more detailed study. directly incorporate the idea that the goal for a corridor is A corridor analysis, for instance, might indicate a problem with not always high-speed travel. It could be used in conjunc- one mode, but the solution may be to improve another mode tion with an areawide planning effort to relate the link or program that is a more cost-effective approach to raising the speeds, used in estimating the TTI, to the outcome measures corridor value to the target. The amount of corridor or area- of door-to-door trip satisfaction. wide person-travel that occurs in conditions worse than the locally determined targets can be used to monitor progress toward transportation goals and identify problem areas. 2.12.3 Impact on Data Collection One outcome of a move to the travel-time-based measures would be the ability to include directly collected travel-time 2.12.2 Relationship to Door-to-Door data from the various transportation system elements. Many Travel-Time Measures areas do not collect this information, but the initial statistics The measure of system performance closest to the concern can be developed from estimates of travel speed. As travel- of travelers is door-to-door travel time. Any performance time studies are conducted or archived data systems devel- measure should relate to door-to-door travel time as closely as oped, the actual data can be used to replace the estimates in possible. Calibrating the user view of system performance with the index, as well as to improve the estimation processes. The measures that can be more readily collected from existing data information derived from systems that automatically collect sources is the key to the efficient and effective presentation of and analyze travel speed over sections of freeways provide a mobility and reliability information. Periodic updates of significant resource for travel-time-based performance meas- public opinion can be used to adjust corridor and areawide ure calculation.