Incorporating Reliability Performance Measures into the Transportation Planning and Programming Processes: Technical Reference (2014) / Chapter Skim
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52 OVERVIEW OF TRAVEL TIME RELIABILITY Color versions of the figures in this chapter are available online: http://www.trb.org/Main/Blurbs/168856.aspx Travel time reliability is a significant aspect of transportation system performance. Reliability is important to travelers and transportation practitioners for a variety of reasons.
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6INCORPORATING RELIABILITY PERFORMANCE MEASURES INTO THE TRANSPORTATION PLANNING AND PROGRAMMING PROCESSES: TECHNICAL REFERENCE • Project C04 (Improving Our Understanding of How Highway Congestion and Pricing Affect Travel Demand) defined reliability as "the level of (un)
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7INCORPORATING RELIABILITY PERFORMANCE MEASURES INTO THE TRANSPORTATION PLANNING AND PROGRAMMING PROCESSES: TECHNICAL REFERENCE In recent years, some non-U.S. reliability research has focused on another aspect of reliability: the probability of failure, where failure is defined in terms of traffic flow breakdown.
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8INCORPORATING RELIABILITY PERFORMANCE MEASURES INTO THE TRANSPORTATION PLANNING AND PROGRAMMING PROCESSES: TECHNICAL REFERENCE times are affected by interaction of demand fluctuations, traffic control devices, traffic incidents, inclement weather, work zones, and physical capacity. Travel time reliability also can be used to compare current conditions with historical conditions: Is the travel time today "typical" of what happens or is it better than the usual or near-worst case.
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10 INCORPORATING RELIABILITY PERFORMANCE MEASURES INTO THE TRANSPORTATION PLANNING AND PROGRAMMING PROCESSES: TECHNICAL REFERENCE Figure 2.1. Reliability metrics within a travel time distribution.
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11 INCORPORATING RELIABILITY PERFORMANCE MEASURES INTO THE TRANSPORTATION PLANNING AND PROGRAMMING PROCESSES: TECHNICAL REFERENCE Figure 2.2. Skew statistic within a travel time distribution.
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12 INCORPORATING RELIABILITY PERFORMANCE MEASURES INTO THE TRANSPORTATION PLANNING AND PROGRAMMING PROCESSES: TECHNICAL REFERENCE • The 80th percentile travel time index has not been widely used. However, SHRP 2 L03 found that it can be more sensitive to operational changes than the 95th percentile and recommended its inclusion.
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14 INCORPORATING RELIABILITY PERFORMANCE MEASURES INTO THE TRANSPORTATION PLANNING AND PROGRAMMING PROCESSES: TECHNICAL REFERENCE relative contribution each of these sources makes on overall congestion in a typical urban area. Historically, the transportation planning process has focused on assessing system performance by comparing the base system capacity with average demand on a typical day in order to generate average travel times that formed the basis for comparison and prioritization of system investments.
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15 INCORPORATING RELIABILITY PERFORMANCE MEASURES INTO THE TRANSPORTATION PLANNING AND PROGRAMMING PROCESSES: TECHNICAL REFERENCE to consider these impacts would result in a greatly understated estimation of project value. Likewise, if practitioners ignore the impacts of congestion on nontypical days, or fail to account for situations where specific congestion mitigation strategies (e.g., incident management systems)
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16 INCORPORATING RELIABILITY PERFORMANCE MEASURES INTO THE TRANSPORTATION PLANNING AND PROGRAMMING PROCESSES: TECHNICAL REFERENCE • It is clear that traditional capacity projects improve reliability, and failure to account for this effect in economic analyses has resulted in the exclusion of these impacts in the accounting of the full benefits to users. • Management and operations strategies designed to minimize disruptions (e.g., incident management)
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17 INCORPORATING RELIABILITY PERFORMANCE MEASURES INTO THE TRANSPORTATION PLANNING AND PROGRAMMING PROCESSES: TECHNICAL REFERENCE Other examples of highway improvements that can improve reliability include ramp metering and electronic toll collection. For arterials, other examples include access management, advanced signal systems, and parking restrictions.
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18 INCORPORATING RELIABILITY PERFORMANCE MEASURES INTO THE TRANSPORTATION PLANNING AND PROGRAMMING PROCESSES: TECHNICAL REFERENCE As travel time reliability performance measures and strategies are incorporated into the transportation planning and programming process, the effects need to be included in the monetized benefits to better understand the project's need given funding constraints. To integrate travel time reliability into a benefit–cost analysis, the following data are needed: • A measure for travel time reliability; • A value of time related to reliability; • A method for predicting future reliability; and • A method for estimating changes in reliability due to a project.
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