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CHAPTER 10 Conclusions and Suggested Research 10.1 Conclusions The study team and stakeholders involved in the Southern California pilot site agree that considering reliability in the decision-making process would be a significant step forward. Travel time reliability is important to customers, so it should be an important decision criterion for transportation agencies. Current tools, such as travel demand and microsimulation models, do not allow agencies to predict how projects will improve reliability. The SHRP 2 research has generated a number of tools and analysis techniques to help agencies understand reliability issues and estimate the reliability impacts of projects. The study team found that that the exercise of identifying reliability factors can help agencies better understand freeway facilities. Reliability factor analysis provides useful information for understanding the causes of reliability issues along freeway facilities. Tool Modifications Calibrating the SHRP 2 tools to baseline conditions is a critical first step before any reliability analysis can occur. The reliability tools show promise for analyzing travel time reliability on highway facilities, but they need modifications to their user interfaces and calculation analytics before they are ready for implementation by transportation agencies. The Southern California pilot site provided the SHRP 2 Reliability program with a list of âquick fixesâ that are critical for moving the tools toward implementation. The detailed findings in this report support those quick fixes. Below are a few common threads of modifications needed across the tools: ⢠There needs to be guidance for how to calibrate each tool to baseline conditions. This guidance should cover how to identify study areas and analysis periods as well as which levers allow the tool to be calibrated. ⢠These calibration levers must be easily accessible to the user. The Southern California study team identified several variables, such as capacity, capacity adjustment factors, and demand by hour of day, that can be used to calibrate models. Other pilot sites may identify other variables. In some tools, these calibration levers are hidden from the user or not presented directly on the input pages. They need to be made more easily accessible. ⢠The tools need modifications to support scenario analysis. These modifications should include the ability to designate specific time periods and highway segments for analysis. Practitioners need the ability to tailor reliability analyses to support existing studies and analysis. In addition, tool users need to be able to adjust time periods and highway segments and then rerun analyses without having to set up the models from scratch. 197
⢠A related need is the ability to import and save data. The reliability tools tested in the Southern California pilot site need varying amounts of data, but generally require the user to enter information through pull-down menus or manual entry. For the tools that require large amounts of information (e.g., FREEVAL-RL), the ability to copy and paste data or import data from external spreadsheet files would significantly aid the calibration and scenario processes. ⢠The tools also need analytics to support specific types of operational improvements. For Southern California, the most common operational projects to test include ramp metering strategies, auxiliary lanes, freeway connectors, and ramp modifications. The other three pilot sites may identify additional project types. The SHRP 2 reliability tools should be able to analyze the projects that users are most likely to want to test. ⢠The tools need to be able to model reliability for highly congested facilities. Practitioners are most likely to want to test reliability on these facilities, yet the extent and duration of congestion in Southern California exceeded the ability of some SHRP 2 tools to model congestion. ⢠The tools need to support life-cycle benefit-cost analysis. Benefit-cost analysis needs the sum of user benefits over an expected life cycle. The current tools either estimate benefits for specific years or assume that the current benefits remain constant over the life cycle. The tools should be modified so that agencies can simply take the sum of the reliability benefits and add them to standard benefit-cost analysis. Benefit-Cost Analysis The study team found that reliability could be added to existing benefit-cost analysis, but only with external analysis of the SHRP 2 tool results. The team was unable to take the reliability benefit calculations directly from any of the SHRP 2 tools analysis, but this shortcoming can be addressed with a few simple tool modifications. The addition of reliability benefits did not change the rank order of the projects tested at the Southern California pilot site. However, reliability can change the ranking of two projects with similar benefits, and it can make marginal projects cost beneficial (i.e., increasing the benefit-cost ratio to above one). The study team discovered that when the C11 tool is used to estimate reliability benefits, these benefits fall within the 29 percent to 36 percent range of the mobility (recurring delay) benefits. This suggests that agencies could adopt one of two techniques for including travel time reliability in benefit-cost analysis: ⢠Use the C11 tool. Agencies model facility performance using traditional tools, such as travel demand or microsimulation models. The C11 tool can be used to estimate reliability changes by logical segments (e.g., defined by bottlenecks or highway geometry). The mobility benefits are adjusted to match the traditional tools and the resulting reliability improvement is reported as part of the benefit-cost analysis. 198
⢠Use a 30-percent rule of thumb. Agencies estimate reliability benefits to the facility by multiplying mobility benefits by 30 percent. Southern California stakeholders have expressed a desire to measure reliability impacts directly rather than use a rule of thumb, but the 30 percent rule was consistent for both facilities and among different project types evaluated using the C11 tool. The FREEVAL-RL tool may produce different results, but the study team was unable to use the model, given the complex geometries along the I-5 and I-210 facilities. The stakeholder agencies involved with the Southern California testing preferred the use of simpler tools for reliability analysis. Reliability Performance Measures There is a need for common reliability performance measures across the planning process. The SHRP 2 reliability research has resulted in different measures and tools, but it has also resulted in inconsistency if they are applied to a practical planning problem. Figure 10.1 summarizes a simple planning process and the inconsistency in measures when the SHRP 2 tools are applied. Figure 10.1. Inconsistency in performance measures for planning process. The L02 project provides a number of methods for analyzing travel time reliability and suggests that users present results in terms of cumulative distribution functions (CDF) or semi- variance. Other SHRP 2 projects (e.g., L07, L08, and C11) provide tools for testing the reliability impacts of potential projects. While these tools summarize the reliability impacts in a variety of performance measures, the measures differ from the ones used to identify the travel time reliability problem in L02. 199
