Incorporating Travel Time Reliability into the Highway Capacity Manual (2014) / Chapter Skim
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1This final report documents the activities performed during SHRP 2 Reliability Project L08: Incorporating Travel Time Reliability into the Highway Capacity Manual. It serves as a supplement to the proposed chapters for incorporating travel time reliability into the Highway Capacity Manual prepared for this same project.
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2that a sufficient history of travel times be present to track travel time performance. This history is described by the travel time distribution for a given trip.
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3been established, including standard statistical measures (e.g., standard deviation, kurtosis) , percentile-based measures (e.g., 95th percentile travel time, buffer index)
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4facility. Performance measures based on the average travel time are therefore deemed to be less appropriate for HCM analysis.
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5The largest shaded oval and dotted line represent the current implementation of the HCM freeway facilities chapter, with study period data specific to the facility being studied entered directly into FREEVAL-RL for analysis of (predominantly) recurring congestion effects.
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6Once all of the scenarios associated with the reliability reporting period have been generated, they are evaluated during the facility evaluation stage. The urban streets computational engine is used to automate the calculations.
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7travel time. In other words, they generate a travel time distribution instead of a deterministic and fixed travel time, as would be obtained by running a single study period.
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8addition, these scenarios tend to have exceptionally large TTI values that significantly shift the tail of the cumulative distribution to the right (i.e., toward higher TTI values)
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9highways, and urban streets that cross the corridor and provide connections between the corridor's facilities will also be affected; however, those effects are beyond the scope of a reliability analysis. The focus of a corridor evaluation is on the parallel facilities.
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10 define level of service. When reliability is considered as a service measure, the research team recommends that the reliability rating (now a performance measure)
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11 Research to Improve the Reliability Submodels Research is needed to understand and quantify the impact of weather, work zones, or special events on traffic demand: • The method assumes that incident rates and weather conditions are independent. Research is needed to develop models that can explain the relationship.
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