Analytical Procedures for Determining the Impacts of Reliability Mitigation Strategies (2012) / Chapter Skim
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1Project Background The fundamental objective of SHRP 2 Project L03 was to develop predictive relationships between highway improvements and travel time reliability. In other words, how can the effect of an improvement on reliability be predicted?
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2reliability has taken on increasing importance as variation in travel times is now understood as a separate component of the public's and business sector's frustration with congestion problems. Reliability is a major part of system performance and of travelers' perceptions of performance.
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3automated equipment is the only feasible method of data collection. Because of the cost of collecting new data, the team relied on data already being collected by transportation agencies, primarily in support of operations programs.
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4events, scheduled and unscheduled construction, and other events that could affect traffic conditions (e.g., severe weather or transit delays)
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5Analysis Approach The analysis was based on a conceptual model previously developed by members of the research team (Figure ES.3)
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6Findings Data Set Compilation and Usage The large and comprehensive data set included many levels of aggregation and summarization. Traffic data from urban freeways comprised the largest portion of the data set and included the original measurements from roadway detectors (5-minute intervals by lane)
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7highway section or trip. An examination of the distributions from the research study section reveals several characteristics: • The shape of the travel time distribution for congested peak times (nonholiday weekdays)
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8travel time reliability (increase in the variance) is extreme and abrupt enough to suggest it is a vertical function, with a nonsingular relationship to further volume increases.
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9Reliability of Urban Trips Based on the Reliability of Links For extended travel on urban freeways (trips of 10 to 12 miles in length) , the reliability of the entire trip can be predicted as a function of the reliability of the links that comprise the trip.
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10 Table ES.5. Summary of Urban Freeway Before-and-After Studies Case No.
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11 Two model forms were developed: simple and complex. The simple model form relates all the reliability metrics to the mean TTI for the three highway types studied (urban freeways, rural freeways, and signalized arterials)
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12 In the Seattle area, many incidents take place during peak periods, causing already existing congestion to grow worse as a result of the interwoven effects of incidents, bad weather, and traffic volumes on travel times. In addition, all types of disruptions to normal roadway performance (rain, crashes, and noncrash incidents)
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13 SHRP 2 contractors suggest that capacity varies even in the absence of disruptions (4)
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