Identification and Evaluation of the Cost-Effectiveness of Highway Design Features to Reduce Nonrecurrent Congestion (2014) / Chapter Skim
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4Background The Reliability area of the second Strategic Highway Research Program (SHRP 2) has focused on the need to improve travel time reliability on freeways and major arterials.
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5 Nonrecurrent congestion has not traditionally been addressed through highway design. In recent decades, operational solutions such as intelligent transportation systems and incident management techniques have been the chief means of combating nonrecurrent congestion.
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6is 60 mph (meaning a vehicle could traverse the segment in 2.0 min) , and a vehicle traverses the segment in 2.4 min, then the TTI for that vehicle is the ratio of 2.4 to 2.0 min (i.e., TTI = 1.2; note that as a ratio of two quantities measured in consistent units of time, TTI is a unitless index)
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7 and extends the discussion to other measures that have been derived from the travel time distribution. Mean-Based Measures Certain measures, such as mean TTI and the lateness index, relate to the mean of the travel time distribution.
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8SeMivariance Although calculating the variance about the mean (as shown by Equation 1.2) is fairly common statistical practice, describing how travel times differ from the mean is potentially not as useful as describing how they differ from the ideal.
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9 PercenTage of TriPS on TiMe Percentage of trips on time (labeled "PTOT" in Figure 1.6) essentially works in the reverse direction of the PTI and MI, in effect specifying a target TTI and then extracting the corresponding percentile from the cumulative TTI curve.
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10 that very different distributions can have identical BIs and skew statistics, respectively. Summary of Reliability Indicators As discussed above, it is not merely unreliability, but undesirable unreliability that must be quantified.
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11 from the National Climactic Data Center; a literature review; and interviews with state highway agency staff. Chapter 3 also lists all the nonrecurrent congestion design treatments considered in this research.
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