Identification and Evaluation of the Cost-Effectiveness of Highway Design Features to Reduce Nonrecurrent Congestion (2014) / Chapter Skim
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Pages 24-39
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24 Overview The cumulative distribution function of the travel time index (TTI-CDF) curve was introduced in Chapter 1 as the fundamental diagram from which reliability statistics can be computed.
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25 actually traveled on the freeway section)
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26 In some cases, the analyst may be aware that, due to extreme volume fluctuations or the presence of major traffic generators, the 30th-highest hour is higher than Equation 4.3 would suggest. One way to illustrate this issue is to consider special events.
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27 the relative proportions of noncrash incidents are based on Project L07 discussions with highway agencies. Diurnal Distribution of Nincidents.
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28 Calculating Nblocked. To calculate Nblocked,i (the average number of lanes blocked per incident for each incident type i)
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29 result in longer queues and therefore queue discharge times (all else being equal)
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30 in the travel time reliability models to predict various TTI percentiles, or points along the cumulative TTI curve. These models are designed to be applied for single-hour timeslices.
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31 Table 4.5. TTI Prediction Model Coefficients N (percentile)
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32 Class II is further subdivided into six cases, as described in the following subsections. These cases are not necessarily exhaustive, but they cover the relevant nonrecurrent congestion design treatments studied and provide a guide that could be extrapolated to other types of ILHL-reducing treatments.
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33 these cases, ILHL* is composed of three terms: one for incidents unaffected by the treatment; one for incidents affected by the treatment but before treatment implementation with a duration until conversion T*
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34 it is assumed that incidents for which this treatment would be deployed have a longer-than-average duration. Ttreatable is used in a different way with Case IIF than it is used in Cases IIB and IIE, as discussed below.
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35 applied to any treatment or operational strategy that can be mapped to at least one of the four variables in the TTI prediction models. Figure 4.5, presented earlier as Figure 4.1, is re–presented here to illustrate the process that leads to the final calculation of operational benefits.
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36 a Italicized values are user-modifiable in the Analysis Tool. b There are many varieties of wildlife collisions, each with its own set of potential treatments and effects.
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37 Figure 4.4. Comparison of treated and untreated TTI curves.
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38 Therefore, the travel time savings (∆TTn) -- and by implication, delay reduction -- at a given percentile n can be calculated as shown by Equation 4.38: L S n n n n nTT TT TT TTI TTI 4.38 FF ( )
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39 However, the standard deviation is the measure suggested by the literature for calculating the economic value of reliability. Change in Other Reliability Measures The cumulative TTI curves for the treated and untreated conditions can also be used to derive any of the remaining reliability indicators presented in Chapter 1, most of which are indices.
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