Incorporating Travel Time Reliability into the Highway Capacity Manual (2014) / Chapter Skim
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Pages 205-215
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205 Incidents Defined and Classified An incident is an unplanned disruption to the capacity of the facility. Incidents do not need to block a travel lane to disrupt the capacity of the facility.
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206 incidents represent the service provided for vehicles. The incident occurrence rate and its duration are fed into the model to fully characterize the problem.
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207 incidents recorded and categorized as defined in a previous section of this appendix, along with their durations. Study period monthly probabilities of different incident types are computed from Equation F.1.
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208 Incident Rates Estimation in the Study Periods in a Month All subsequent discussions in this section refer to items numbered 1, 2, and 4 in Figure F.1. Two possible approaches for incident rate estimation can be carried out.
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209 in Appendix D and depicted in Table F.4. Equation F.3 gives a definition of G(i)
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210 Core Incident Methodology Model This model begins with the widely used assumption that the number of incidents in a given study period is Poisson distributed (Skabardonis et al.
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211 is also Poisson with the rate nj × ;i. Thus, the probability of incident type i in month j is computed using Equation F.12: Prob incident type in an SP in month 1 1 (F.12)
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212 Figure F.2. Proposed process flow.
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213 100 million VMT, and the CTI factor is 7 based on local data (Khattak and Rouphail 2005)
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214 shown to be 7.64%. All computations are shown in Equation F.20: P e e n E ij i { } = − = − = − = ( )
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215 The methodology could benefit from a number of enhancements, notably in acknowledging the correlation between incidents and weather conditions. The team is aware of a parallel effort under the auspices of SHRP 2 Project L04 in which a model is being tested in the New York area that provides conditional incident probabilities based on weather events.
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