Incorporating Travel Time Reliability into the Highway Capacity Manual (2014) / Chapter Skim
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Pages 193-204
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193 Weather is one of the seven sources of congestion on the transportation network. Weather has a twofold effect on roadways: influencing driver behavior and increasing the likelihood of incidents (another source of congestion)
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194 that occurs between scheduled hourly transmissions. A SPECI will be issued if any of the following occurs: • The ceiling decreases to 1,500 ft or less, or when a cloud layer, previously not reported, appears below 1,000 ft (or below the highest minimum for straight-in instrument flight rules [IFR]
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195 2010) includes 15 weather categories with an average and range of capacity effects on freeways.
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196 of a certain category. This approach may seem more data intensive initially, but it eliminates any stochastic element in the application of the method.
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197 event durations. This duration is used in order to model each weather event in the computational engine.
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198 probabilities by month of the year and hour of day for each weather category created from the 10 years of METARs, as described earlier. Once a study period is selected, the scenario generator averages the probabilities across the study period hours (weighted by the fraction of each hour included in the study period in the case of partial hours)
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199 enough probability compared with the remainder of the study period with no weather and incident effects. Weather scenarios modeled in the computational engines are assigned a CAF and free-flow SAF for each weather type for the duration of the modeled weather event taken from the sources mentioned in the introduction.
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200 Although rain event characteristics were fairly well estimated, snow event characteristics had more issues. Average snow event intensities were overestimated, and durations were under estimated.
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201 probabilities. For the metropolitan area of Chicago, Figure E.9 shows the annual average probabilities by category; Figure E.10 shows average probabilities for January; and Figure E.11 shows average probabilities for April.
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202 future-year weather probabilities. Both 2011 and 2010 were withheld as estimation years, and average probabilities of the 10 weather categories were calculated on a monthly (12 probabilities per weather category)
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203 Figure E.13. 2011 Chicago annual weather probability sample size sensitivity.
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204 compared with monthly probabilities of each year of historical data, as well as the 10-year average. Figure E.15 shows the same relationship on an annual level, where error is lower.
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