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OCR for page 39
39 where forecasts, which tends to result in agencies "painting them- selves into corners" by planning very precisely for ultimately x = The mean of the performance measure as measured in uncertain future conditions. the field; For a first attempt to introduce the concept of uncertainty q = The maximum acceptable value for the performance measure; into the forecasting process, the analyst can use the known s = The standard deviation of the performance measure as and measured uncertainty of direct field measurements of measured in the field; travel time and delay. It is assumed the variance of the fore- n = The number of measurements of the performance casts is at least equal to, if not actually greater than that measure made in the field; and measured in the field, since in addition to all the other un- t = The Student's t distribution for a level of confidence of certainties in the field, forecasts have uncertainty as to the (1-alpha) and (n - 1) degrees of freedom (see standard actual number of vehicles present. So as a first approxima- statistics text book, spreadsheet function, or Exhibit 5.1 tion, the analyst might use the field measured variance in below for values to use). travel time and delay (if available) and perform the hypoth- esis tests described above for field measured data. One merely substitutes the forecasted values for the field meas- 5.4.3 Second Null Hypothesis ured mean values into the equations and uses the standard (Cry Fire at the First Hint of Smoke) deviation of the field measured values for the standard de- Reject the null hypothesis that the system meets agency viation in the equations. standards (with confidence level equal to 1-alpha) if the fol- The effect of introducing the above described hypothesis lowing equation is true. tests into the assessment of future deficiencies is to provide for a margin of error in planning for the future. s x > q - t (1- );(n-1) (Eq. 5.2) n 5.6 Diagnosing the Causes where all variables are as explained above for previous equation. Once one or more deficiencies have been identified, it is valuable to be able to assign a primary cause to the defi- ciency. This will aid the analyst later in generating alterna- tive improvement strategies to mitigate the problem. 5.5 Comparing Forecasted Exhibit 5.2 below provides some initial suggestions for iden- Performance to tifying the root causes of travel time, delay, and variability Performance Standards deficiencies. Other significant resource documents are Generally the degree of uncertainty present in forecasts available for this purpose, and the reader is referred to Sec- or estimates of travel time or delay in not known. Common tion 7.3 for several references that cover both highway and practice is to completely ignore any uncertainty in the transit modes. Confidence Level N 50% 85% 90% 95% 99% 5 0.74 1.78 2.13 2.78 4.60 10 0.70 1.57 1.83 2.26 3.25 50 0.68 1.46 1.68 2.01 2.68 100 0.68 1.45 1.66 1.98 2.63 1,000 0.67 1.44 1.65 1.96 2.58 Exhibit 5.1. Student's t values.
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40 Deficiency Proximate Causes Likely Root Causes Travel Time is excessive, but Free-flow speeds are too low or Low-speed facility perhaps due to no significant delay or travel distances are too great. inadequate design speed (not a reliability deficiencies. freeway). Road System does not provide a straight line path between origin and destination (such as in mountainous terrain). Delay is regular but excessive. Inadequate capacity when compared Insufficient number of lanes. (There may be excessive to demand. Inadequate design. variability in travel time, but Poor signal timing. delay recurs regularly.) Too much demand. Lack of alternative routes or modes for travelers. Excessive Variability in Delay. Facility is prone to incidents and/or Facility is accident prone due to response to incidents is inadequate. poor design. There may be surges in demand. Frequent days of poor weather. Incident detection and response is poorly managed or nonexistent. Travelers not provided with timely information to avoid segments with problems. There are unmetered surges of demand (often from large special generators). Exhibit 5.2. Diagnosis chart for travel time, delay, and variability deficiencies.