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47 Step 5: Conduct a benefit-cost analysis. Where a When the overdispersion parameter is 2.2, decrease in nighttime nonintersection crashes is expected, apply a unit crash cost to the expected change. Compare ( ) ^ = 0.003366( AADT 0.6392 )e ( -0.25 DOC1+ 0.675 DOC 2 ) (6-2) E K this benefit with the cost of PRPM installation and main- tenance, using conventional life cycle economic analy- Where sis tools. ( ) ^ = Nighttime nonintersection crashes per E K mile-year, 6.6 ILLUSTRATION OF THE ANALYTICAL DOC1 = 0 and DOC2 = 1 for degree of curve > ENGINEERING PROCEDURE FOR TWO-LANE ROADWAYS 3.5, and DOC1 = 1 and DOC2 = 0 for 0 < degree of This section illustrates the engineering procedure for a curve < 3.5. two-lane roadway section that is 1 mile long. This roadway section is mostly curved, with all curves having a degree of d) Perform local and annual recalibration of base SPFs curvature less than 3.5. For this roadway section, crash data for roadway sections "with PRPMs" and "without are available for 5 years (1998 to 2002). The base year is PRPMs." The SPFs provided in this report must be 2002, which is the last full year for which data are available. recalibrated for each jurisdiction and for each year of the analysis period. The recalibration procedure is taken from Harwood et al. (40) and has recently been 6.6.1 Step 1: Assemble Data to Use SPFs tested by Persaud et al. (49). Recalibration requires annual crash counts and a) Determine the number of nighttime nonintersection AADTs for a sample of roadway segments in the crashes per year. The counts of all nonintersection jurisdiction that are typical of roadways that tend to nighttime crashes in each year of the analysis be considered for PRPM installations. First, the period are shown in Row 2 of Table 6-5. There were SPF (see Equation 6-1) is used to estimate the num- 10 crashes over 5 years, or an average of 2 crashes ber of crashes for each year for each roadway seg- per year. ment in the sample. For each year, the sum of the b) Obtain or estimate the AADT per year. AADTs are observed crash counts for each year collected is estimated for each year, including the first year after divided by the sum of the SPF estimates (for the PRPM implementation, using methods suitable to same year of data) to give an annual calibration fac- the jurisdiction's practices. For this illustration, tor ( f ). The calibration factor is applied as a mul- AADTs are listed in Row 3 of Table 6-5. tiplier ( f ) to Equation 6-1 to recalibrate the base If actual AADTs are not available for each year, model to local SPF. The annual values of f are most jurisdictions have trend factors that can be shown in Row 4 of Table 6-5 for the example illus- applied to estimate AADTs for each year. More for- trated here. mal and accurate methods for estimating missing AADTs (48) are available and can be applied by the 6.6.2 Step 2: Estimate Expected Nighttime more sophisticated analyst. Nonintersection Crashes without PRPMs c) Use SPFs as base models for two-lane roadways (from Table 6-3). When the overdispersion parame- Using Equation 6-1 and 1998 data, calculate the expected ter is 2.1, number of nighttime crashes: ( ) ^ = 0.001444( AADT 0.7345 )e ( 0.0811DOC1+ 0.457 DOC 2 ) (6-1) E K E(Ky) = y 0.001444(AADT y 0.7345 ) e (0.0811DOC1 + 0.457DOC2) TABLE 6-5 Summary of Step 1 of the illustration of analytical engineering procedure for two-lane roadways Row Data and Estimation Parameters 1 Year (y) 1998 1999 2000 2001 2002 2002 (With PRPM) 2 Crashes in year (X ) 2 0 4 1 3 To be estimated Sum = Xb = 10 3 AADT 10900 12000 11500 9800 10400 10400 4 Calibration factor f 1.1 1.04 1.01 0.95 1.04 1.04 5 Overdispersion parameter k 2.10 2.10 2.10 2.10 2.10 2.20