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46 plowable PRPMs on two-lane roadways and four-lane a) For the past 3 to 5 years, determine the number of freeways. The MUTCD should refer the reader to this nighttime nonintersection crashes per year for the report for additional details. roadway section under analysis. b) For the past 3 to 5 years, obtain or estimate the AADT 2. Add the following paragraph under Guidance on page for each year. Estimate the AADT for the year after 3B-29: PRPM installation. c) Use SPFs, as base models, for roadway sections "with The use of any raised pavement markers as a supple- ment or replacement to standard pavement markings PRPMs" and "without PRPMs" (see Tables 6-3 and should be based on an analytical engineering study of 6-4 for two- and four-lane roadways, respectively). the potential safety impacts and costs. d) Perform local and annual recalibration of the base models using the procedure presented by Harwood The purpose of the above paragraph is to recommend et al. (40). that an analytical engineering procedure be performed Step 2: Estimate expected nighttime nonintersection to establish the cost-effectiveness of using raised pave- crashes without PRPMs. Use the empirical Bayes pro- ment markers. Although this research study has deter- cedure with the data from Step 1 and the "without mined the procedure for snowplowable PRPMs on two- PRPMs" local SPFs to estimate the expected annual lane roadways, there is a need to research analytical number of nighttime nonintersection crashes that would engineering procedures for conventional and other occur without PRPMs for the last full year for which PRPMs on other types of road. data are available (i.e., the base year). Step 3: Estimate expected nighttime nonintersection 6.5 OVERVIEW OF THE ANALYTICAL crashes with PRPMs. Use the "with PRPMs" local SPFs ENGINEERING PROCEDURE and the AADTs from Step 1 to estimate the expected The analytical engineering procedure estimates the likely annual number of nighttime nonintersection crashes that safety effect of installing PRPMs. The procedure contains a would occur with PRPMs had they been installed in the last series of steps to be undertaken by an analyst in carrying out full year for which data are available (i.e., the base year). the engineering procedure: Step 4: Compare expected crashes with and without PRPMs. Calculate the difference between the expected Step 1: Assemble data to use SPFs. Include the annual number of nighttime nonintersection crashes esti- following: mates from Steps 2 and 3. TABLE 6-3 Two-lane roadways: SPFs (base models) for the analytical engineering procedure SPF Without PRPMs With PRPMs Nighttime nonintersection (AADT)1exp(2DOC1+ 3DOC2) (AADT)1exp(2DOC1+ 3DOC2) crashes per mile-yr ln() (s.e.) -6.5400 (0.3880) -5.6940 (0.5370) 1 (s.e.) 0.7345 (0.0415) 0.6392 (0.0574) 2 (s.e.) 0.0811 (0.0908) -0.2570 (0.1210) 3 (s.e.) 0.4570 (0.1100) 0.6750 (0.1430) K 2.1 2.2 DOC1 = 0 and DOC2 = 1 for degree of curve > 3.5. DOC1 = 1 and DOC2 = 0 for 0 < degree of curve 3.5. TABLE 6-4 Four-lane freeways: SPFs (base models) for the analytical engineering procedure SPF Without PRPMs With PRPMs Nighttime nonintersection (AADT)1 (AADT)1 crashes per mile-yr ln() (s.e.) -11.5230 (0.7480) -12.0360 (0.9060) 1 (s.e.) 1.1013 (0.0947) 1.1530 (0.1190) K 3.9 2.4