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Safety Evaluation of Permanent Raised Pavement Markers (2004)

Chapter: Chapter 6 - Guidelines for the Use of Snowplowable PRPMs

« Previous: Chapter 5 - Discussion of Study Results
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Suggested Citation:"Chapter 6 - Guidelines for the Use of Snowplowable PRPMs." National Academies of Sciences, Engineering, and Medicine. 2004. Safety Evaluation of Permanent Raised Pavement Markers. Washington, DC: The National Academies Press. doi: 10.17226/13724.
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Suggested Citation:"Chapter 6 - Guidelines for the Use of Snowplowable PRPMs." National Academies of Sciences, Engineering, and Medicine. 2004. Safety Evaluation of Permanent Raised Pavement Markers. Washington, DC: The National Academies Press. doi: 10.17226/13724.
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Suggested Citation:"Chapter 6 - Guidelines for the Use of Snowplowable PRPMs." National Academies of Sciences, Engineering, and Medicine. 2004. Safety Evaluation of Permanent Raised Pavement Markers. Washington, DC: The National Academies Press. doi: 10.17226/13724.
×
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Suggested Citation:"Chapter 6 - Guidelines for the Use of Snowplowable PRPMs." National Academies of Sciences, Engineering, and Medicine. 2004. Safety Evaluation of Permanent Raised Pavement Markers. Washington, DC: The National Academies Press. doi: 10.17226/13724.
×
Page 47
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Suggested Citation:"Chapter 6 - Guidelines for the Use of Snowplowable PRPMs." National Academies of Sciences, Engineering, and Medicine. 2004. Safety Evaluation of Permanent Raised Pavement Markers. Washington, DC: The National Academies Press. doi: 10.17226/13724.
×
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Suggested Citation:"Chapter 6 - Guidelines for the Use of Snowplowable PRPMs." National Academies of Sciences, Engineering, and Medicine. 2004. Safety Evaluation of Permanent Raised Pavement Markers. Washington, DC: The National Academies Press. doi: 10.17226/13724.
×
Page 49
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Suggested Citation:"Chapter 6 - Guidelines for the Use of Snowplowable PRPMs." National Academies of Sciences, Engineering, and Medicine. 2004. Safety Evaluation of Permanent Raised Pavement Markers. Washington, DC: The National Academies Press. doi: 10.17226/13724.
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44 CHAPTER 6 GUIDELINES FOR THE USE OF SNOWPLOWABLE PRPMS This chapter describes some of the current warrants and guidelines for pavement markings and markers as background for the development of guidelines for the use of PRPMs. It then proposes guidelines for the use of snowplowable PRPMs based on the research study findings documented in Chap- ter 5. The analytical engineering procedures included in the proposed guidelines are illustrated for two-lane roadways. 6.1 BACKGROUND The “Manual on Uniform Traffic Control Devices” (Sec- tion 1A.13) (3) defines a warrant for a traffic control device as follows: A warrant describes threshold conditions to the engineer in evaluating the potential safety and operational benefits of traffic control devices and is based upon average or normal conditions. Warrants are not a substitute for engineering judg- ment. The fact that a warrant for a particular traffic control devices is met is not conclusive justification for the installa- tion of the device. The MUTCD contains a limited number of warrants. There are warrants for • Traffic signal installation (Section 4C.01), • Centerline markings and left edgeline pavement mark- ings (Section 3B.01), • No-passing zone pavement markings (Section 3B.02), • White lane line and right edgeline pavement markings (Section 3B.04), and • Edgelines (Section 3B.07). For traffic signalization, there are eight specific warrants, each of which provides conditions to be met to justify a traf- fic signal. The MUTCD emphasizes, however, that the satis- faction of a traffic signal warrant or warrants shall not in itself require the installation of a traffic signal. In compari- son, the warrants for the pavement markings noted above are less clear and are embedded or inferred within statements under the Standard, Guidance, and Option sections. For exam- ple, there are conditions where a centerline marking is required (Standard), where they are recommended (Guidance), and where they are permitted (Option). It could be argued whether or not there are warrants for pavement markings when con- sidering the definition above. For PRPMs, there are no statements in the MUTCD as to the conditions that would warrant their use. A PRPM is rec- ognized as a “device that is intended to be used as a posi- tioning guide or to supplement or substitute for pavement markings.” Standard, Guidance, and Option statements relate how to use PRPMs once it is decided to use them. (It could be argued that since PRPMs may be used as a “substitute for pavement markings” [Section 3B.14], then if an agency pre- fers to use PRPMs over painted pavement markings, there is a requirement [Standard] for when PRPMs are to be used for centerline markings [Section 3B.01].) The 2001 edition of the Traffic Control Devices Hand- book (47) recognizes PRPMs as providing excellent visibility at night and in the rain. PRPMs are discussed from a materials standpoint, and no guidance is provided as to when PRPMs should be used. The “Roadway Delineation Practices Handbook” (2) devotes an entire chapter to PRPMs. The advantages of PRPMs are noted. Principal disadvantages are a high initial cost and the need for more expensive snowplowable markers in snowfall areas. Because of the high cost of PRPMs, the “Handbook” notes that their use is limited to important roadways where additional delineation is needed and to roadways having a sur- face that will not soon be subjected to major repair, replace- ment, or excavation. Little information is provided on how to determine what is an important roadway or where additional delineation is needed. Narrow bridges on two-lane rural roads are mentioned as a special type of location where PRPMs were found to be effective in reducing nighttime speeds and centerline encroachments. A project performed by the University of Iowa for the FHWA resulted in “Guidelines for the Use of Raised Pave- ment Markers” (5). These guidelines suggest that PRPMs should be installed • To supplement double yellow centerlines on two-lane curves; • To delineate centerlines and edgelines where there are pavement width reductions at a narrow bridge; • At painted gores, exits, and bifurcations; • On all freeways and Interstate highways (snowplowable PRPMs); and

45 • On state highways at locations determined by the Bureau of Traffic Engineering on the basis of accident data (snowplowable PRPMs). The guidelines suggest that snowplowable PRPMs should not be installed on interchange ramps (5). The suggestions above should be considered cautiously because the authors likely did not intend that freeways and Interstate highways in regions with no snowfall should have snowplowable markers. 6.2 PROPOSED GUIDELINES FOR PRPMS ON TWO-LANE ROADWAYS The following guidelines are proposed: • AMFs shown in Table 6-1 should be used to guide deci- sions on where not to install PRPMs (i.e., when an AMF is greater than 1). An AMF is the ratio between the num- ber of crashes per unit of time expected after a measure is implemented and the number of crashes per unit of time estimated if the implementation does not take place. An AMF less than 1 would indicate a positive safety effect (i.e., a reduction in crashes), while an AMF greater than 1 would indicate a negative safety effect (i.e., an increase in crashes). • Given the negative safety impacts that are demonstrated to be associated with curves with more than 3.5 degrees of curvature, and given the findings of speed increases in association with PRPMs, it would seem prudent to avoid placing PRPMs well in advance of roadway sec- tions with substandard geometry or where the feature is unexpected because of the character of the road previ- ously encountered by the driver. • An analytical engineering procedure should be under- taken at locations where an AMF is less than 1 to assess the cost-effectiveness of the PRPM installation. • The results of the analytical engineering procedure should form part of the decision-making process for whether to install PRPMs at a given location. Other issues to be considered with this information are – Other measures for improving nighttime crashes that may result in higher benefit-cost effectiveness and – Other locations that may result in a higher-than- expected cost-effectiveness from the installation of PRPMs (thus, the results of the engineering study should be entered into the safety resource allocation process). 6.3 PROPOSED GUIDELINES FOR PRPMS ON FOUR-LANE FREEWAYS The following guidelines are proposed: • AMFs shown in Table 6-2 should be used to guide deci- sions on where to install PRPMs (i.e., when an AMF is less than 1). An AMF is the ratio between the number of crashes per unit of time expected after a measure is implemented and the number of crashes per unit of time estimated if the implementation does not take place. An AMF less than 1 would indicate a positive safety effect (i.e., a reduction in crashes), while an AMF greater than 1 would indicate a negative safety effect (i.e., an increase in crashes). • If a cost-effectiveness study is required, the analytical engineering procedure illustrated for two-lane roadways can be used in a similar manner. 6.4 PROPOSED REVISIONS TO THE MUTCD In light of the findings of the composite and disaggregate analyses of PRPMs on two-lane roadways and four-lane divided freeways, the following changes to the MUTCD are proposed: 1. Add the following paragraph after the initial paragraph under Support for Section 3B.11 on top of page 3B-29: Retroreflective raised pavement markers enhance guid- ance for drivers by providing longer delineation of the travel path during nighttime and wet pavement condi- tions. They also provide auditory feedback when the motorist approaches the edge of the travel lane, although snowplowable raised markers do so to a much lesser extent. These positive effects can be offset some- times by inducing higher speeds, which under certain conditions, such as on sharp curves, can result in an overall negative safety benefit. The purpose of the above paragraph is to recognize the positive and potential negative effects of snow- AMF AADT (veh/day) When DOC ≤ 3.5 When DOC > 3.5 0–5000 1.16 1.43 5001–15000 0.99 1.26 15001–20000 0.76 1.03 DOC = degree of curvature. TABLE 6-1 AMFs (nighttime crashes) derived from Table 4-3 AADT (veh/day) AMF ≤ 20000 1.13 20001–60000 0.94 > 60000 0.67 TABLE 6-2 AMFs (nighttime crashes) derived from Table 4-7

46 plowable PRPMs on two-lane roadways and four-lane freeways. The MUTCD should refer the reader to this report for additional details. 2. Add the following paragraph under Guidance on page 3B-29: The use of any raised pavement markers as a supple- ment or replacement to standard pavement markings should be based on an analytical engineering study of the potential safety impacts and costs. The purpose of the above paragraph is to recommend that an analytical engineering procedure be performed to establish the cost-effectiveness of using raised pave- ment markers. Although this research study has deter- mined the procedure for snowplowable PRPMs on two- lane roadways, there is a need to research analytical engineering procedures for conventional and other PRPMs on other types of road. 6.5 OVERVIEW OF THE ANALYTICAL ENGINEERING PROCEDURE The analytical engineering procedure estimates the likely safety effect of installing PRPMs. The procedure contains a series of steps to be undertaken by an analyst in carrying out the engineering procedure: • Step 1: Assemble data to use SPFs. Include the following: a) For the past 3 to 5 years, determine the number of nighttime nonintersection crashes per year for the roadway section under analysis. b) For the past 3 to 5 years, obtain or estimate the AADT for each year. Estimate the AADT for the year after PRPM installation. c) Use SPFs, as base models, for roadway sections “with PRPMs” and “without PRPMs” (see Tables 6-3 and 6-4 for two- and four-lane roadways, respectively). d) Perform local and annual recalibration of the base models using the procedure presented by Harwood et al. (40). • Step 2: Estimate expected nighttime nonintersection crashes without PRPMs. Use the empirical Bayes pro- cedure with the data from Step 1 and the “without PRPMs” local SPFs to estimate the expected annual number of nighttime nonintersection crashes that would occur without PRPMs for the last full year for which data are available (i.e., the base year). • Step 3: Estimate expected nighttime nonintersection crashes with PRPMs. Use the “with PRPMs” local SPFs and the AADTs from Step 1 to estimate the expected annual number of nighttime nonintersection crashes that would occur with PRPMs had they been installed in the last full year for which data are available (i.e., the base year). • Step 4: Compare expected crashes with and without PRPMs. Calculate the difference between the expected annual number of nighttime nonintersection crashes esti- mates from Steps 2 and 3. SPF Without PRPMs With PRPMs Nighttime nonintersection crashes per mile-yr α(AADT)β1exp(β2DOC1+ β3DOC2) α(AADT)β1exp(β2DOC1+ β3DOC2) ln(α) (s.e.) -5.6940 (0.5370)-6.5400 (0.3880) β1 (s.e.) 0.7345 (0.0415) 0.6392 (0.0574) β2 (s.e.) 0.0811 (0.0908) -0.2570 (0.1210) β3 (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. SPF Without PRPMs With PRPMs Nighttime nonintersection crashes per mile-yr α(AADT)β1 α(AADT)β1 ln(α) (s.e.) -11.5230 (0.7480) -12.0360 (0.9060) β1 (s.e.) 1.1013 (0.0947) 1.1530 (0.1190) K 3.9 2.4 TABLE 6-3 Two-lane roadways: SPFs (base models) for the analytical engineering procedure TABLE 6-4 Four-lane freeways: SPFs (base models) for the analytical engineering procedure

47 • Step 5: Conduct a benefit-cost analysis. Where a decrease in nighttime nonintersection crashes is expected, apply a unit crash cost to the expected change. Compare this benefit with the cost of PRPM installation and main- tenance, using conventional life cycle economic analy- sis tools. 6.6 ILLUSTRATION OF THE ANALYTICAL ENGINEERING PROCEDURE FOR TWO-LANE ROADWAYS This section illustrates the engineering procedure for a two-lane roadway section that is 1 mile long. This roadway section is mostly curved, with all curves having a degree of curvature less than 3.5. For this roadway section, crash data are available for 5 years (1998 to 2002). The base year is 2002, which is the last full year for which data are available. 6.6.1 Step 1: Assemble Data to Use SPFs a) Determine the number of nighttime nonintersection crashes per year. The counts of all nonintersection nighttime crashes in each year of the analysis period are shown in Row 2 of Table 6-5. There were 10 crashes over 5 years, or an average of 2 crashes per year. b) Obtain or estimate the AADT per year. AADTs are estimated for each year, including the first year after PRPM implementation, using methods suitable to the jurisdiction’s practices. For this illustration, AADTs are listed in Row 3 of Table 6-5. If actual AADTs are not available for each year, most jurisdictions have trend factors that can be applied to estimate AADTs for each year. More for- mal and accurate methods for estimating missing AADTs (48) are available and can be applied by the more sophisticated analyst. c) Use SPFs as base models for two-lane roadways (from Table 6-3). When the overdispersion parame- ter is 2.1, (6-1)E K eˆ . . ( . . )( ) ( )= +0 001444 0 7345 0 0811 1 0 457 2AADT DOC DOC When the overdispersion parameter is 2.2, (6-2) Where = Nighttime nonintersection crashes per mile-year, DOC1 = 0 and DOC2 = 1 for degree of curve > 3.5, and DOC1 = 1 and DOC2 = 0 for 0 < degree of curve < 3.5. d) Perform local and annual recalibration of base SPFs for roadway sections “with PRPMs” and “without PRPMs.” The SPFs provided in this report must be 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 tested by Persaud et al. (49). Recalibration requires annual crash counts and AADTs for a sample of roadway segments in the jurisdiction that are typical of roadways that tend to be considered for PRPM installations. First, the SPF (see Equation 6-1) is used to estimate the num- ber of crashes for each year for each roadway seg- ment in the sample. For each year, the sum of the observed crash counts for each year collected is divided by the sum of the SPF estimates (for the same year of data) to give an annual calibration fac- tor (α f). The calibration factor is applied as a mul- tiplier (α f) to Equation 6-1 to recalibrate the base model to local SPF. The annual values of α f are shown in Row 4 of Table 6-5 for the example illus- trated here. 6.6.2 Step 2: Estimate Expected Nighttime Nonintersection Crashes without PRPMs Using Equation 6-1 and 1998 data, calculate the expected number of nighttime crashes: E(Ky) = αy ∗ 0.001444(AADT y0.7345)e(0.0811DOC1 + 0.457DOC2) E Kˆ( ) E K eˆ . . ( . . )( ) ( )= − +0 003366 0 6392 0 25 1 0 675 2AADT DOC DOC 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 αf 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

48 E(K1998) = 1.10 ∗ 0.001444 ∗ (109000.7345) ∗ e(0.0811 ∗ 1 + 0.457 ∗ 0) = 1.591 nighttime nonintersection crashes per mile-year The estimates of nighttime nonintersection crashes per mile for each year are shown in Row 6 of Table 6-6. Calculate the annual correction factors (Cy) between the annual estimated number of crashes for each year and the annual estimated number of crashes for 2002 (the base year). The annual correction factors for this example are shown in Row 7 of Table 6-6 and are summed in Row 8. Using the values in Rows 2 through 8 (Table 6-6) and the empirical Bayes formula, estimate a value of the expected annual number of crashes without PRPMs (and its variance) for the base year (2002): (6-3) Where y = Subscript to represent the year, αf = Recalibrated annual factor, k = Overdispersion parameter, E(Ky) = Predicted number of crashes on this road- way section for Year y using SPF, ˆ . . . . . . ˆ . . . . . K k X k E K C K k X k E K C b b b b 2002 2002 2002 2002 2 2 1 000 2 10 10 2 10 1 453 5 126 1 841 1 000 2 10 10 2 10 1 453 5 126 0 = + + = + + = = + + = + + = ( ) ( )( )[ ] ( ) ( )[ ] ( ) ( ) ( ){ }[ ] ( ) ( ){ }[ ] crashes per mile-year for roads “without PRPMs” VAR .280 Cy = Annual correction factor for Year y relative to the base year, = Expected number of crashes during 2002 (the base year) if PRPMs were not installed, and E(K2002)PRPM = Expected number of crashes during 2002 (the base year) were PRPMs to be installed in that year. These values are summarized in Row 9 of Table 6-6. 6.6.3 Step 3: Estimate Expected Nighttime Nonintersection Crashes with PRPMs The number of crashes (in the base year, 2002) if the PRPMs were to be installed is estimated using the “with PRPMs” SPF model (Equation 6-2). (6-4) These values are shown in Row 10 of Table 6-6. E K e E K e K E K k y i i( ) ( ) ( ) ( ) ( ) = ∗ = ∗ ∗ = = = = − + − ∗ + ∗ PRPM DOC DOC PRPM PRPM AADT crashes per mile-year for roads “with PRPMs” VAR α 0 003366 1 04 0 003366 10400 1 005 1 0005 2 2 0 455 0 6392 0 257 1 0 675 2 2002 0 6392 0 257 1 0 675 0 2002 2002 2 2 . . . ( ) . . . . . ( . . ) . ( . . ) E Kˆ( ) Row Data and Estimation Results 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 αf 1.10 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 6 Model Prediction E(Ky) 1.5910 1.6140 1.5190 1.2710 1.4530 1.0005 7 Cy = E(Ky)/E(K2002) 1.095 1.111 1.046 0.874 1.000 1.000 8 Comparison ratio for period Sum = Cb = 5.126 Ca = 1.000 9 2002 ˆK )]ˆ([ 2002KVAR 1.842 [0.280] 10 E(K2002)PRPM [VAR(K2002)PRPM] 1.0005 [0.455] TABLE 6-6 Summary of engineering study procedure illustration for two-lane roadways

6.6.4 Step 4: Compare Expected Crashes with and without PRPMs The difference between the expected number of nighttime nonintersection crashes estimates from Steps 2 and 3 is cal- culated as (6.5) 6.6.5 Step 5: Conduct a Benefit-Cost Analysis A sample benefit-cost analytical procedure is presented here: a) Estimate the relative injury cost (RIC) for all two- lane roadways (or a large sample) in the jurisdiction: (6-6) Where Fat = Number of fatal injury crashes, Inj = Number of nonfatal injury crashes, PDO = Number of property-damage-only crashes, wfat = Weighting factor for fatal injury crashes, and winj = Weighting factor for nonfatal injury crashes. The weighting factor for fatal injury crashes (wfat) is the ratio of the cost of a fatal injury crash to the cost of a PDO crash. The weighting factor for non- fatal injury crashes is the ratio of the cost of a non- fatal injury crash to the cost of a PDO crash. The two most commonly used accident cost figures are those contained in the AASHTO Roadside Design Guide (50) and the FHWA comprehensive cost figures based on the willingness-to-pay concept. Users should choose the set of accident cost figures that best suits their particular study. Table 6-7 shows the crash costs used by the FHWA (51) updated to 2002 dollars and the RIC values estimated from these crash cost values. Assume that on all two-lane roadways within this jurisdiction, the average number of crashes per year RIC (Fat) (Inj) (PDO) (Fat) (Inj) (PDO) fat inj = + + + + w w ∆ = − ( ) = − = ˆ . . . K E K2002 2002 1 841 1 0005 0 841 PRPM crashes per mile-year 49 is 10 fatal, 1,200 injury, and 4,200 PDO. Therefore, the RIC can be calculated as follows: (6-7) b) Estimate the potential reduction in crash costs. The average cost of a crash is the product of the RIC and the cost of a PDO crash: Cost per crash = 9.25 ∗ $2,300 = $21,275 per crash (6-8) The total savings per year is the product of the aver- age cost of a crash and the expected crash reduction per year: Crash savings benefit per year = $21,275 * 0.841 (6-9) = $17,892 per mile-year c) Determine an annual cost of installing and maintain- ing PRPMs. Each jurisdiction should obtain its own annual cost estimate for the installation and mainte- nance of PRPMs at the study locations. The annual cost estimate will consist of three components: 1. Indirect installation cost. This includes the cost of providing work zone signs, attenuation vehi- cles, special law enforcement, and so forth to implement PRPMs. This cost is incurred at each site where PRPM lenses are replaced or PRPMs are installed. The indirect cost per PRPM can be determined by dividing the total indirect cost by the total number of PRPMs to be installed during the contract. The following equation can be used to determine the annual indirect cost, A: (6-10) Where COST = Indirect cost of installation per PRPM, i = Annual discount rate, and n = Number of years in lens replacement cycle. If the indirect cost of implementing 500 PRPMs is $5,000, then the indirect cost per PRPM = $5,000/500 = $10. Assuming a discount rate of 5 percent, the annual cost = 10 × 0.05 × A i i i n n = ∗ + + − COST ( ) ( ) 1 1 1 RIC = ∗ + ∗ + + + = ( ) ( ) 1 304 3 10 27 4 1 200 4 200 10 1 200 4 200 9 25 , . . , , , , . Crash severity Cost (2002) RIC Fatal $3,000,000 1304.3 Injury $63,000 27.4 PDO $2,300 1.0 TABLE 6-7 Crash costs and relative injury cost values

(1.05)3/[(1.05)3 − 1] = $3.67 per PRPM for a 3-year lens replacement cycle. 2. Direct installation cost. This refers to the actual cost of installing or replacing the PRPM and includes the cost of material, equipment, and labor. Equation 6-10 can be used to calculate the annual direct installation cost, in which case n is equal to the number of years in the PRPM replacement cycle. For example, in Missouri (52), the cost of installing one PRPM is about $42.50, and PRPMs are replaced every 10 years. Assuming a discount rate of 5 percent, the annual cost equals 42.50 × 0.05 × (1.05)10/[(1.05)10 − 1] = $5.50 per PRPM for a 10-year PRPM replacement cycle. 3. Maintenance cost. This refers to the average annual cost of replacing the lens according to the replacement cycle and includes the cost of mate- rials, equipment, and labor. Equation 6-10 can be used to calculate the annual maintenance cost, in which case n is equal to the number of years in the lens replacement cycle. For example, Missouri DOT replaces the lens every 3 years at a cost of approximately $6.20 per lens (52). Assuming a 50 discount rate of 5 percent, the annual cost equals 6.20 × 0.05 × (1.05)3/[(1.05)3 − 1] = $2.28 per PRPM is estimated for a 3-year lens replacement cycle. If justified by jurisdictional procedures, any annual costs incurred by lens replacement outside the typical cycle should be included in the overall maintenance costs. The total annual cost per PRPM is the sum of these three cost compo- nents: $3.67 + $5.50 + $2.28 = $11.45. Based on this assumed cost, the cost per mile (1 mile = 5,280 ft) for 40 ft and 80 ft PRPM spacings are $1,511 and $756, respectively. For the example illustrated here, if PRPMs are implemented at a 40-ft (12-m) spacing (at curves) for 80 percent of the length of the 1-mile-long section and at 80-ft (24-m) spacing for the remainder of the length, the annual PRPM implementation and mainte- nance cost will be 0.8($1,511) + 0.2($756) = $1,360 for a 1-mile-long, two-lane roadway section. d) Determine benefit-cost ratio. The benefit-cost ratio equals the present value of crash savings divided by the implementation cost ($17,892/$1,360 = 13.16).

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 518: Safety Evaluation of Permanent Raised Pavement Markers examines the safety performance of snowplowable permanent raised pavement markers on two-lane roadways and four-lane freeways.

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