Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects (2021) / Chapter Skim
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From page 74... ...
63 Chapter 7. Application of Benefit–cost Analysis for 3R Projects Benefit–cost analysis enables highway agencies to assess the cost-effectiveness of design alternatives for 3R projects and decide (a)
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64 Every highway agency has established procedures for estimating the cost of geometric design alternatives, both for cost estimates that are sufficiently accurate for planning-level analyses and for detailed cost estimates prepared in final design. It is assumed that, in most cases, highway agencies will prefer to use their own project cost estimation procedures as the basis for 3R project benefit–cost analyses.
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65 data are available, to combine the predicted and observed crash frequencies using the Empirical Bayes (EB) procedure presented in the Appendix to HSM Part C
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66 𝑁 𝑒 C CMF CMF … CMF /𝑛 (27) where: a,b = coefficients presented in HSM Chapter 11 In the HSM Chapter 11 procedure, Equation (27)
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67 𝑁 ∑ 𝑒 , , 𝐶 𝐶𝑀𝐹 𝐶𝑀𝐹 … 𝐶𝑀𝐹 /𝑛 (29) where: Npredicted iavg = predicted average crash frequency for a particular intersection for a particular year a,b,c = coefficients presented in HSM Chapter 10, 11, and 12 AADTy,maj = annual average daily traffic volume on the major road (veh/day)
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68 Values for the overdispersion parameter, k, can be determined from: HSM Equation (10-7) for roadway segments on rural two-lane highways Text accompanying HSM Equations (10-8)
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69 The CMF representing the effectiveness of a single geometric design improvement is determined as: 𝐶𝑀𝐹 𝐶𝑀𝐹 ,𝐶𝑀𝐹 , (34) where: CMFj,after = crash modification factor for improvement j in the condition after improvement CMFj,before = crash modification factor for improvement j in the condition before improvement The CMF representing the combined effectiveness for a design alternative that incorporates several geometric design improvements is determined as: 𝐶𝑀𝐹 𝐶𝑀𝐹 ,𝐶𝑀𝐹 , 𝐶𝑀𝐹 , 𝐶𝑀𝐹 , … 𝐶𝑀𝐹 , 𝐶𝑀𝐹 , (35)
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70 7.1.6 Improvement Service Life Pavement resurfacing typically has a service life of 7 to 12 years, depending upon construction and material quality and traffic volume, until resurfacing is needed again. However, the service life for the pavement surface does not typically enter directly into benefit–cost analyses concerning geometric design improvements, because the pavement will require resurfacing at the same interval whether geometric design improvements are incorporated in a 3R project or not.
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71 P/F, 𝑖%,𝑛 1 𝑖100 (36) where: (P/F, i%, n)
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72 NB 𝐶𝑅 𝐶 𝑃 𝐴⁄ , 𝑖%,𝑛 𝐼𝐶 𝑃 𝐹⁄ , 𝑖%,𝑛 (39) where: NB = net benefit The net benefit is often the most useful form of benefit–cost analysis results for identifying the design alternative that will maximize the safety benefits for any given level of expenditure on geometric design improvements in 3R projects.
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73 Table 46. Input Data for Safety Benefits Calculation Example Geometric Improvement Lane Widening from 10 to 12 ft Lane Widening Service Life 20 yrs Discount Rate 7% Roadway Type Rural Two-lane Highway Shoulder Width 2 ft Shoulder Type Paved Roadside Slope 1V:3H Centerline Rumble Strip No Shoulder Rumble Strip No Section Length 3 mi AADT (does not change)
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74 The predicted annual average crash frequency for the roadway prior to the 3R project is 0.942 crashes per year, as shown in Equations (43) through (45)
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75 𝐶𝑅 1 0.934 0.942 0.062 𝑐𝑟𝑎𝑠ℎ𝑒𝑠 𝑟𝑒𝑑𝑢𝑐𝑒𝑑 𝑝𝑒𝑟 𝑦𝑒𝑎𝑟 (50) 7.2.2.2 Using Observed Crash Data The EB methodology, described in Section 7.1.3.5 is used to incorporate observed crash data into the calculation of the expected reduction in crash frequency.
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76 severity distributions from HSM Chapter 10 are used to transform total annual crash reduction into annual crash reduction by severity level in Table 48. Table 48.
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77 calculated in Section 7.2.2. The benefit–cost ratio of widening the example roadway section can now be computed, which is shown in Equation (63)
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78 Consider, for example, a similar site, identical in most respects to the previous example, but with an AADT of 4,000 veh/day. In this case, the net implementation cost remains the same at $475,889.
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79 Table 49. Example of Benefit–cost Calculations for Lane Widening from 10 to 12 ft in Level Terrain on a Rural Two-Lane Highway Lane Width (ft)
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80 Table 51. Example of Benefit–cost Calculations for Lane Widening from 9 to 11 ft in Level Terrain on a Rural Two-Lane Highway Lane Width (ft)
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81 Table 54. Example of Benefit–cost Calculations for Lane Widening from 11 to 12 ft in Level Terrain on a Rural Two-Lane Highway Lane Width (ft)
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82 Table 55. Example of Incremental Analysis to Determine Net Benefits of Lane Widening for Existing Rural Two-Lane Highways with 9-ft Lanes in Level Terrain AADT (veh/day)
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83 For a roadway with an AADT of 1,000 veh/day, none of the lane widening alternatives are cost-effective. For a roadway with an AADT of 2,000 or 3,000 veh/day, lane widening from 9 to 11 ft has the maximum net benefit.
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84 Table 56. Examples of Incremental Analysis to Determine Net Benefits of Lane Widening for Existing Rural Two-Lane Highways with 10-ft Lanes in Level Terrain AADT (veh/day)
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85 The minimum AADT levels for lane widening can be expanded to include rolling and mountainous terrain types, as shown in Table 57. Minimum AADT levels can be established for shoulder widening using the same procedure described above, as shown in Table 58.
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86 Table 57. Example of AADT Levels at which Lane Widening Becomes Cost-Effective Rural Two-Lane Highway Segments Assuming 2-ft Paved Shoulders, 1V:3H Roadside Foreslopes, and Moderate Horizontal Curvature Proposed Improvement Minimum AADT level (veh/day)
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87 Benefit–cost analysis for multiple design alternatives can be performed with Spreadsheet Tool 2 presented below in Section 8.2. 7.5.3 Benefit–cost Analysis to Develop Agency-Specific Minimum AADT Guidelines for Application in Design Decisions Highway agencies can develop minimum AADT guidelines for application in 3R project design decisions, analogous to those shown in Tables 57 and 58.
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88 Similarly, TRB Special Report 214 (4) presents the dimensional design criteria for lane and shoulder width shown in Table 19, which are, in some cases, less than the Green Book criteria for new construction or reconstruction.
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89 The expected crash frequency by crash severity level for the existing conditions at the site is determined using the applicable HSM predictive method together, where crash history data are available, with the EB method presented in the Appendix to HSM Part C Candidate improvements to be considered for implementation in the 3R project are identified; dimensional elements like lane width this would include a range of potential dimensions; for example, for an existing site with 9-ft lanes, potential lane widening to 10-ft, 11-ft, and 12-ft lanes would be considered.
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90 Then, the most cost-effective alternative is simply the alternative with the largest net benefit, as determined with Equation (39)
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91 Widen lanes on roadways where the AADT exceeds the minimum AADT criteria presented in Table 57 Widen lanes where the net present benefits of the project exceeds zero (i.e., where the benefits exceed the costs) based on the benefit–cost analysis procedure presented in Chapter 5 The benefits and cost of lane widening in this example are based on assumptions concerning crash costs, unit construction costs, project service life, and minimum attractive rate of return presented in Section 7.1.
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92 This improvement program would provide benefits of $57,180,686 at a cost of $200,107,835, equivalent to a benefit–cost ratio of 0.29. Of the 832.28 mi of roadway improved, 96 percent consisted of projects with benefit–cost ratios less than 1.0.
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93 costs. This strategy does the best job at focusing on the best projects and avoids many of the projects that are not cost-effective.
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