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Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects (2021)

Chapter: Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types

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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
×
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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Suggested Citation:"Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types." National Academies of Sciences, Engineering, and Medicine. 2021. Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/26199.
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34 Chapter 5. Crash Reduction Effectiveness of Specific Design Improvement Types 3R programs are managed by highway agencies as part of their effort to preserve pavements and extend their service life. However, 3R projects also provide an opportunity to consider the need for design improvements to reduce crash frequency and severity. The Federal 3R program requires consideration of safety improvement needs in 3R projects, and most highway agencies also consider safety improvement needs in 3R projects that are not part of the Federal 3R program. This chapter presents the crash reduction effectiveness measures for many of the design improvements that are frequently implemented on rural two-lane highways, rural multilane undivided and divided highways, and freeways. These crash reduction effectiveness measures form a key part of cost-effectiveness or benefit–cost analyses that can be used as the basis for selecting appropriate design improvements. Design improvements are made in 3R improvements on urban and suburban highways as well, but there are few documented crash reduction effectiveness measures for such improvements on urban and suburban arterials. As a result, the design guidelines for 3R projects on urban and suburban arterials suggest decision-making approaches not based on benefit–cost analysis tools. 5.1 Quantifying Crash Reduction Effectiveness of 3R Improvements: Crash Modification Factors Management of a 3R program to effectively reduce crashes requires an understanding of the crash reduction effectiveness of design improvements that can potentially be made as part of 3R projects. The following discussion introduces the concept of crash modification factors (CMFs) to represent the crash reduction effectiveness of design improvements, presents their definition, and describes their use. The values of CMFs for specific 3R improvement types are presented below in Section 5.2. 5.1.1 Introduction to the Concept of Crash Modification Factors Crash modification factors (CMFs) represent the relative change in crash frequency due to a change in a roadway design feature or any other specific condition. A CMF represents the ratio of the crash frequency of a site under two different conditions, such as after a project in comparison to before a project. Therefore, a CMF serves as an estimate of the effect of a particular geometric design or traffic control feature or the effectiveness of a particular treatment or condition. Some CMFs apply to specific crash severities, crash types, roadway types and/or may only be applicable to specific ranges of traffic volumes. As an example, if a change in a specific design feature were expected to reduce the crash frequency at a site from 10 to 8 crashes per year, the corresponding CMF value would be 8/10 = 0.80. A CMF value less than 1.0 indicates that the design feature or treatment is expected to

35 reduce crashes. A CMF value greater than 1.0 indicates that the design feature or treatment would be expected to increase crashes. A CMF value equal to 1.0 indicates that the design feature or treatment would not be expected to have any effect on crashes. CMFs are a useful form for expressing the expected crash reduction effectiveness of design features or treatments because the expected crash frequency for a site after a project can be estimated as the expected crash frequency before the project multiplied by the CMF. The crash reduction effectiveness of design features or treatments can also be expressed as a crash reduction factor, which represents the expected percentage reduction in crashes resulting from a design feature or treatment. The CMF value of 0.80 presented above corresponds to a crash reduction factor of 20 percent, determined as follows: Crash Reduction Factor 1.00 0.80 100 20 percent (1) Similarly, a CMF value of 1.20 corresponds to a crash reduction factor of -20 percent (in other words, an increase in crash frequency of 20 percent), determined as follows: Crash Reduction Factor 1.00 1.20 100 20 percent (2) 5.1.2 Quality and Uncertainty Issues Related to Crash Modification Factors CMFs represent the expected or average effects of geometric design features on crash frequency and severity. CMFs are developed from data for individual sites at which the crash counts and the change in crash counts from before to after a project may vary substantially. CMFs represent the average effect of future infrastructure improvements on crash counts, but as in the data used to develop CMFs, such effects are likely to vary substantially from site to site. Some CMFs are more precise and reliable than others. Each CMF presented in Part D of the Highway Safety Manual (6) is accompanied by a standard error that represents the degree of uncertainty associated with predictions made with that CMF. For example, the CMF applicable to all types and all severities of crashes for installing centerline rumble strips on rural two-lane highways is presented in HSM Part D as: Crash Modification Factor Standard Error 0.86 0.05 The CMF value of 0.86 indicates that installation of centerline rumble strips would be expected to reduce crashes by 14 percent. This can be demonstrated by replacing the value of 0.80 in Equation (1) with 0.86. The standard error represents the precision of the CMF value. The confidence range around the CMF value permits assessment of the accuracy of the CMF value. Confidence limits at the 95-percent confidence level typically range from the CMF value minus twice the standard error to the CMF value plus twice the standard error. This CMF value of 0.86 is considered accurate because a CMF value of 1.0 (no effect on crashes) is not included within the range 0.86 ± 2*(0.05), or 0.76 to 0.96. This range indicates that the effectiveness of

36 centerline rumble strips can range from a CMF of 0.76 to a CMF of 0.96, or a reduction of crashes in the range from 4 to 24 percent. Such uncertainties are typical of even the most reliable CMFs, given the variations inherent in crash data. While the crash reduction effectiveness of centerline rumble strips at individual sites may vary over the range shown (4 to 24 percent), the overall effectiveness of applying centerline rumble strips to a broad range of rural two-lane highway sites is likely to be about 14 percent. Highway agencies can plan 3R programs with the expectation that the available CMFs will represent the average results that will be obtained. The HSM uses a formal inclusion rule (2,31) to determine whether CMFs are of sufficient quality to be applied reliably. With limited exceptions, only CMFs that meet the inclusion rule are presented in HSM Part D. Thus, CMFs from the HSM can be relied upon in planning 3R programs. CMFs are also available from the FHWA Crash Modification Factors Clearinghouse (32). The FHWA Clearinghouse assigns star ratings to CMFs to represent their quality and identifies the facility type(s) to which each CMF applies. Star ratings for CMFs range from one star (lowest quality) to five stars (highest quality). The ratings are based on an assessment of the research approach and the data used in developing the CMFs. The philosophy of the FHWA Clearinghouse is to include all CMFs reported in the literature, regardless of quality, and to let the star ratings indicate the quality of individual CMFs. Thus, CMFs rated as one or two stars in the FHWA Clearinghouse are derived from research or data of low quality and should not be relied upon unless no better information is available. CMFs rated one or two stars, and some rated three stars, in the FHWA Clearinghouse would likely not pass the inclusion rule for incorporation in HSM Part D. The FHWA Clearinghouse also indicates which CMFs presented in the Clearinghouse also meet the HSM inclusion rule. Application of CMFs from the FHWA Clearinghouse should focus on those CMFs rated three stars, or preferably four or five stars, or those identified in the FHWA Clearinghouse as meeting the HSM inclusion rule. 5.1.3 Development of Crash Modification Factors CMFs are developed primarily through analysis of crash data. The two most common types of crash analyses to develop CMFs are:  observational before-after studies  cross-sectional studies Observational before-after studies are conducted by identifying projects where a design improvement has been implemented at multiple sites. Crash history and traffic volume data are then obtained for time periods before and after implementation of each project (typically at least three years of data for the period before project implementation and three years of data for the period after project implementation. Observational before-after evaluations are typically conducted with a technique known as the Empirical Bayes (EB) method, which compensates for potential biases in crash data. An observational before-after evaluation compares the observed crash count in the period after implementation of each project to an estimate of the crash count that would have been expected if the project had not been implemented. The crash count that

37 would have been expected if the project had not been implemented is estimated with a crash prediction model known as a safety performance function (SPF). SPFs are developed with a modeling technique known as Negative Binomial regression analysis, which is well suited to modeling crash data. HSM Chapter 9 presents a computational procedure for observational before-after evaluations using the Empirical Bayes method. The analysis results provide a CMF that represents the crash reduction effectiveness of the project. Figure 1 illustrates a typical SPF for a roadway segment developed with Negative Binomial regression that could be used in developing CMFs with the EB method. This particular SPF is from HSM Figure 11-3 and applies to roadway segments on rural multilane undivided highways (nonfreeways). SPFs can have either linear or curvilinear functional forms. The horizontal axis for the SPF represents the annual average daily traffic volume (AADT) for the roadway segment. The vertical axis represents the predicted number of crashes per mile per year on the roadway segment. Figure 1. Example of Safety Performance Function for Undivided Roadway Segments on Rural Multilane Highways (6) Cross-sectional studies are conducted when observational before-after evaluations are not feasible. Cross-sectional modeling uses Negative Binomial regression modeling to quantify the effect of a specific geometric design feature on crash counts by considering sites with and without that feature or where that feature is present in varying dimensions. Cross-sectional analyses must be carefully designed and conducted because, where specific design features or other factors are correlated with one another, the effect of other factors can potentially be mistaken for an effect of the geometric design feature of interest. HSM Chapter 9 also presents a computational procedure for developing CMFs with cross-sectional modeling.

38 Further guidance on CMF development is found, in HSM Chapter 9 (6) and in the FHWA report, A Guide to Developing Crash Modification Factors (33). 5.1.4 Sources for Obtaining Crash Modification Factor Values As indicated above, the two most extensively used catalogs of CMF values for design and traffic control improvements are the HSM (6) and the FHWA Crash Modification Factors Clearinghouse (www.cmfclearinghouse.org). All of the CMFs in the HSM and the FHWA Clearinghouse were developed in research projects and then reviewed and assessed prior to inclusion. CMFs can also be obtained from review of individual research reports, although this should generally be done only for new sources that have not yet been assessed for inclusion in the HSM or the FHWA Clearinghouse. 5.2 Crash Modification Factors for Specific 3R Improvement Types This section presents the CMF values most commonly used to represent the crash reduction effectiveness of specific 3R improvement types. The following discussion is organized by roadway type, and then, within roadway type, by design feature. The CMFs presented here are, whenever possible, those used in the HSM (6,7), which is the most widely utilized source for such information. Where no CMF is available in the HSM, high- quality CMFs from other sources are utilized. While these other CMFs were identified in the FHWA Crash Modification Factors Clearinghouse (32), the original sources have also been reviewed. 5.2.1 Rural Two-Lane Highways 5.2.1.1 Lane Width HSM Chapter 10 (Rural Two-Lane Highways) presents CMFs for lane widths on rural two-lane highways (6). The CMF is calculated using the equations shown in Table 24 based on the lane width and the average annual daily traffic (AADT) for the roadway in question. A 12-ft lane is considered to be the base condition (CMF = 1.0). The lane width CMF is illustrated graphically in Figure 2.

39 Table 24. CMF for Lane Width on Rural Two-Lane Roadway Segments (6, 10, 34, 35) Lane width Average annual daily traffic (AADT) (veh/day) < 400 400 to 2000 > 2000 9 ft or less 1.05 1.05 + 2.81 x 10-4(AADT-400) 1.50 10 ft 1.02 1.02 + 1.75 x 10-4(AADT-400) 1.30 11 ft 1.01 1.01 + 2.5 x 10-5(AADT-400) 1.05 12 ft or more 1.00 1.00 1.00 NOTE: The collision types related to lane width to which these CMFs apply are single-vehicle run-off-the-road and multiple-vehicle head-on, opposite-direction sideswipe, and same-direction sideswipe crashes. Standard error of the CMF is unknown. To determine the CMF for changing lane width and/or AADT, divide the “new” condition CMF by the “existing” condition CMF. NOTE: Based on HSM Table 10-8. NOTE: Based on HSM Figure 10-7. Figure 2. CMFra for Lane Width on Undivided Roadways on Rural Two-Lane Roadway Segments (6, 34) The lane-width CMF illustrated in Table 24 and Figure 2 applies only to single-vehicle run-off- the-road, and multiple-vehicle head-on, opposite-direction sideswipe, and same-direction sideswipe crashes. Equation (3) is used to adjust the lane-width CMF for these target or “related” crash types to a lane-width CMF applicable to total crashes:

40 𝐶𝑀𝐹 𝐶𝑀𝐹 1.0 𝑝 1.0 (3) where: CMFra = crash modification factor for the effect of lane width on target or “related” crashes (i.e., single-vehicle run-off-the-road and multiple-vehicle head-on, opposite-direction sideswipe, and same-direction sideswipe crashes), such as the crash modification factor for lane width shown in Table 24 pra = proportion of total crashes constituted by crash types related to lane and shoulder width The proportion of related crashes, pra, (i.e., single-vehicle run-off-the-road, and multiple-vehicle head-on, opposite-direction sideswipe, and same-direction sideswipes crashes) is estimated as 0.574 (i.e., 57.4 percent) based on the default distribution of crash types presented in HSM Table 10-4. This default crash type distribution, and therefore, the value of pra, may be updated from local data as part of the calibration process. 5.2.1.2 Shoulder Width and Shoulder Type HSM Chapter 10 (Rural Two-Lane Highways) presents CMFs for shoulder width and shoulder type on rural two-lane roadways (6). The shoulder width effect, represented by CMFwra, is calculated using the equations shown in Table 25. A 6-ft shoulder represents the base condition (CMF = 1.0). Shoulders wider than 6-ft (e.g., 8-ft shoulders) have CMFs less than 1.0, and shoulders narrower than 6 ft have CMFs greater than 1.0. The shoulder width CMF for rural two- lane highways (CMFwra) is illustrated in Figure 3. The base condition for shoulder type is a paved shoulder (CMF = 1.0). Table 26 presents values for CMFtra which adjusts for the safety effects of shoulder types (paved, gravel, turf, and composite shoulders). Table 25. CMF for Shoulder Width on Rural Two-Lane Roadway Segments (CMFwra) (6, 10, 34, 36) Shoulder width Average annual daily traffic (AADT) (vehicles/day) < 400 400 to 2000 > 2000 0 ft 1.10 1.10 + 2.5 x 10-4(AADT-400) 1.50 2 ft 1.07 1.07 + 1.43 x 10-4(AADT-400) 1.30 4 ft 1.02 1.02 + 8.125 x 10-5(AADT-400) 1.15 6 ft 1.00 1.00 1.00 8 ft or more 0.98 0.98 - 6.875 x 10-5(AADT-400) 0.87 NOTE: The collision types related to lane width to which this CMFs applies include single- vehicle run-off-the-road and multiple-vehicle head-on, opposite-direction sideswipe, and same-direction sideswipe crashes. Standard error of the CMF is unknown. To determine the CMF for changing paved shoulder width and/or AADT, divide the “new” condition CMF by the “existing” condition CMF. NOTE: Based on HSM Table 10-9. The values from this table are used as CMFwra in Equation (4).

41 NOTE: Based on HSM Figure 10-8. Figure 3. Crash Modification Factor for Shoulder Width on Rural Two-Lane Highway Roadway Segments (6, 10, 34, 36) A combined CMF for shoulder width and type is computed as: 𝐶𝑀𝐹 𝐶𝑀𝐹 𝐶𝑀𝐹 1.0 𝑝 1.0 (4) where: CMFwra = crash modification factor for shoulder width from the equations in Table 25. CMFtra = crash modification factor for shoulder type from Table 26. If the shoulder types and/or widths for the two directions of a roadway segment differ, the CMFs are determined separately for the shoulder type and width in each direction of travel and the resulting CMFs are then averaged. The CMFs for shoulder width and type shown above apply only to the collision types that are most likely to be affected by shoulder width and type: single-vehicle run-off-the-road and multiple-vehicle head-on, opposite-direction sideswipe, and same-direction sideswipe crashes. The CMFs expressed on this basis are, therefore, adjusted to total crashes using Equation (4). The HSM default value for pra for two-lane highways in Equation (4) is 0.574.

42 Table 26. CMFs for Shoulder Types and Shoulder Width on Roadway Segments (CMFtra) (6, 10, 34, 36) Shoulder type Shoulder width (ft) 0 1 2 3 4 6 8 Paved 1.00 1.00 1.00 1.00 1.00 1.00 1.00 Gravel 1.00 1.00 1.01 1.01 1.01 1.02 1.02 Composite 1.00 1.01 1.02 1.02 1.03 1.04 1.06 Turf 1.00 1.01 1.03 1.04 1.05 1.08 1.11 NOTE: The values for composite shoulders in this table represent a shoulder for which 50 percent of the shoulder width is paved and 50 percent of the shoulder width is turf. NOTE: Based on HSM Exhibit 10-10. 5.2.1.3 Realignment of Horizontal Curves HSM Chapter 10 (Rural Two-Lane Highways) presents the following CMF for radius and length of horizontal curves on rural two-lane roads (6): 𝐶𝑀𝐹 1.55 𝐿 80.2 𝑅 0.012 𝑆 1.55 𝐿 (5) where: Lc = Length of horizontal curve including length of spiral transitions, if present (mi); R = Radius of curvature (ft); and S = 1 if spiral transition curve is present: 0 if spiral transition curve is not present. The base condition (CMF = 1.0) is a tangent alignment with no curvature. This CMF applies to total crashes and is based on research by Zegeer et al. (37). 5.2.1.4 Superelevation Restoration/Improvement on Horizontal Curves HSM Chapter 10 presents a CMF for superelevation variance on rural two-lane highways (6), as follows: 𝐶𝑀𝐹 1.00 𝑓𝑜𝑟 𝑆𝑉 0.01 (6) 𝐶𝑀𝐹 1.00 6 𝑆𝑉 0.01 𝑓𝑜𝑟 0.01 𝑆𝑉 0.02 (7) 𝐶𝑀𝐹 1.06 3 𝑆𝑉 0.02 𝑓𝑜𝑟 𝑆𝑉 0.02 (8) where: SV = superelevation variance (ft/ft), which represents the superelevation rate presented in the AASHTO Green Book minus the actual superelevation rate of the curve. This CMF applies to total roadway segment crashes located on horizontal curves.

43 5.2.1.5 Centerline Rumble Strips HSM Chapter 10 includes a CMF for the effects of a centerline rumble strip on a rural two-lane highway (6). The value of the CMF is 0.94 for crashes of all types. 5.2.1.6 Shoulder Rumble Strips NCHRP Report 641 (38) found that shoulder rumble strips on rural two-lane highways reduce the target crash type, single-vehicle run-off-road crashes by 15 percent, corresponding to a CMF of 0.85. Based on the estimates of crash-type proportions from HSM Chapter 10, the estimated CMF for the effect of shoulder rumble strips on total crashes is 0.92. 5.2.1.7 Striping and Delineation 3R projects often include improving the striping and delineation of the roadway. Replacement of pavement markings is an inherent part of most 3R projects, since pavement markings must be renewed when a roadway is resurfaced. No long-term safety benefit is likely if the existing pavement markings are simply replaced with equivalent pavement markings. Research for the Missouri Department of Transportation (MoDOT) in the report, Benefit–cost Evaluation of MoDOT’s Total Striping and Delineation Program (39) presents CMFs for striping and delineation improvement packages that include replacement of conventional painted markings with more durable markings that often have higher retroreflectivity. These striping and delineation packages often included placement of shoulder rumble strips as well, so the CMFs presented in this section should not be combined with the CMFs for shoulder rumble strips. The MoDOT evaluation (39) found a 24.5 percent crash reduction in fatal and injury crashes on rural two-lane highways that were improved with striping and delineation packages, equivalent to a CMF of 0.76. A CMF of 0.76 also represents the best available estimate of the effectiveness of striping and delineation packages for property-damage-only crashes and for total crashes. 5.2.1.8 Roadside Slope Flattening Flattening the roadside slope refers to lessening the steepness of the slope, such as adjusting a 1V:3H slope to a 1V:4H or 1V:6H slope. Roadside slope flattening may be considered as a crash countermeasure in 3R projects. Flatter roadside slopes provide the driver with a higher probability of a safe recovery maneuver in a vehicle that has departed the roadway, thus lessening the chance of a rollover or a collision with a roadside object. Roadside slope flattening appears to be primarily applicable to roadside foreslopes in fill sections, but flattening of cut slopes may be considered as well. Research by Fitzpatrick et al. (40) quantified the effect of roadside slope flattening on total crash reduction based on the CMFs shown in Table 27. A roadside slope of 1V:3H represents the baseline condition in Table 27.

44 Table 27. Roadside Slope CMFs for Rural Two-Lane Highways (40) Roadside slope CMF 1V:2H 1.01 1V:3H 1.00 1V:4H 0.95 1V:6H 0.89 5.2.1.9 Guardrail Installation/Restoration 3R projects frequently include installation of new guardrail where it is found to be needed or restore existing guardrail by providing needed maintenance or replacing older guardrail or guardrail end terminals with current designs. There are no formal CMFs for guardrail installation or restoration. The cost-effectiveness of guardrail improvements is typically assessed with the Roadside Safety Analysis Program (RSAP) (12,13,14) which incorporates methods for quantifying the effectiveness of guardrail improvements. 5.2.1.10 Intersection Left- and Right-Turn Lanes HSM Chapter 10 presents CMFs for the installation of left- and right-turn lanes at an intersection on rural two-lane highways (6). Table 28 presents CMFs for installing left-turn lanes and Table 29 presents CMFs for installing right-turn lanes. These CMFs apply to crashes of all crash severity levels. Table 28. CMFs for Installation of Left-Turn Lanes on Intersection Approaches (6) Intersection Type Intersection Traffic Control Number of Approaches with Left-Turn Lanesa One Approach Two Approaches Three Approaches Four Approaches Three-leg Intersection Minor road stop controlb 0.56 0.31 — — Four-leg Intersection Minor road stop control b 0.72 0.52 — — Traffic signal 0.82 0.67 0.55 0.45 a Stop-controlled approaches are not considered in determining the number of approaches with left-turn lanes b Stop signs present on minor road approaches only NOTE: Based on HSM Table 10-13. Table 29. CMFs for Installation of Right-Turn Lanes on Intersection Approaches (6) Intersection Type Intersection Traffic Control Number of Approaches with Right-Turn Lanesa One Approach Two Approaches Three Approaches Four Approaches Three-leg Intersection Minor road stop controlb 0.86 0.74 — — Four-leg Intersection Minor road stop control b 0.86 0.74 — — Traffic signal 0.96 0.92 0.88 0.85 a Stop-controlled approaches are not considered in determining the number of approaches with right-turn lanes b Stop signs present on minor road approaches only NOTE: Based on HSM Table 10-14.

45 These CMFs apply to left- or right-turn lanes installed on signalized intersection approaches or uncontrolled major-road approaches to stop-controlled intersections, but do not apply to stop- controlled approaches to stop-controlled intersections. 5.2.2 Rural Multilane Undivided Highways 5.2.2.1 Lane Width The CMF for lane widths on multilane undivided roadways is determined using the equations in Table 30 (6). The base condition for lane width on multilane undivided highways is 12 ft. Figure 4 illustrates graphically the CMFs for lane widths on rural multilane undivided highways shown in Table 30. Table 30. CMF for Lane Width on Undivided Rural Multilane Roadway Segments (6, 41) Lane width Average annual daily traffic (AADT) (veh/day) < 400 400 to 2000 > 2000 9 ft or less 1.04 1.04 + 2.13 x 10-4(AADT-400) 1.38 10 ft 1.02 1.02 + 1.31 x 10-4(AADT-400) 1.23 11 ft 1.01 1.01 + 1.88 x 10-5(AADT-400) 1.04 12 ft or more 1.00 1.00 1.00 NOTE: The collision types related to lane width to which these CMFs apply are single-vehicle run-off-the-road and multiple-vehicle head-on, opposite-direction sideswipe, and same-direction sideswipe crashes. Standard error of the CMF is unknown. To determine the CMF for changing lane width and/or AADT, divide the “new” condition CMF by the “existing” condition CMF. NOTE: Based on HSM Table 11-11.

46 NOTE: Based on HSM Figure 11-8. Figure 4. CMFra for Lane Width on Undivided Roadway Segments on Rural Multilane Highways (6, 41) The CMFs shown in Table 30 and Figure 4 are applicable to single-vehicle run-off-the-road, multiple-vehicle head-on, opposite-direction sideswipe, and same-direction sideswipe crashes. Equation (3) can be used to convert these crash-type-specific CMFs to a CMF for total crashes. The default value of pra in Equation (3) is 0.27 for rural multilane undivided highways. 5.2.2.2 Shoulder Width and Shoulder Type HSM Chapter 11 (Rural Multilane Highways) presents CMFs for shoulder type and width on rural multilane undivided highways (6). CMFs for undivided sections of multilane highways are calculated using the same equations as two-lane highways, using the shoulder width effect shown in Table 25 (see also Figure 3) and the shoulder type effect shown in Table 26. As in the case of rural two-lane highways, the base condition for the shoulder width CMF on a rural multilane highway is 6-ft and the base condition for shoulder type is a paved shoulder. Also, as is the case for rural two-lane highways, the shoulder width and type CMF for rural multilane highways is adjusted from related crashes to total crashes using Equation (4). The HSM default value for pra for rural multilane undivided highways used in Equation (4) is 0.27. 5.2.2.3 Superelevation Restoration/Improvement on Horizontal Curves No CMFs are available for superelevation variance or superelevation restoration for horizontal curves on rural multilane undivided highways. The CMFs shown in Equations (6) through (8)

47 represent the best available estimate of the effect of superelevation variance on crash frequency for curves on rural multilane undivided highways. 5.2.2.4 Centerline Rumble Strips No CMFs are available for the effect of centerline rumble strips on rural multilane highways. For purposes of these guidelines, the safety effect of centerline rumble strips on rural multilane undivided highways has been estimated as a CMF of 0.94, the same value used in the HSM for rural two-lane highway segments. 5.2.2.5 Shoulder Rumble Strips No CMF for the effect of shoulder rumble strips on rural multilane undivided highways has been found, but for purposes of these guidelines the CMF has been estimated as 0.92, the same as the CMF for rural two-lane undivided highways. 5.2.2.6 Striping and Delineation The MoDOT evaluation (39) presents striping and delineation evaluation results for rural multilane undivided highways. The MoDOT evaluation found that fatal and injury crashes were reduced by 29.8 percent on rural multilane undivided highways that were improved with striping and delineation packages. This is equivalent to a CMF of 0.70. For purposes of these guidelines, we have assumed that the CMFs for property-damage-only (PDO) crashes are the same as the CMF for fatal and injury crashes. Thus, the total crash CMF for striping and delineation packages on rural multilane undivided highways is 0.70. 5.2.2.7 Roadside Slope Flattening HSM Chapter 11 presents CMFs for roadside slope flattening on rural multilane undivided highways (6), shown in Table 31. These CMFs apply to total crashes. The base condition for these crashes is a roadside slope of 1V:7H.

48 Table 31. Roadside Slope CMFs for Rural Multilane Highways (6) Roadside slope CMF 1V:2H 1.18 1V:3H 1.15 1V:4H 1.12 1V:5H 1.09 1V:6H 1.05 1V:7H 1.00 NOTE: Based on HSM Table 11-14. 5.2.2.8 Guardrail Installation/Restoration 3R projects frequently include installation of new guardrail where it is found to be needed or restore existing guardrail by providing needed maintenance or replacing older guardrail or guardrail end terminals with current designs. There are no formal CMFs for guardrail installation or restoration. The cost-effectiveness of guardrail improvements is typically assessed with the Roadside Safety Analysis Program (RSAP) (12,13,14) which incorporates methods for quantifying the effectiveness of guardrail improvements. 5.2.2.9 Intersection Left- and Right-Turn Lanes HSM Chapter 11 presents CMFs for the installation of left- and right-turn lanes at intersections on rural multilane highways (6). CMF values for installation of left- and right-turn lanes are presented in Table 32 and Table 33, respectively. Separate CMFs apply to total and fatal and injury crashes. Table 32. CMFs for Installation of Left-Turn Lanes on Intersection Approaches (6) Intersection Type Crash Severity Level Number of Approaches with Left-Turn Lanes a One Approach Two Approaches Three-leg minor road stop controlb Total 0.56 —Fatal and Injury 0.45 — Four-leg minor road stop controlb Total 0.72 0.52Fatal and Injury 0.65 0.42 a Stop-controlled approaches are not considered in determining the number of approaches with left-turn lanes. b Stop signs present on minor road approaches only. NOTE: Based on HSM Table 11-22. Table 33. CMFs for Installation of Right-Turn Lanes on Intersection Approaches (6) Intersection Type Crash Severity Level Number of Approaches with Left-Turn Lanesa One Approach Two Approaches Three-leg minor road stop controlb Total 0.86 —Fatal and Injury 0.77 — Four-leg minor road stop controlb Total 0.86 0.74Fatal and Injury 0.77 0.59 a Stop-controlled approaches are not considered in determining the number of approaches with right-turn lanes. b Stop signs present on minor road approaches only. NOTE: Based on HSM Table 11-23.

49 5.2.3 Rural Multilane Divided Highways (Nonfreeways) 5.2.3.1 Lane Width The CMF for lane width on multilane divided roadways is determined using the equations in Table 34 (6). The base condition for lane width on multilane undivided highways is 12 ft. Figure 5 illustrates graphically the CMFs for lane widths on rural multilane divided highways. Table 34. CMF for Lane Width on Divided Rural Multilane Roadway Segment (6, 41) Lane width Average annual daily traffic (AADT) (vehicles/day) < 400 400 to 2000 > 2000 9 ft or less 1.03 1.03 + 1.381 x 10-4(AADT-400) 1.25 10 ft 1.01 1.01 + 8.75 x 10-4(AADT-400) 1.15 11 ft 1.01 1.01 + 1.25 x 10-5(AADT-400) 1.03 12 ft or more 1.00 1.00 1.00 NOTE: The collision types related to lane width to which these CMFs apply are single-vehicle run-off-the-road and multiple-vehicle head-on, opposite-direction sideswipe, and same-direction sideswipe crashes. Standard error of the CMF is unknown. To determine the CMF for changing lane width and/or AADT, divide the “new” condition CMF by the “existing” condition CMF. NOTE: Based on HSM Table 11-16. NOTE: Based on HSM Figure 11-10. Figure 5. CMFra for Lane Width on Divided Roadway Segments on Rural Multilane Highways (6, 41)

50 The CMFs shown in Table 34 and Figure 5 are applicable to single-vehicle run-off-the-road, multiple-vehicle head-on, opposite-direction sideswipe, and same-direction sideswipe crashes. Equation (3) can be used to convert these crash-type-specific CMFs to a CMF for total crashes. The default value of pra in Equation (3) is 0.50 for rural multilane divided highways. 5.2.3.2 Shoulder Width HSM Chapter 11 (Rural Multilane Highways) presents CMFs for shoulder width on rural multilane divided highways (6). CMFs for the width of the right (outside) shoulder of a rural multilane divided highway are given by the values presented in Table 35. The base condition (CMF = 1.0) for the divided highway shoulder width is represented by an 8-ft shoulder. This CMF applies to total crashes and does not need to be adjusted using a pra value. There is no CMF for shoulder type. Table 35. CMFs for Paved Right (Outside) Shoulder Width on Rural Multilane Divided Highway Segments (6, 42) Average paved shoulder width (ft) 0 2 4 6 8 or more 1.18 1.13 1.09 1.04 1.00 NOTE: Based on HSM Table 11-17. There is no CMF available for the width or type of the left (inside) shoulder of a rural multilane highway (nonfreeway). 5.2.3.3 Realignment of Horizontal Curves No CMFs for horizontal curve length, radius, or superelevation on rural multilane divided highways (nonfreeways) are presented in HSM Chapter 11. 5.2.3.4 Superelevation Restoration/Improvement on Horizontal Curves No CMFs were found for superelevation variance or superelevation restoration for horizontal curves on rural multilane divided highways. The CMFs shown in Equations (6) through (8) for rural two-lane highways represent the best available estimate of the effect of superelevation variance on crash frequency for curves on rural multilane divided highways. 5.2.3.5 Shoulder Rumble Strips A CMF for the effect of shoulder rumble strips on total crashes on multilane divided highways presented in HSM Chapter 13 is 0.84 (6).

51 5.2.3.6 Striping and Delineation The MoDOT evaluation (39) also presents results for rural multilane divided highways. On rural multilane divided highways that were improved with striping and delineation packages, fatal and injury crashes were reduced by 13.8 percent, equivalent to a CMF of 0.86. For the purpose of this research, we have assumed that the CMFs for PDO crashes are the same as the CMF for fatal and injury crashes. Thus, the total crash CMF for striping and delineation packages on rural multilane divided highways is 0.86. 5.2.3.7 Roadside Slope Flattening No CMFs are available for rural multilane divided highways, but the CMFs presented in Table 31 likely represent the best available estimate. 5.2.3.8 Guardrail Installation/Restoration 3R projects frequently install new guardrail where it is found to be needed or restore existing guardrail by providing needed maintenance or replacing older guardrail with current designs. There are no formal CMFs for guardrail installation or restoration. The cost-effectiveness of guardrail improvements is typically assessed with the Roadside Safety Analysis Program (RSAP) (12,13,14) which incorporates methods for quantifying the effectiveness of guardrail improvements. 5.2.3.9 Intersection Left- and Right-Turn Lanes HSM Chapter 11 presents CMFs for the installation of left- and right-turn lanes at intersections on rural multilane highways. CMF values for installation of left- and right-turn lanes are presented in Table 36 and Table 37, respectively. Separate CMFs apply to total and fatal and injury crashes. Table 36. CMFs for Installation of Left-Turn Lanes on Intersection Approaches (6) Intersection Type Crash Severity Level Number of Approaches with Left-Turn Lanes a One Approach Two Approaches Three-leg minor road stop controlb Total 0.56 —Fatal and Injury 0.45 — Four-leg minor road stop controlb Total 0.72 0.52Fatal and Injury 0.65 0.42 a Stop-controlled approaches are not considered in determining the number of approaches with left-turn lanes. b Stop signs present on minor road approaches only. NOTE: Based on HSM Table 11-22.

52 Table 37. CMFs for Installation of Right-Turn Lanes on Intersection Approaches (6) Intersection Type Crash Severity Level Number of Approaches with Left-Turn Lanesa One Approach Two Approaches Three-leg minor road stop controlb Total 0.86 — Fatal and Injury 0.77 — Four-leg minor road stop controlb Total 0.86 0.74 Fatal and Injury 0.77 0.59 a Stop-controlled approaches are not considered in determining the number of approaches with right-turn lanes. b Stop signs present on minor road approaches only. NOTE: Based on HSM Table 11-23. 5.2.4 Urban and Suburban Arterials 5.2.4.1 Lane Width There are no CMFs for lane width on urban and suburban arterials in HSM Chapter 12. The AASHTO Green Book (1) provides broad flexibility for the use of 10-, 11-, and 12-ft lanes on urban and suburban arterials. Recent research (43) found no substantial differences in safety performance between 10-, 11-, and 12-ft lanes, except in limited cases. 5.2.4.2 Shoulder Type and Width There are no CMFs for shoulder width on urban and suburban arterials in HSM Chapter 12. The AASHTO Green Book (6) provides broad flexibility in choosing shoulder widths on urban and suburban arterials, including use of curb-and-gutter sections with no shoulder on low- and intermediate-speed arterials. Research is underway as part of the FHWA Highway Safety Information System (HSIS) program to assess the safety performance of shoulder widths and curb-and-gutter sections on urban and suburban arterials. 5.2.4.3 Realignment of Horizontal Curves There are no CMFs for horizontal curve length or radius on urban and suburban arterials in HSM Chapter 12 and there is no definitive research on this topic. Research is underway as part of the FHWA HSIS program to assess the effects of horizontal curve design elements on urban and suburban arterials. 5.2.4.4 Superelevation There are no CMFs for superelevation of horizontal curves on urban and suburban arterials in HSM Chapter 12 and there is no definitive research on this topic.

53 5.2.4.5 Centerline Rumble Strips NCHRP Report 641, Guidance for the Design and Application of Shoulder and Centerline Rumble Strips (38), found that centerline rumble strips on urban two-lane arterials reduce total target crashes by 40 percent, corresponding to a CMF of 0.60. Target crashes for centerline rumble strips are those involving a lane departure to the left. Based on the estimates of crash- type proportions from HSM Chapter 12 and the assumption that 50 percent of lane departures on rural two-lane highways are to the left, the estimated CMF for the effect of centerline rumble strips on total crashes on urban and suburban two-lane arterials is 0.96. No CMFs are available for the safety effects of centerline rumble strips on multilane urban and suburban arterials. 5.2.4.6 Shoulder Rumble Strips No research was found on CMFs for shoulder rumble strips on urban and suburban arterials. For the purposes of these guidelines, it is estimated that the CMF for shoulder rumble strips is 0.92 for undivided arterials and 0.84 for divided arterials, consistent with the values found in research for rural highways. It is assumed that shoulder rumble strips would only be installed on urban and suburban arterials with open cross sections (i.e., with shoulder present) and not on urban and suburban arterials with curb-and-gutter sections. 5.2.4.7 Striping and Delineation There are no accepted CMFs for striping and delineation packages on urban and suburban arterials. 5.2.4.8 Intersection Left- and Right-Turn Lanes HSM Chapter 12 presents CMFs for the installation of left- and right-turn lanes at intersections on urban and suburban arterials (6). CMF values for installation of left- and right-turn lanes are shown in Table 38 and Table 39, respectively. These CMFs apply to crashes of all severity levels. Table 38. CMFs for Installation of Left-Turn Lanes on Intersection Approaches (6) Intersection Type Intersection Traffic Control Number of Approaches with Left-Turn Lanesa One Approach Two Approaches Three Approaches Four Approaches Three-leg intersection Minor road stop control b 0.67 0.45 — — Traffic signal 0.93 0.86 0.80 — Four-leg intersection Minor road stop control b 0.73 0.53 — — Traffic signal 0.90 0.81 0.73 0.66 a Stop-controlled approaches are not considered in determining the number of approaches with left-turn lanes. b Stop signs present on minor road approaches only. NOTE: Based on HSM Table 11-24.

54 Table 39. CMFs for Installation of Right-Turn Lanes on Intersection Approaches (6) Intersection Type Intersection Traffic Control Number of Approaches with Right-Turn Lanesa One Approach Two Approaches Three Approaches Four Approaches Three-leg intersection Minor road stop control b 0.86 0.74 — — Traffic signal 0.96 0.92 — Four-leg intersection Minor road stop control b 0.86 0.74 — — Traffic signal 0.96 0.92 0.88 0.85 a Stop-controlled approaches are not considered in determining the number of approaches with right-turn lanes. b Stop signs present on minor road approaches only. NOTE: Based on HSM Table 11-26. 5.2.5 Rural and Urban Freeways 5.2.5.1 Lane Width HSM Chapter 18 (7) presents CMFs for lane width on through traffic lanes on freeways. The CMFs are the same for fatal and injury multiple-vehicle crashes and fatal and injury single- vehicle crashes and are determined using the following equations. The CMF is applicable to lane widths in the range of 10.5 to 14 ft. 𝐶𝑀𝐹 𝑒 , 𝐼𝑓 𝑊 13 𝑓𝑡 (9) 𝐶𝑀𝐹 𝑏, 𝐼𝑓 𝑊 13 𝑓𝑡 (10) where: a = -0.0376 b = 0.963 Wl = lane width (ft) The HSM shows no quantifiable effect of lane width on property-damage-only crashes. 5.2.5.2 Inside Shoulder Width HSM Chapter 18 (7) presents a CMF for paved shoulder width on the inside or median side of freeways. The CMF is calculated using the following equation. 𝐶𝑀𝐹 𝑒 (11) where: a = CMF coefficient (see Table 40) Wis = paved inside shoulder width (ft) This CMF is applicable to paved shoulder widths in the range of 2 to 12 ft. The CMF is computed separately for fatal and injury crashes and property-damage-only crashes. Table 40

55 shows the coefficient values that are used in Equation (11). Note that for a given crash severity level, the coefficient values are the same for multiple- and single-vehicle crashes. Table 40. Coefficients for Inside Shoulder Width CMF on Freeways (7) Crash Type Crash Severity Level CMF Coefficient (a) Multiple vehicle Fatal and injury -0.0172Property damage only -0.0153 Single vehicle Fatal and injury -0.0172 Property damage only -0.0153 5.2.5.3 Outside Shoulder Width HSM Chapter 18 (7) presents a CMF for paved shoulder width on the outside or right side of freeway roadways. The CMF is calculated using the following equation: 𝐶𝑀𝐹 1.0 𝐿 ,2𝐿 𝑒 𝐿 , 2𝐿 𝑒 (12) where: a,b = CMF coefficients (see Table 41) Ws = paved inside shoulder width (ft) Lc,i = length of curve i on freeway segment (mi) L = length of freeway segment (mi) m = total number of curves on freeway section for both directions of travel combined The curve lengths need to be summed for both directions of the freeway. The coefficient values used in Equation (12) are shown in Table 41. This CMF is only applicable to paved outside shoulder widths in the range of 4 to 14 ft. Also, this CMF is only applicable to single-vehicle crashes of all severity levels. Equation (12) is presented here in a slightly different form than it appears in the HSM so that the effect of all curves for both direction so travel combined can be estimated in a single calculation. Table 41. Coefficients for Outside Paved Shoulder Width CMF on Freeways (7) Crash Type Crash Severity Level CMF Coefficients a b Single-vehicle Fatal and injury -0.0647 -0.0897Property damage only 0.00 -0.0840 5.2.5.4 Shoulder Rumble Strips HSM Chapter 18 (7) presents a CMF for the presence of shoulder rumble strips on the inside and outside shoulders of freeway roadways. The CMF is presented in the following equations:

56 𝐶𝑀𝐹 1.0 𝐿 ,2𝐿 𝑓 𝐿 , 2𝐿 (13) 𝑓 0.5 1.0 𝑃 0.811𝑃 0.5 1.0 𝑃 0.811𝑃 (14) where: Pir = proportion of segment with rumble strips present on the inside shoulders Por = proportion of segment with rumble strips present on the outside shoulders Equation (13) is presented here in a slightly different form than it appears in the HSM so that the effect of all curves for both directions of travel combined can be estimated in a single calculation. 5.2.5.5 Median Barrier HSM Chapter 18 (7) presents a CMF for the presence of a median barrier on a freeway. The CMF is computed using the following equations. This CMF is applicable to both for single- and multiple-vehicle collisions of all crash severity levels. 𝐶𝑀𝐹 1.0 𝑃 𝑃 𝑒 (15) where: a = CMF coefficient (see Table 42) Pib = proportion of segment length with a barrier present in the median (see discussion below for calculation of Pib) Wicb = distance from edge of inside shoulder to barrier face (see discussion below for calculation of Wicb) If a continuous median barrier is centered in the median, use Equations (16) and (17) to calculate Pib and Wicb. 𝑊 2𝐿 ∑ 𝐿 ,𝑊 , , 𝑊 2𝐿 ∑𝐿 , 0.5 𝑊 2𝑊 𝑊 (16) 𝑃 1.0 (17) where: L = length of segment (mi) Lib,i = length of lane paralleled by inside barrier i (mi) Woff,in,i = horizontal clearance from the edge of the traveled way to the face of inside barrier i (ft) Wis = paved inside shoulder width (ft) Wm = median width (measured from near edges of traveled way in both directions) (ft) Wib = inside barrier width (measured from barrier face to barrier face) (ft)

57 If a continuous median barrier is adjacent to one of the directions of travel, but not centered in the median, use Equation (18) and (19) to calculate Pib and Wicb. 𝑊 2𝐿𝐿 𝑊 𝑊 ∑ 𝐿 ,𝑊 , , 𝑊 𝐿 ∑𝐿 ,𝑊 2𝑊 𝑊 𝑊 (18) 𝑃 1.0 (19) where: Wnear = near horizontal clearance from the edge of the traveled way to the continuous median barrier (measure for both travel directions and use the smaller distance) (ft) For segments with some short sections of barrier in the median, use Equation (20) and (21) to determine Pib and Wicb. 𝑊 ∑𝐿 ,∑𝑊 , , 𝑊 (20) 𝑃 ∑𝐿 ,2𝐿 (21) Values for the median barrier CMF coefficient, a, are presented in Table 42. This coefficient is used in Equation (15). Table 42. Coefficients for Presence of Median Barrier CMF on Freeways (7) Crash Type Crash Severity Level CMF Coefficient (a) Multiple vehicle Fatal and injury 0.131 Property damage only 0.169 Single vehicle Fatal and injury 0.131 Property damage only 0.169 5.2.5.6 Traffic Barrier on the Outside or Right Side of a Freeway HSM Chapter 18 (7) presents a CMF for the presence of a traffic barrier on the outside or right roadside of a freeway. The CMF is calculated using the following equations. This CMF is only applicable to single-vehicle collisions. The HSM shows no quantifiable effect of the presence of a median barrier on multiple-vehicle collisions. 𝐶𝑀𝐹 1.0 𝑃 𝑃 𝑒 (22) where: a = CMF coefficient (see Table 43) Pob = proportion of segment length with a barrier present on the roadside Wocb = distance from edge of outside shoulder to barrier face (ft)

58 Use Equations (23) and (24) to calculate Pob and Wocb. 𝑊 ∑𝐿 , ∑ 𝐿 ,𝑊 , , 𝑊 (23) 𝑃 ∑𝐿 ,2𝐿 (24) where: L = length of segment (mi) Lob,i = length of lane paralleled by outside barrier i (mi) Woff,o,i = horizontal clearance from the edge of the traveled way to the face of outside barrier i (ft) Ws = paved outside shoulder width (ft) Values for the outside barrier CMF coefficient, a, are presented in Table 43. This coefficient is used in Equation (22). Table 43. Coefficients for Presence of Outside Barrier CMF on Freeways (7) Crash Type Crash Severity Level CMF Coefficient (a) Single vehicle Fatal and injury 0.131 Property damage only 0.169 5.2.5.7 Median Width HSM Chapter 18 (7) presents a CMF for median width on a freeway. Median width is not likely to be changed in a 3R project. However, this CMF is presented here because the median width CMF is influenced by the paved inside shoulder width (Wis), which may be changed in a 3R project. The median width CMF is calculated using the following equation. This CMF is applicable to both single- and multiple-vehicle collisions of all crash severity levels. 𝐶𝑀𝐹 1.0 𝑃 𝑒 𝑃 𝑒 (25) where: a = CMF coefficient (see Table 44) Wis = paved inside shoulder width (ft) Wm = median width (measured from near edges of traveled way in both directions) (ft) Pib = proportion of segment length with a barrier present in the median (see Equations (16) through (21) for calculation of this variable) Wicb = distance from edge of inside shoulder to barrier face (ft) (see Equations (16) through (21) for calculation of this variable) Values for the median width CMF coefficient, a, are presented in Table 44. This coefficient is used in Equation (25).

59 Table 44. Coefficients for Median Width on Freeways (7) Crash Type Crash Severity Level CMF Coefficient (a) Multiple vehicle Fatal and injury -0.00302 Property damage only -0.00291 Single vehicle Fatal and injury 0.00102 Property damage only -0.00289

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Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects Get This Book
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The aging U.S. highway system, coupled with fiscal constraints, is placing increased pressures on highway agencies to maintain the highway system in a cost-effective manner and is, thus, creating greater needs for 3R projects.

The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 244: Developing Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects presents the results of research to develop improved design guidelines for 3R projects. The guidelines were developed to replace the older guidance presented in TRB Special Report 214: Designing Safer Roads: Practices for Resurfacing, Restoration, and Rehabilitation.

Supplementary to the Document is NCHRP Research Report 876: Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Two spreadsheet tools for benefit–cost analysis in support of design decisions for 3R projects also accompany the report. Spreadsheet Tool 1 is a tool for analysis of a single design alternative or combination of alternatives. Spreadsheet Tool 2 is a tool for comparison of several design alternatives or combinations of alternatives.

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