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Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects (2018)

Chapter: Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity

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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
×
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
×
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
×
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
×
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
×
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
×
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
×
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
×
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
×
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
×
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
×
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
×
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
×
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
×
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
×
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
×
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
×
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Suggested Citation:"Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity." National Academies of Sciences, Engineering, and Medicine. 2018. Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. Washington, DC: The National Academies Press. doi: 10.17226/25206.
×
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17 Chapter 4. Managing a 3R Program to Reduce Crash Frequency and Severity 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 shows how 3R programs can be managed not only to preserve pavements and extend their service life, but also as a key part of each highway agency’s efforts to reduce the frequency and severity of traffic crashes on the roadway system. This goal can be achieved through a structured design process that considers the need for design improvements in each 3R project, with the objective of implementing such improvements where a crash history review identifies an existing crash pattern or where an economic analysis indicates that a design improvement would be cost-effective. Prioritizing improvement needs is critical to using available funds efficiently and maximizing the crash reduction benefits from 3R improvements, so it is important that design improvements be made as part of 3R projects where needed and also that design improvements not be made where there is no demonstrated need and where such improvements would not be cost-effective. 4.1 Role of 3R Projects in Overall Safety Management Programs of Highway Agencies Highway agencies and their safety partners undertake a broad range of activities to reduce crash frequency and severity. These may include:  Monitoring the crash history of the roadway system and making design or traffic control improvements where potentially correctable crash patterns are identified  Conducting systemic reviews of the roadway system and making design or traffic control improvements where such improvements appear to be cost-effective based on their potential reduction of potential future crashes, whether or not there is a history of observed crashes  Reviewing plans for highway infrastructure projects, as they develop, to identify opportunities for incorporating design or traffic control changes in specific projects to reduce crash frequency or severity  Coordinating with law enforcement agencies to prioritize enforcement activities and focus enforcement at locations where it is most needed  Conducting public education and driver training programs to promote highway safety awareness and safe driving practices among the traveling public  Encouraging improvement of emergency medical services to assist crash victims in receiving on-site and in-hospital treatment as soon as practical after a crash occurs

18 All types of highway infrastructure improvement projects, including 3R projects, help to address the first three aspects of highway safety management listed above. The list of highway safety activities presented above makes clear that, while highway safety management includes monitoring crash history and developing projects to address identified crash patterns, it is in no way limited to that. Similarly, highway safety management should not be seen as limited to projects funded from safety specific funding sources such as the Highway Safety Improvement Program (HSIP). Any project, regardless of why it is initiated and how it is funded, can include features intended specifically to reduce crash frequency and severity. Thus, all types of projects, including 3R projects, can play an important role in reducing crash frequency and severity. Rational management of safety considerations in 3R projects requires an understanding of the crash reduction effectiveness of specific design features that may be considered as safety improvements in 3R projects. The crash reduction effectiveness of design features is typically presented in the form of crash modification factors, as explained in Section 4.2. 4.2 Quantifying Crash Reduction Effectiveness of 3R Improvements: Crash Modification Factors (CMFs) 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 to represent the crash reduction effectiveness of design improvements, presents their definition, and describes their use. The values of crash modification factors for specific 3R improvement types are presented in Section 4.3. 4.2.1 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 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

19 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) 4.2.2 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 (2) 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 reliable 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 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

20 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 proceed with planning of 3R programs with the expectation that the available CMFs will represent the average results to be expected. The HSM uses a formal inclusion rule (2, 12) 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 (13). 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. 4.2.3 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 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 modeling before-af that repre Figure 2 regressio from HSM (nonfreew for the SP The verti segment. Figure Cross-sec feasible. effect of without t analyses other fac mistaken computat technique k crash data. ter evaluatio sents the cr illustrates a n that could Figure 11 ays). SPFs F represent cal axis repr 2. Example tional studi Cross-sectio a specific ge hat feature o must be care tors are corr for an effec ional proced nown as Ne HSM Chap ns using the ash reductio typical SPF be used in d -3 and appli can have ei s the annual esents the p of Safety P es are condu nal modelin ometric des r where tha fully design elated with o t of the geom ure for dev gative Bino ter 9 present Empirical n effectiven for a roadw eveloping C es to roadw ther linear o average dai redicted num erformance Rural Mult cted when o g uses Nega ign feature o t feature is p ed and cond ne another, etric desig eloping CM 21 mial regress s a computa Bayes metho ess of the pr ay segment MFs with t ay segments r curvilinear ly traffic vo ber of cras Function ilane Highw bservationa tive Binom n crash cou resent in va ucted becau the effect o n feature of Fs with cros ion analysis tional proce d. The anal oject. developed w he EB meth on rural mu functional lume (AAD hes per mile for Undivid ays (2) l before-afte ial regressio nts by cons rying dimen se, where s f other facto interest. HS s-sectional m , which is w dure for obs ysis results ith negative od. This par ltilane undi forms. The h T) for the ro per year on ed Roadwa r evaluation n modeling idering sites sions. Cross pecific desig rs can poten M Chapter 9 odeling. ell suited to ervational provide a C Binomial ticular SPF vided highw orizontal ax adway segm the roadwa y Segments s are not to quantify with and -sectional n features o tially be also presen MF is ays is ent. y on the r ts a

22 Further guidance on CMF development is found, in HSM Chapter 9 (2) and in the FHWA report, A Guide to Developing Crash Modification Factors (14). 4.2.4 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 (2) 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 for 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. 4.3 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 (2), 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 (13), the original sources have also been reviewed. 4.3.1 Rural Two-Lane Highways 4.3.1.1 Lane Width HSM Chapter 10 (Rural Two-Lane Highways) presents CMFs for lane widths on rural two-lane highways (2). The CMF is calculated using the equations shown in Table 3 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 3.

Table Lan 9 ft or less 10 ft 11 ft 12 ft or mo NOTE: The multiple-ve Standard e To determi condition C NOTE: Base F The lane- the-road, sideswip crash typ 3. CMF fo e width re collision types hicle head-on, rror of the CM ne the CMF fo MF. d on HSM Ta NOTE: Based igure 3. CM width CMF and multipl e crashes. Eq es to a lane- r Lane Wid < 400 1.05 1.02 1.01 1.00 related to lane opposite-direc F is unknown. r changing lan ble 10-8. on HSM Figure Fra for La Two illustrated i e-vehicle he uation (3) i width CMF th on Rura Averag width to whic tion sideswipe e width and/or 10-7. ne Width o -Lane Roa n Table 3 an ad-on, oppo s used to adj applicable t 23 l Two-Lan e annual dail 40 1.05 + 2.81 1.02 + 1.75 1.01 + 2.5 x h these CMFs , and same-dir AADT, divide t n Undivide dway Segm d Figure 3 a site-directio ust the lane o total crash e Roadway y traffic (AAD 0 to 2000 x 10-4(AADT-4 x 10-4(AADT-4 10-5(AADT-4 1.00 apply are sing ection sideswi he “new” cond d Roadway ents (2, 15) pplies only n sideswipe -width CMF es: Segments ( T) (veh/day) 00) 00) 00) le-vehicle run-o pe crashes. ition CMF by t Segments to single-ve , and same- for these ta 2, 15, 16, 17 > 2000 1.50 1.30 1.05 1.00 ff-the-road an he “existing” on Rural hicle run-of direction rget or “rela ) d f- ted”

24 1.0 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 3 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. 4.3.1.2 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 (2). The shoulder width effect, represented by CMFwra, is calculated using the equations shown in Table 4. 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 4. The base condition for shoulder type is a paved shoulder (CMF = 1.0). Table 5 presents values for CMFtra which adjusts for the safety effects of shoulder types (paved, gravel, turf, and composite shoulders). Table 4. CMF for Shoulder Width on Rural Two-Lane Roadway Segments (2, 15, 16, 18) 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).

A combin where: C C If the sho are determ resulting The CMF most like multiple- The CMF The HSM NOTE: Based Figure 4. ed CMF fo MFwra = c T MFtra = c ulder types ined separ CMFs are th s for should ly to be affe vehicle head s expressed default val on HSM Figure Crash Mod Segment r shoulder w rash modifi able 4. rash modifi and/or width ately for the en average er width an cted by shou -on, opposi on this basi ue for pra fo 10-8. ification F s for Two-l idth and typ cation facto cation facto s for the tw shoulder ty d. d type show lder width te-direction s are, theref r two-lane h 25 actor for Sh ane Highwa e is comput 1.0 r for should r for should o directions pe and width n above app and type: sin sideswipe, a ore, adjuste ighways in oulder Wi y (2, 15, 16 ed as: 1. er width fro er type from of a roadwa in each dir ly only to th gle-vehicle nd same-di d to total cra Equation (4 dth on Roa , 18) 0 m the equati Table 5. y segment d ection of tra e collision t run-off-the rection sides shes using E ) is 0.574. dway ons in iffer, the C vel and the ypes that ar -road and wipe crashe quation (4) (4) MFs e s. .

26 Table 5. CMFs for Shoulder Types and Shoulder Width on Roadway Segments (CMFtra) (2, 15, 16, 18) 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. 4.3.1.3 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 (2): 1.55 80.2 0.012 1.55 (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. (19). 4.3.1.4 Superelevation Restoration/Improvement on Horizontal Curves HSM Chapter 10 presents a CMF for superelevation variance on rural two-lane highways (2), as follows: 1.00 0.01 (6) 1.00 6 0.01 0.01 0.02 (7) 1.06 3 0.02 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.

27 4.3.1.5 Centerline Rumble Strips HSM Chapter 10 includes a CMF for the effects of a centerline rumble strip on a rural two-lane highway (2). The value of the CMF is 0.94 for crashes of all types. 4.3.1.6 Shoulder Rumble Strips NCHRP Report 641 (20) 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. 4.3.1.7 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 (21) 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 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 (21) 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. 4.3.1.8 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. (22) quantified the effect of roadside slope flattening on total crash reduction based on the CMFs shown in Table 6. A roadside slope of 1V:3H represents the baseline condition in Table 6.

28 Table 6. Roadside Slope CMFs for Rural Two-Lane Highways (22) Roadside slope CMF 1V:2H 1.01 1V:3H 1.00 1V:4H 0.95 1V:6H 0.89 4.3.1.9 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) (23, 24, 25) which incorporates methods for quantifying the effectiveness of guardrail improvements. 4.3.1.10 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 (2). Table 7 presents CMFs for installing left-turn lanes and Table 8 presents CMFs for installing right-turn lanes. These CMFs apply to crashes of all crash severity levels. Table 7. CMFs for Installation of Left-Turn Lanes on Intersection Approaches (2) 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 controlb 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 8. CMFs for Installation of Right-Turn Lanes on Intersection Approaches (2) 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 controlb 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.

29 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. 4.3.2 Rural Multilane Undivided Highways 4.3.2.1 Lane Width The CMF for lane widths on multilane undivided roadways is determined using the equations in Table 9 (2). 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 undivided highways shown in Table 9. Table 9. CMF for Lane Width on Undivided Rural Multilane Roadway Segments (2, 26) 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. The CMFs shown in Table 9 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 0.27 for rural multilane undivided highways. 4.3.2.2 Shoulder Width and Shoulder Type HSM Chapter 11 (Rural Multilane Highways) presents CMFs for shoulder type and width on rural multilane undivided highways (2). 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 4 (see also Figure 4) and the shoulder type effect shown in Table 5. 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.

F 4.3.2.3 No CMF curves on represent for curve 4.3.2.4 No CMF purposes undivide rural two 4.3.2.5 No CMF found, bu CMF for NOTE: Based igure 5. CM Supereleva s are availab rural multi the best ava s on rural m Centerline s are availab of these gui d highways -lane highw Shoulder R for the effe t for purpos rural two-la on HSM Figure Fra for La tion Resto le for super lane undivid ilable estim ultilane und Rumble St le for the ef delines, the has been est ay segments umble Stri ct of shoulde es of these g ne undivide 11-8. ne Width o Multilane ration/Impr elevation va ed highway ate of the ef ivided highw rips fect of cente safety effec imated as a . ps r rumble str uidelines th d highways. 30 n Undivide Highways ovement o riance or su s. The CMF fect of supe ays. rline rumbl t of centerlin CMF of 0.9 ips on rural e CMF has d Roadway (2, 26) n Horizonta perelevation s shown in E relevation v e strips on ru e rumble st 4, the same multilane u been estima Segments o l Curves restoration quations (6 ariance on c ral multilan rips on rura value used i ndivided hig ted as 0.92, n Rural for horizon ) through (8 rash frequen e highways l multilane n the HSM f hways has the same as tal ) cy . For or been the

31 4.3.2.6 Striping and Delineation The MoDOT evaluation (21) 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 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. 4.3.2.7 Roadside Slope Flattening HSM Chapter 11 presents CMFs for roadside slope flattening on rural multilane undivided highways (2), shown in Table 10. These CMFs apply to total crashes. The base condition for these crashes is a roadside slope of 1V:7H. Table 10. Roadside Slope CMFs for Rural Multilane Highways (2) 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. 4.3.2.8 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) (23, 24, 25) which incorporates methods for quantifying the effectiveness of guardrail improvements. 4.3.2.9 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 (2). CMF values for installation of left- and right-turn lanes are presented in Table 11 and Table 12, respectively. Separate CMFs apply to total and fatal-and- injury crashes.

32 Table 11. CMFs for Installation of Left-Turn Lanes on Intersection Approaches (2) 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 0.52 Fatal 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 12. CMFs for Installation of Right-Turn Lanes on Intersection Approaches (2) 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. 4.3.3 Rural Multilane Divided Highways (Nonfreeways) 4.3.3.1 Lane Width The CMF for multilane divided roadways is determined using the equations in Table 13 (2). The base condition for lane width on multilane undivided highways is 12 ft. Figure 6 illustrates graphically the CMFs for lane widths on rural multilane divided highways. Table 13. CMF for Lane Width on Divided Rural Multilane Roadway Segment (2, 26) 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. The CMFs shown in Table 13 and Figure 6 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.

4.3.3.2 HSM Ch multilane multilane (CMF = CMF app for shoul There is n divided h NOTE: Based Figure 6. C Shoulder W apter 11 (Ru divided hig divided hig 1.0) for the d lies to total der type. Table NO o CMF ava ighway (non on HSM Figure MFra for L idth ral Multilan hways (2). C hway are gi ivided high crashes and 14. CMFs Rural Mul 0 1.18 TE: Based on ilable for th freeway). 11-10. ane Width Multilane e Highways MFs for th ven by the v way should does not ne for Paved R tilane Divid Average pav 2 1.13 HSM Table 11 e width or ty 33 on Divided Highways ) presents C e width of th alues presen er width is r ed to be adju ight (Outs ed Highwa ed shoulder w 4 1.09 -17. pe of the le Roadway S (2, 26) MFs for sho e right (out ted in Tabl epresented b sted using ide) Should y Segments idth (ft) 6 1.04 ft (inside) sh egments on ulder width side) should e 14. The ba y an 8-ft sh a pra value. er Width o (2, 27) 8 or more 1.00 oulder of a Rural on rural er of a rural se condition oulder. This There is no C n rural multil MF ane

34 4.3.3.3 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. 4.3.3.4 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) represent the best available estimate of the effect of superelevation variance on crash frequency for curves on rural multilane divided highways. 4.3.3.5 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 (2). 4.3.3.6 Striping and Delineation The MoDOT evaluation (21) 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. 4.3.3.7 Roadside Slope Flattening No CMFs are available for rural multilane divided highways, but the CMFs presented in Table 10 likely represent the best available estimate. 4.3.3.8 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) (23, 24, 25) which incorporates methods for quantifying the effectiveness of guardrail improvements.

35 4.3.3.9 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 15 and Table 16, respectively. Separate CMFs apply to total and fatal-and- injury crashes. Table 15. CMFs for Installation of Left-Turn Lanes on Intersection Approaches (2) 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 0.52 Fatal 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 16. CMFs for Installation of Right-Turn Lanes on Intersection Approaches (2) 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. 4.3.4 Urban and Suburban Arterials 4.3.4.1 Lane Width There are no CMFs for lane width on urban and suburban arterials in HSM Chapter 12. The AASHTO Green Book (2) provides broad flexibility for the use of 10-, 11-, and 12-ft lanes on urban and suburban arterials. Recent research (28) found no substantial differences in safety performance between 10-, 11-, and 12-ft lanes, except in limited cases. Research is currently underway in NCHRP Project 3-112 to document the safety performance of lane widths on urban and suburban arterials and provide design guidance. 4.3.4.2 Shoulder Type and Width There are no CMFs for shoulder width on urban and suburban arterials in HSM Chapter 12. The AASHTO Green Book (2) 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

36 Information System (HSIS) program to assess the safety performance of shoulder widths and curb-and-gutter sections on urban and suburban arterials. 4.3.4.3 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. 4.3.4.4 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. 4.3.4.5 Centerline Rumble Strips NCHRP Report 641, Guidance for the Design and Application of Shoulder and Centerline Rumble Strips (20), 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 10 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 roads is 0.96. No CMFs are available for the safety effects of centerline rumble strips on multilane urban and suburban arterials. 4.3.4.6 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. 4.3.4.7 Striping and Delineation There are no accepted CMFs for striping and delineation packages on urban and suburban arterials.

37 4.3.4.8 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 (2). CMF values for installation of left- and right-turn lanes are shown in Table 17 and Table 18, respectively. These CMFs apply to crashes of all severity levels. Table 17. CMFs for Installation of Left-Turn Lanes on Intersection Approaches (2) 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 0.67 0.45 — — Traffic signal 0.93 0.86 0.80 — Four-leg intersection Minor-road stop controlb 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. Table 18. CMFs for Installation of Right-Turn Lanes on Intersection Approaches (2) 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 — — Traffic signal 0.96 0.92 — Four-leg intersection Minor-road stop controlb 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. 4.3.5 Rural and Urban Freeways 4.3.5.1 Lane Width HSM Chapter 18 (3) presents CMFs for lane width on through traffic lanes on freeways (2). 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. , 13 (9) , 13 (10)

38 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. 4.3.5.2 Inside Shoulder Width HSM Chapter 18 (3) presents a CMF for paved shoulder width on the inside or median side of freeways. The CMF is calculated using the following equation. (11) where: a = CMF coefficient (see Table 19) 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 19 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 19. Coefficients for Inside Shoulder Width CMF on Freeways (3) Crash Type Crash Severity Level CMF Coefficient (a) Multiple vehicle Fatal and injury -0.0172 Property damage only -0.0153 Single vehicle Fatal and injury -0.0172 Property damage only -0.0153 4.3.5.3 Outside Shoulder Width HSM Chapter 18 (3) presents a CMF for paved shoulder width on the outside or right side of freeway roadways (2). The CMF is calculated using the following equation: 1.0 ,2 , 2 (12) where: a,b = CMF coefficients (see Table 20) 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

39 The curve lengths need to be summed for both directions of the freeway. The coefficient values used in Equation (12) are shown in Table 20. 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 20. Coefficients for Outside Paved Shoulder Width CMF on Freeways (3) Crash Type Crash Severity Level CMF Coefficients a b Single-vehicle Fatal and injury -0.0647 -0.0897 Property damage only 0.00 -0.0840 4.3.5.4 Shoulder Rumble Strips HSM Chapter 18 (3) presents a CMF for the presence of shoulder rumble strips on the inside and outside shoulders of freeway roadways (2). The CMF is presented in the following equations: 1.0 ,2 , 2 (13) 0.5 1.0 0.811 0.5 1.0 0.811 (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. 4.3.5.5 Median Barrier HSM Chapter 18 (3) presents a CMF for the presence of a median barrier on a freeway (2). 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. 1.0 (15) where: a = CMF coefficient (see Table 21) 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)

40 If a continuous median barrier is centered in the median, use Equations (16) and (17) to calculate Pib and Wicb. 2 ∑ , , , 2 ∑ , 0.5 2 (16) 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) 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. 2 ∑ , , , ∑ ,2 (18) 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. ∑ , ∑ , , (20) ∑ , 2 (21) Values for the median barrier CMF coefficient, a, are presented in Table 21. This coefficient is used in Equation (15).

41 Table 21. Coefficients for Presence of Median Barrier CMF on Freeways (3) 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 4.3.5.6 Traffic Barrier on the Outside or Right Side of a Freeway HSM Chapter 18 (3) 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 the presence of a of median barrier on multiple-vehicle collisions. 1.0 (22) where: a = CMF coefficient (see Table 22) Pob = proportion of segment length with a barrier present on the roadside Wocb = distance from edge of outside shoulder to barrier face (ft) Use Equations (23) and (24) to calculate Pob and Wocb. ∑ , ∑ , , , (23) ∑ , 2 (24)

42 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 22. This coefficient is used in Equation (22). Table 22. Coefficients for Presence of Outside Barrier CMF on Freeways (3) Crash Type Crash Severity Level CMF Coefficient (a) Single vehicle Fatal and injury 0.131 Property damage only 0.169 4.3.5.7 Median Width HSM Chapter 18 (3) presents a CMF for median width on a freeway (2). 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. 1.0 (25) where: a = CMF coefficient (see Table 25) 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 23. This coefficient is used in Equation (25). Table 23. Coefficients for Median Width on Freeways (3) 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

43 4.4 Investing Available 3R Funds for Maximum Reduction of Crash Frequency and Severity Since funds available for highway infrastructure improvements are limited, it is important that those funds, including funds for 3R projects, be invested to accomplish the project objectives, including preserving the pavement and extending the pavement service life, improving traffic operations, and, to the extent practical, maximizing the potential reduction in crash frequency and severity. Maximizing the reduction in crash frequency and severity requires that the available funds be invested in a logical priority fashion, focusing on those projects where the maximum crash reduction benefit can be obtained for the least cost. Recent research (6) has shown that investing available funds in safety improvement without careful planning can lead to suboptimal results. This is illustrated by the following example. 4.4.1 Example of Optimal and Suboptimal Strategies for Investing Available Funds in Design Improvements to Reduce Crash Frequency and Severity Highway agency users who apply the 3R design guidelines will apply them to one site at a time, but their appropriateness can be tested by applying them systemwide to rural highway systems and reviewing the results. As an example, four potential strategies for lane widening to the rural two-lane highway system of an actual state using data from the FHWA Highway Safety Information System (HSIS) were tested in recent research (7). This rural two-lane highway system consists of 4,630.71 mi of road with AADTs up to 25,000 veh/day. The current lane width distribution for this road system is: Lane Width (ft) Total Length (mi) 9 18.29 10 251.74 11 2,087.38 12 2,273.30 TOTAL 4,630.71 The following improvement strategies were considered and were applied as if the entire road system were a candidate for resurfacing in a single year:  Widen all lanes with widths less than 12 ft to 12 ft  Widen lanes where a need is indicated by the TRB Special Report 214 (1) lane width criteria presented in Table 24  Widen lanes on roadways where the AADT exceeds the minimum AADT criteria presented in Table 24  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

44 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 5.1. These values vary widely in current practice, but the assumptions used here are typical of the middle range of values currently used by highway agencies. Table 24. Minimum Lane and Shoulder Widths for Rural Two-Lane Highways from TRB Special Report 214 (1) 10 percent ormore trucksc Less than 10 percent trucks Design year volume (ADT)a Running speedb (mph) Lane width (ft) Combined lane and shoulder width (ft)d Lane width (ft) Combined lane and shoulder width (ft)d 1-750 Under 50 10 12 9 11 50 and over 10 12 10 12 751-2,000 Under 50 11 13 10 12 50 and over 12 15 11 14 Over 2,500 All 12 18 11 17 a Design volume for a given highway feature should match average traffic anticipated over the expected performance of that feature. b Highway segments should be classified as “under 50” only if most vehicles have an average speed of less than 50 mph over the length of the segment. c For this comparison, trucks are defined as heavy vehicles with six or more tires. d One foot less for highways in mountainous terrain. NOTE: This table is presented as an example of historical practice but, as shown below, its use is no longer recommended. 4.4.1.1 Lane Widening Strategy—Widen All Lanes to 12 ft This strategy would select for widening the 2,357.41 mi of roadway with existing lane widths less than 12 ft. This would change the lane-width distribution on the roadway system as shown below: Lane Width (ft) Total Length (mi) Before After 9 18.29 0.00 10 251.74 0.00 11 2,087.38 0.00 12 2,273.30 4,630.71 TOTAL 4,630.71 4,630.71 This improvement program would provide benefits of $68,911,192 at a cost of $516,336,773, equivalent to a benefit-cost ratio of 0.13. Of the 2,357.41 mi of roadway improved, 97 percent consisted of projects with benefit-cost ratios less than 1.0. This high proportion of projects that were not cost-effective occurred because most of the extensive mileage of two-lane roadways consisted of 11-ft lanes, where lane widening provides only a limited benefit (see Section 4.3.1.1)

45 4.4.1.2 Lane Widening Strategy—Widen Lanes Based on TRB Special Report 214 Criteria This strategy would select for widening the 832.28 mi of roadway that do not meet the TRB Special Report 214 lane width criteria shown in Table 24. This would change the lane-width distribution on the roadway system as shown below: Total Length (mi) Lane Width (ft) Before After 9 18.29 0.00 10 251.74 175.20 11 2,087.38 1,432.58 12 2,273.30 3,022.93 TOTAL 4,630.71 4,630.71 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. This strategy does a better job at focusing on the best projects and avoided many of the projects included in the 12-ft lane strategy that were not cost-effective, but not all of them. 4.4.1.3 Lane Widening Strategy—Minimum AADT Levels This strategy would select for widening the 54.88 mi of roadway that meet the minimum AADT criteria presented in Section 5.4. This would change the lane-width distribution on the roadway system as shown below: Total Length (mi) Lane Width (ft) Before After 9 18.29 18.29 10 251.74 231.25 11 2,087.38 2,052.99 12 2,273.30 2,328.18 TOTAL 4,630.71 4,630.71 This improvement program would provide benefits of $12,391,936 at a cost of $14,824,774, equivalent to a benefit-cost ratio of 0.84. Of the 54.88 mi of roadway improved, 60 percent consisted of projects with benefit-cost ratios less than 1.0. This strategy does a better job at focusing on the best projects and avoided many of the projects that are not cost-effective. 4.4.1.4 Lane Widening Strategy—Benefit-Cost Analysis for Individual Projects This strategy would select for widening 35.34 mi of roadway for which a need for lane widening was identified by a benefit-cost analysis using the benefit-cost analysis procedure presented in

46 Section 5.2.2. This would change the lane-width distribution on the roadway system as shown below: Total Length (mi) Lane Width (ft) Before After 9 18.29 18.29 10 251.74 219.36 11 2,087.38 2,076.96 12 2,273.30 2,301.32 TOTAL 4,630.71 4,630.71 This improvement program would provide benefits of $7,817,183 at a cost of $5,603,567, equivalent to a benefit-cost ratio of 1.40. Every portion of the 35.34 mi of roadway improved in this program, was a cost-effective project, and the benefits for the program as a whole exceed the costs. This strategy does the best job at focusing on the best projects and avoids many of the projects that are not cost-effective. 4.4.1.5 Summary of Findings from the Example of Lane Widening Strategies The example of lane widening strategies for a statewide system of rural two-lane highways shows that an improvement program based on benefit-cost analysis for individual projects would provide the greatest net present benefits and the greatest return per dollar spent and would avoid improvements that are not cost-effective. However, benefit-cost analysis results are not necessarily exact, and not every highway agency will have the data available and in a suitable form for a benefit-cost analysis. Minimum AADT criteria developed through a benefit-cost analysis come the closest of the other alternatives to providing optimal results. Both blanket lane widening to a minimum 12-ft lane width and lane widening based on the design criteria from TRB Special Report 214 (1) result in many lane widening investments that would not be cost- effective. The primary drawback of strategies developed before publication of the HSM is likely to be a focus on widening 11 ft lanes to 12 ft, which is unlikely to be cost-effective except at very high volumes. Finally, it should be noted that in actual practice some roadways might be found to have experienced patterns of single-vehicle run-off-road, head-on, opposite-direction sideswipe, or same-direction sideswipe crashes that could indicate a need for lane widening regardless of the results of the benefit-cost analysis.

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Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects Get This Book
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TRB's National Cooperative Highway Research Program (NCHRP) has released a pre-publication, non-edited version of Research Report 876: Guidelines for Integrating Safety and Cost-Effectiveness into Resurfacing, Restoration, and Rehabilitation (3R) Projects. The report presents an approach for estimating the cost-effectiveness of including safety and operational improvements in a resurfacing, restoration, or rehabilitation (3R) project. The approach uses the performance of the existing road in estimating the benefits of a proposed design improvement and in determining if it is worthwhile. These guidelines are intended to replace TRB Special Report 214: Designing Safer Roads: Practices for Resurfacing, Restoration, and Rehabilitation. The guidelines are accompanied by two spreadsheet tools available for download through a .zip file: one for analyzing a single design alternative and one for comparing several alternatives or combinations of alternatives.

Disclaimer: This software is offered as is, without warranty or promise of support of any kind either expressed or implied. Under no circumstance will the National Academy of Sciences or the Transportation Research Board (collectively "TRB") be liable for any loss or damage caused by the installation or operation of this product. TRB makes no representation or warranty of any kind, expressed or implied, in fact or in law, including without limitation, the warranty of merchantability or the warranty of fitness for a particular purpose, and shall not in any case be liable for any consequential or special damages.

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