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From page 3... ... 3 2 Highway Safety Manual Overview The HSM provides analytical tools and techniques for quantifying the potential effects on crashes because of decisions made in planning, design, operations, and maintenance. The information provided in the manual will assist agencies in their efforts to integrate safety into their decision-making processes. Read the entire page →
From page 4... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 4 2.2 HSM Part B: Roadway Safety Management Process HSM Part B discusses the process of monitoring and reducing crash frequency on existing roadway networks. The roadway safety management process consists of six steps: network screening (HSM Chapter 4) Read the entire page →
From page 5... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 5 • The Haddon Matrix is used to identify crash contributing factors before, during, and after a crash from the perspective of human, vehicle, and roadway. • Regression-to-the-mean (RTM) Read the entire page →
From page 6... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 6 Source: HSM, 1st Edition Figure 1: Stability of Performance Measures The selected performance measure can be applied to roadway segments, intersections, and facilities using different screening methods. Generally, roadway segments can be screened using either a slidingwindow or peak-searching method, while intersections can be screened using only a simple ranking method. Read the entire page →
From page 7... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 7 analysis. HSM Part B Section 5.3 (HSM p. Read the entire page →
From page 8... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 8 2.2.4 HSM Chapter 7: Economic Appraisal The main objectives for the economic appraisal of a safety countermeasure or combination of countermeasures are to determine whether a project is economically justifiable, and determine which project or alternative is the most cost-effective. There are two methods for conducting economic appraisals, benefit-cost analysis, and cost-effectiveness analysis. Read the entire page →
From page 9... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 9 used to prioritize safety projects including Linear Programming optimization, Integer Programming Optimization and Dynamic Programming Optimization. HSM Appendix 8A (HSM p. Read the entire page →
From page 10... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 10 data needs and input, pre-evaluation activities, and computational procedures are provided in HSM Part B Section 9.4 (HSM p. Read the entire page →
From page 11... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 11 detailed explanation of observed crash frequency, predicted average crash frequency, and expected average crash frequency is provided in Section 2.3.3 of this guide. Figure 3 describes the facility type definitions included in each HSM Part C chapter. Read the entire page →
From page 12... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 12 2.3.2 HSM Part C Relationship to HSM Parts A, B, and D HSM Part A – Introduction, Human Factors, and Fundamentals. This section presents background information to understand the methods provided in the HSM to analyze and evaluate crash frequencies. Read the entire page →
From page 13... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 13 Figure 4: Illustration of Observed, Predicted, and Expected Crash Frequency Estimates 2.3.4 Safety Performance Functions SPFs are regression models for estimating the predicted average crash frequency of individual roadway segments or intersections. SPFs are developed through statistical regression techniques using historical crash data collected over several years at "base" sites with similar characteristics. Read the entire page →
From page 14... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 14 Figure 5: Sample SPF – Colorado Department of Transportation (Source: Kononov, 2011) Multivariate models, or Level II SPFs, incorporate a variety of variables other than traffic volume only. Read the entire page →
From page 15... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 15 TABLE 3 List of SPFs in HSM Part C Chapter Facility Type SPF for Collision Type SPF for Crash Severity Level • Multiple-vehicle nondriveway collision • All severity levels • Fatal-and-injury crashes • PDO crashes • Multiple-vehicle driveway-related collision • All severity levels • Vehicle-pedestrian collision • All severity levels • Vehicle-bicycle collision • All severity levels Intersection • Multiple-vehicle collision • All severity levels • Fatal-and-injury crashes • PDO crashes • Single-vehicle crashes • All severity levels • Fatal-and-injury crashes • PDO crashes • Vehicle-pedestrian collision • All severity levels • Vehicle-bicycle collision • All severity levels 2.3.5 Crash Modification Factors HSM Part C base models are developed using a given set of site characteristics and are used to estimate the predicted average crash frequency. The Part C CMFs are used to adjust the base models to local conditions. Read the entire page →
From page 16... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 16 The EB method uses a weighted factor (w) which is a function of the SPF's overdispersion parameter (k) Read the entire page →
From page 17... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 17 • Method 3: For cases where HSM Part C predictive method is not available, but an SPF for a facility not included in the HSM is available. Apply the SPF to calculate the predicted average crash frequency of existing conditions and apply an appropriate HSM Part D CMF to estimate the safety performance of the proposed condition. Read the entire page →
From page 18... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 18 2.3.11 HSM Part C Summary HSM Part C provides the basic methodology for calculating the predicted and/or expected crash frequency for selected highway facilities under given traffic and geometric conditions. The following concepts (Figure 6) Read the entire page →
From page 19... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 19 2.3.12 HSM Chapter 10: Predictive Method for Rural Two-Lane, Two-Way Roads HSM Chapter 10 provides a methodology to estimate the predicted and/or expected average crash frequency, crash severity, and collision types for rural two-lane, two-way facilities. Crashes involving vehicles of all types, bicycles, and pedestrians are included, except for crashes between bicycles and pedestrians. Read the entire page →
From page 20... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 20 HSM Chapter 10 also provides guidance on how to define roadway segments and intersections (HSM Section 10.5, p. Read the entire page →
From page 21... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 21 2.3.13 Calculating the Crash Frequency for Rural Two-Lane, Two-Way Roads HSM Chapter 10 provides the methodology for calculating the predicted and/or expected crash frequency for roadway segments and intersections on rural two-lane, two-way roads. The calculation is for a given period during which the geometric design and traffic control features are unchanged and traffic volumes are known. Read the entire page →
From page 22... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 22 Figure 10: Rural Two-Lane, Two-Way Roads Base Conditions CMFs are applied to account for the differences between the specific site under investigation and the base condition for the facility type. CMFs are used to adjust the SPF estimate of predicted average crash frequency for the effect of individual geometric design and traffic control features. Read the entire page →
From page 23... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 23 TABLE 6 CMFs for Rural Two-Lane Highway Segments and Intersections Facility Type CMF CMF Description HSM CMF Equations and Tables HSM Table 10-11 (HSM p. Read the entire page →
From page 24... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 24 𝐶𝑀𝐹 = CMFs specific to site type x and specified geometric design and traffic control features y 𝐶 = calibration factor to adjust SPF for local conditions for site type x Step 3: Expected Crash Frequency with Empirical Bayes Method This step can be omitted if no recorded crash data for the specific site under investigation were available or the data are considered unreliable. When historical crash data are available, the EB method (either site-specific or project-level) Read the entire page →
From page 25... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 25 Figure 11 shows the HSM Chapter 10 predictive method flowchart for calculating the predicted and expected average crash frequency for rural two-lane, two-way roads. Figure 11: Flowchart for Calculating Expected Crash Frequency on Rural Two-Lane, Two-Way Roads 2.3.14 Data Requirements for Rural Two-Lane, Two-Way Roads For the study period, it is important to determine the availability of AADT volumes and, for an existing roadway, the availability of observed/reported crash data to determine whether the EB method is applicable. Read the entire page →
From page 26... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 26 TABLE 9 Intersection Data Requirements for Rural Two-Lane, Two-Way Roads Intersections Units/Description Intersection type Unsignalized three-leg (3ST) , unsignalized four-leg (4ST) Read the entire page →
From page 27... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 27 TABLE 10 Roadway Segment Data Requirements for Rural Two-Lane, Two-Way Roads Roadway Segments Units/Description Passing lanes Present (1 lane) , present (2 lanes) Read the entire page →
From page 28... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 28 TABLE 11 Roadway Segment and Intersection Types and Descriptions for Rural Two-Lane, Two-Way Roads Facility Type Site Types with SPFs in HSM Chapter 11 Roadway Segments Rural four-lane undivided segments (4U) Rural four-lane divided segments (4D) Read the entire page →
From page 29... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 29 To apply the predictive method, the roadway within the defined study area limits must be divided into homogenous individual sites, segments and intersections. Roadway segment boundaries begin at the center of an intersection and end at either the center of the next intersection, or where there is a change in the segment's cross-section (homogeneous segment) Read the entire page →
From page 30... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 30 TABLE 12 Rural Multilane Highways SPFs in HSM Chapter 11 Facility Type Equation in HSM AADT Range Rural four-lane undivided segments (4U) HSM Equation 11-7 Up to 33,200 vpd Rural four-lane divided segments (4D) Read the entire page →
From page 31... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 31 CMFs are applied to account for the differences between the specific site under investigation and the base condition for the facility type. CMFs are used to adjust the SPF estimate of predicted average crash frequency for the effect of individual geometric design and traffic control features. Read the entire page →
From page 32... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 32 TABLE 13 CMFs for Rural Multilane Highway Segments and Intersections Facility Type CMF CMF Description HSM CMF Equations and Tables Three- and FourLeg StopControlled Intersections CMF1i Intersection angle (3ST and 4ST) Definition (HSM p. Read the entire page →
From page 33... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 33 where: 𝑤 = the weighted adjustment to be placed on the predictive model estimate. This value can be calculated using Equation 6: 𝑤 = ×∑ (Eq. Read the entire page →
From page 34... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 34 Figure 16 shows the HSM Chapter 11 predictive method flowchart for calculating the predicted and expected crash frequency for rural multilane highways. Figure 16: Flowchart for Calculating Predicted and Expected Crash Frequency on Rural Multilane Highways 2.3.17 Data Requirements for Rural Multilane Highways For the study period, it is important to determine the availability of AADT volumes, and for an existing roadway, the availability of observed crash data to determine whether the EB method is applicable. Read the entire page →
From page 35... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 35 TABLE 16 Intersection Data Requirements for Rural Multilane Highways Intersections Units/Description Intersection type Unsignalized three-leg (3ST) , unsignalized four-leg (4ST) Read the entire page →
From page 36... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 36 TABLE 17 Roadway Segment Data Requirements for Rural Multilane Highways Roadway Segments Units/Description Calibration factor (Cr) Derived from the calibration process Observed crash data Applicable only with the EB method; crashes that occur between intersections and are not related to the presence of an intersection during the period of study Notes: AADT = average annual daily traffic Cr = roadway segment calibration factor vpd = vehicles per day 2.3.18 HSM Chapter 12: Predictive Method for Urban and Suburban Arterials HSM Chapter 12 provides a structured methodology for estimating the predicted and/or expected average crash frequency, crash severity, and collision types for urban and suburban arterial facilities. Read the entire page →
From page 37... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 37 Figure 17: Urban and Suburban Arterials Facility Types and Definitions Commonly, a roadway consists of a contiguous group of sites (intersections and roadway segments) Read the entire page →
From page 38... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 38 An intersection is defined as the junction of two or more roadway segments. The intersection predictive models estimate the predicted and/or expected average crash frequencies within the intersection limits (Region A in Figure 18) Read the entire page →
From page 39... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 39 TABLE 19 Urban and Suburban Arterials Facility Types and AADT Ranges Item Facility Type AADT Range Four-lane undivided arterials (4U) Up to 40,100 vpd Four-lane divided arterials (4D) Read the entire page →
From page 40... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 40 Step 2: Predicted Crash Frequency under Real Conditions Each SPF listed in Table 20 is used to estimate the predicted crash frequency of a roadway segment or intersection under base conditions, which is later adjusted to site-specific conditions. Base conditions are a specific set of geometric design and traffic control features under which the SPFs were developed and are not necessarily the same for all facilities. Read the entire page →
From page 41... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 41 TABLE 21 CMFs for Urban and Suburban Arterials Roadway Segments and Intersections Facility Type CMF CMF Description CMF Equations and Tables HSM Tables 12-20 and 12-21 (HSM p. Read the entire page →
From page 42... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 42 The SPFs were developed in HSM-related research from the most complete and consistent available data sets. However, the predicted crash frequencies may vary substantially from one jurisdiction to another for a variety of reasons. Read the entire page →
From page 43... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 43 where: 𝑘 = the overdispersion parameter of the associated SPF used to estimate 𝑁 . Table 22 lists the overdispersion values for urban and suburban arterials. Read the entire page →
From page 44... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 44 Figure 20: Flowchart for Calculating Expected Crash Frequency on Urban and Suburban Arterials 2.3.20 Data Requirements for Urban and Suburban Arterials For the study period, it is important to determine the availability of AADT volumes, and for an existing roadway, the availability of observed crash data to determine whether the EB method is applicable. To determine the relevant data needs and avoid unnecessary data collection, it is important to understand the SPFs' base conditions. Read the entire page →
From page 45... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 45 TABLE 24 Intersection Data Requirements for Urban and Suburban Arterials Intersections Units/Description Intersection type Include unsignalized three-leg (3ST) , signalized three-leg (3SG) Read the entire page →
From page 46... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 46 Roadway Segment Data The effect of traffic volume (AADT) on crash frequency is incorporated through an SPF, while the effects of geometric design and traffic control features are incorporated through the CMFs. Read the entire page →
From page 47... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 47 2.4 HSM Part D: CMF Applications Guidance HSM Part D provides information on estimating how effective a treatment, geometric characteristic, and operational characteristic will be in reducing crashes or injuries at a specific location. The effectiveness is expressed in terms of CMFs, trends, or no effect. Read the entire page →
From page 48... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 48 • CMF confidence interval – It can be used to consider the possible range of the CMFs. For CMFs with high standard errors, the upper end of the confidence interval could be greater than 1.0 even if the CMF itself is relatively small, which means that the treatment could potentially result in an increase in crashes. Read the entire page →
From page 49... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 49 The CMFs for different treatments are usually provided in the format of figures, equations, or tables. Users may then determine the CMFs and relevant standard errors based on the treatment and the facility characteristics. Read the entire page →
From page 50... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 50 The crash effects of interchange design elements are included in this chapter. The list of treatments included under interchange design elements and availability of relevant CMFs for different facility types are presented in HSM Table 15-1 at the beginning of HSM Section 15.4 (see Table 29) Read the entire page →
From page 51... ... SECTION 2 – HIGHWAY SAFETY MANUAL OVERVIEW 51 on a particular facility type. The HSM table numbers for treatment summary information are listed in Table 31. Read the entire page →

From page 3...

... 3 2 Highway Safety Manual Overview The HSM provides analytical tools and techniques for quantifying the potential effects on crashes because of decisions made in planning, design, operations, and maintenance. The information provided in the manual will assist agencies in their efforts to integrate safety into their decision-making processes.