HSM Practitioner's Guide for Two-Lane Rural Highways Workshop

HSM Practitioner's Guide for Two-Lane Rural Highways Workshop
August 2010 Predicting Crash Frequency for Two-Lane Rural Highway Segments Session #2 – Predicting Safety for Two-Lane Rural Highway Segments Reference material is Highway Safety Manual – 1st Edition Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Predicting Crash Frequency for Two-Lane Rural Highway Segments
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Learning Outcomes: Describe the Safety Performance Functions (SPFs) for predicting Crash Frequency for Base Conditions Describe the Quantitative Safety Effects of Crash Modification Factors (CMFs) Apply CMFs to the SPF Base Equation Learning Objectives for Session #1 Introduction and Background for Intersection Safety Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Cross Sectional Elements
Predicting Crash Frequency for Two-Lane Rural Highway Segments HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Cross Sectional Elements When we think of safety, the entire cross section is of interest. This includes lanes and shoulders, as well as the roadside. This module focuses on the safety relationships associated with lanes and shoulders. Discuss the “tradeoffs” of widening shoulder which increases steepness of slope of ditches (foreslope to backslope) Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Two-Lane Rural Highway Segments August 2010 What should you expect would be the safety and operational influence of cross sectional elements? Lane Width Shoulder Width Sideslope Clear Zone Crashes Operations Head-on Capacity Wider is “better” Wider means “faster” Run-off-Road Capacity Wider is “better” Functionality (peds, bikes, emergency stops, capacity, maintenance) Run-off-road Maintenance (severity) Flatter is better Flatter is better Run-off-road Horizontal sight distance (frequency and severity) The instructor can use built in animation in the slide to work with participants in defining the crash types and operational attributes of each element. Sideslope and clear zone are covered here for completeness. >>for “wider means faster”, an increase in speed on existing vertical and horizontal alignment is not good Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Functions of shoulders in a rural environment
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop Two-Lane Rural Highway Segments August 2010 Functions of shoulders in a rural environment Clear zone (recovery) Highway Capacity Clear zone (horizontal sight distance) Store vehicles in emergency Pedestrians, bicyclists Protection for turns off the roadway Provide pavement support Store snow Provide space for maintenance activities Enforcement activities In the rural environment, the primary function of a shoulder is to serve as the initial (and hence most important) portion of the clear zone. The instructor can ask the participants to recall the distribution of crash types on rural highways (primarily single vehicle, with a large number of run-off-road crashes) Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Key Findings of FHWA Cross Section Study on Two-Lane Hwys (Zegeer)
Two-Lane Rural Highway Segments HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Key Findings of FHWA Cross Section Study on Two-Lane Hwys (Zegeer) Traffic volume influences crash rate Both lane and shoulder width have influence Roadside Hazard next biggest influence on crashes Alignment affects cross section crashes (terrain is surrogate for alignment) Zegeer, et al conducted a major study for FHWA in late 1980s of safety on 2-lane highways with traffic volumes less than 10,000 vehicles per day. The slide summarizes the key findings. Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Crash Severity for Two-Lane Rural Highways
Two-Lane Rural Highway Segments HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Crash Severity for Two-Lane Rural Highways Rural 2 Lane Highway Segment Severity Ratio = 32.1% for Injury + Fatal Crashes Note the default severity for two-lane Rural Highways Fatalities + Injury = 32.1% PDO = 67.9% Tables 10-3 and 10-4 provide the default proportions for crash severity and for collision type by crash severity level, respectively. These tables may be used to separate the crash frequencies from Equation 10-6 into components by crash severity level and collision type. Tables 10-3 and 10-4 are applied sequentially. First, Table 10-3 is used to estimate crash frequencies by crash severity level, and then Table 10-4 is used to estimate crash frequencies by collision type for a particular crash severity level. The default proportions for severity levels and collision types shown in Tables 10-3 and 10-4 may be updated based on local data for a particular jurisdiction as part of the calibration process described in Appendix A to Part C. Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

HSM Crash Prediction: 18 Steps for Two-Lane Rural Roadways
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Rural Area Definition: Places outside urban boundaries Populations of 5,000 persons, or less Applicable to: 10.4. Predictive Method for Rural Two-Lane, Two-Way Roads The predictive method for rural two-lane, two-way road is shown in Figure Applying the predictive method yields an estimate of the expected average crash frequency (and/or crash severity and collision types) for a rural two-lane, two-way facility. The components of the predictive models in Chapter 10 are determined and applied in Steps 9, 10, and 11 of the predictive method. The information that is needed to apply each step is provided in the following sections and in the Part C, Appendix A. Speed is a indicator of rural vs urban/suburban settings. High speed usually indicates rural conditions and slower speeds indicates a urban/suburban driving enviornment. There are 18 steps in the predictive method. In some situations, certain steps will not be needed because the data is not available or the step is not applicable to the situation at hand. In other situations, steps may be repeated, such as if an estimate is desired for several sites or for a period of several years. In addition, the predictive method can be repeated as necessary to undertake crash estimation for each alternative design, traffic volume scenario, or proposed treatment option within the same period to allow for comparison. The basic components of the procedures for crash prediction includes the SPF Base Models, crash modification factors (CMFs) and calibration factors. The HSM walks the reader through each of the 18 steps in detail of the crash prediction method. In this presentation/workshop we will cover steps 9, 10 and 16 – 18, in detail. We are assuming previous steps have been completed and a Calibration Factor = Steps 12 – 15 are repeats of Steps 9 & 10 for additional sites in a segment. Existing Roadways Design Alternatives for existing or new roadways Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Predicting Crash Frequency Performance - Analysis Sections
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop Predicting Crash Frequency Performance - Analysis Sections August 2010 Organizing information for Safety Analysis: Separate project lengths (and crashes) into homogeneous units: Average daily traffic (AADT) volume (vehicles/day) Lane width (ft) Shoulder width (ft) Shoulder Type Driveway Density (driveways per mile) Roadside Hazard Rating Beginning/End of Horizontal Curves Beginning/End of Segments on Grade (>3%) Key Message: The CPM divides the highway into homogeneous analysis sections (i.e., roadway segments with like geometric features (lane widths, shoulder widths, shoulder types, roadside hazard rating, sideslopes, etc). Additional Info: Analysis sections include both (1) homogeneous highway segments, and (2) individual intersections. Each analysis section is homogenous with respect to geometry and traffic conditions. Homogeneous highway segments have uniform horizontal, vertical, cross section, traffic characteristics, and roadside geometry. At any location where there is a change in geometry (e.g., changing from a horizontal curve to a tangent or a change in shoulder width) or a change in traffic volume, a new highway segment begins. Each intersection is also defined as a separate, homogenous analysis section. Question/Interactivity: Ask participants to study the roadway plans for IHSDM Pike and to identify the first few homogenous highway segments. Show how the first tangent and horizontal curve would be different homogeneous segments. (refer to sheet 2 of the plan/profile) Reference: Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Subdividing Roadway Segments
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Subdividing Roadway Segments Homogeneous Roadway Segments: Lane Width Instructor: From Chapter 10 of the Highway Safety Manual Lane width - For lane widths measured to a 0.1-ft level of precision or similar, the following rounded lane widths are recommended before determining “homogeneous” segments and should be used in evaluating the safety effects for lane width. Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Subdividing Roadway Segments
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Subdividing Roadway Segments Homogeneous Roadway Segments: Shoulder Width Instructor: From Chapter 10 of the Highway Safety Manual Shoulder Width - For shoulder widths measures to a 0.1-ft level of precision or similar, the following rounded paved shoulder widths should be used in evaluating the safety effects for shoulder width. Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

HSM Crash Prediction Method
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 HSM Crash Prediction Method Three Basic Elements: 1. Safety Performance Functions (SPF) Equations Predict safety performance for set base conditions 2. Crash Modification Factors (CMFs) Adjust predicted safety performance from base conditions to existing/proposed conditions Are greater or less than 1: < lower crash frequency > increased crash frequency Calibration, Cr or Ci Accounts for local conditions/data Safety Performance Functions Safety Performance Functions (SPFs) are regression equations that estimate the average crash frequency for a specific site type (with specified base conditions) as a function of annual average daily traffic (AADT) and, 770 in the case of roadway segments, the segment length (L). Base conditions are specified for each SPF and may include conditions such as lane width, presence or absence of lighting, presence of turn lanes etc. Calibration of Safety Performance Functions to Local Conditions The predictive models in Chapters 10, 11, and 12 have three basic elements, Safety Performance Functions, Accident Modification Factors and a calibration factor. The SPFs were developed as part of HSM-related research from the most complete and consistent available data sets. However, the general level of crash frequencies may vary substantially from one jurisdiction to another for a variety of reasons including crash reporting thresholds, and crash reporting system procedures. These variations may result in some jurisdictions experiencing substantially more reported traffic accidents on a particular facility type than in other jurisdictions. In addition, some jurisdictions may have substantial variations in conditions between areas within the jurisdiction (e.g. snowy winter driving conditions in one part of the state and only wet winter driving conditions in another part of the state). Therefore, for the predictive method to provide results that are reliable for each jurisdiction that uses them, it is important that the SPFs in Part C be calibrated for application in each jurisdiction. Methods for calculating calibration factors for roadway segments Cr and intersections Ci are included in the Part C Appendix to allow highway agencies to adjust the SPF to match local conditions. The calibration factors will have values greater than 1.0 for roadways that, on average, experience more accidents than the roadways used in developing the SPFs. Roadways that, on average, experience fewer accidents than the roadways used in the development of the SPF, will have calibration factors less than 1.0. The calibration factors, Cr and Ci, are based on the ratio of the total observed accident frequencies for a selected set of sites to the total expected average crash frequency estimated for the same sites, during the same time period, using the applicable Part C predictive method. Thus, the nominal value of the calibration factor, when the observed and predicted crash frequencies happen to be equal, is When there are more accidents observed than are predicted by the Part C predictive method, the computed calibration factor will be greater than When there are fewer accidents observed than are predicted by the Part C predictive method, the computed calibration factor will be less than 1.00. Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

HSM Crash Prediction Method
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 HSM Crash Prediction Method Total estimated crashes within the limits of the roadway being analyzed: Ntotal = ∑Npredicted-rs + ∑ Npredicted-int Ntotal = Total expected number of crashes within the limits of the roadway facility ∑Npredicted-rs = Expected crash frequency for all roadway segments (sum of individual segments) ∑ Npredicted-int = Expected crash frequency for all intersections (sum of individual intersections) Focus in this section will be on predicting the safety performance of rural two-lane highway segments. The total estimated number of crashes within the network or facility limits during a study period of n years is calculated using Equation 10-4: (Equation 10-4) Where: Ntotal = total expected number of crashes within the limits of a rural two-lane, two-way facility for the period of interest. Or, the sum of the expected average crash frequency for each year for each site within the defined roadway limits within the study period; Nrs = expected average crash frequency for a roadway segment using the predictive method for one specific year; and Nint = expected average crash frequency for an intersection using the predictive method for one specific year. Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Roadway Segment Prediction Model
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Roadway Segment Prediction Model Npredicted-rs = Nspf-rs x (CMF1r … CMFxr) Cr Where: Npredicted-rs = predicted average crash frequency for an individual roadway for a specific year (crashes per year) Nspf-rs = predicted average crash frequency for base conditions for an individual roadway segment (crashes per year) CMF1r ... CMFxr = Crash Modification Factors for individual design elements Cr = calibration factor Key Message:This slide shows how the components on the previous slide are combined in equation form to estimate the expected total crash frequency, Npredicted-rs, on a particular homogenous highway segment. Predictive models for rural two-lane, two-way roadway Segments The predictive models can be used to estimate total predicted average crash frequency (i.e., all crash severities and collision types) or can be used to predict average crash frequency of specific crash severity types or specific collision types. The predictive model for an individual roadway segment or intersection combines a SPF with CMFs and a calibration factor. For rural two-lane, two-way undivided roadway segments the predictive model is shown in Equation 10-2: Npredicted rs = Nspf rs × Cr × (CMF1r × CMF2r × … × CMF12r) (Equation 10-2) Where: Npredicted rs = predicted average crash frequency for an individual roadway segment for a specific year; Nspf rs = predicted average crash frequency for base conditions for an individual roadway segment; Cr = calibration factor for roadway segments of a specific type developed for a particular jurisdiction or geographical area; and CMF1r …CMF12r = crash modification factors for rural two-lane, two-way roadway segments. This model estimates the predicted average crash frequency of non-intersection related crashes (i.e., crashes that would occur regardless of the presence of an intersection). Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Safety Performance Function (SPF)
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Safety Performance Function (SPF) SPF for Two-Lane Rural Highway Segment Crashes for Base Conditions: Nspf-rs = (AADTn) (L) (365) (10-6) e-0.312 Where: Nspf-rs = predicted total crash frequency for a roadway segment for base conditions, crashes per year AADTn = average annual daily two-way traffic volume for specified year n (veh/day) L = length of roadway segment (miles) Key Message: This equation is the highway segment base model. Additional Info: This equation is used to predict the expected crashes per year on individual highway segments based upon traffic volume and segment length. It assumes base conditions for geometric and traffic control elements (shown on the next slide). The inputs to the base model are traffic volume and segment length. The model results in a prediction of crashes for a generic roadway segment with nominal (or base) geometry conditions. Every segment of the same length and with the same ADT on any roadway would have the same number of expected crashes. The SPF for predicted average crash frequency for rural two-lane, two-way roadway segments is shown in Equation 10-6 and presented graphically in Figure 10-3: Nspf rs = AADT × L × 365 × 10-6 × exp(-0.312) (Equation 10-6) Where: Nspf rs = predicted total crash frequency for roadway segment base conditions; AADT = average annual daily traffic volume (vehicles per day); and L = length of roadway segment (miles). Guidance on the estimation of traffic volumes for roadway segments for use in the SPFs is presented in Step 3 of the predictive method described in Section The SPFs for roadway segments on rural two-lane highways are applicable to the AADT range from zero to 17,800 vehicles per day. Application to sites with AADTs substantially outside this range may not provide reliable results. Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Base Conditions for Rural Two-Lane Roadway Segments (CMF = 1.0)
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Base Conditions for Rural Two-Lane Roadway Segments (CMF = 1.0) Lane Width: feet Shoulder Width: feet Shoulder Type: Paved Roadside Hazard Rating: 3 Driveway Density: <5 driveways/mi Grade: <3%(absolute value) Horizontal Curvature: None Vertical Curvature: None Centerline rumble strips: None TWLTL, climbing, or passing lanes: None Lighting: None Automated Enforcement: None Key Message: These are the nominal geometry or base conditions represented by the base model. The predictive model for predicting average crash frequency for base conditions on a particular rural two-lane, two-way roadway segment was presented in Equation 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. The base conditions for roadway segments on rural two-lane, two-way roads are stated on this slide. Additional Info: The reason for selecting these dimensions as the base conditions is statistical; it is not that these are the “safest.” The statistical reasons for these dimensions are: (1) they are (approximately) the mean values for these variables in the database used to fit the base model, and (2) regression model estimates have the least standard error near the mean values of the independent variables in the model. Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Applying SPF for Base Conditions – Example:
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Safety Performance Function (SPF) Applying SPF for Base Conditions – Example: Nspf-rs = (AADTn) (L) (365) (10-6) e-0.312 2-lane state highway connecting a US marked route to a primary State marked route in a rural county; Where: AADT = 3,500 vpd Length = 26,485 feet = 5.02 miles Example calculation using the SPF base model for 2-lane rural highway segments without effect of lane width, shoulder width and type of shoulder, hazard rating, access, or grade. Does not include crashes for curves nor for intersections The model estimates crashes per mile per year Note the effect of AADT (non linear) Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Applying SPF for Base Conditions – Example:
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Safety Performance Function (SPF) Applying SPF for Base Conditions – Example: Where: AADT = 3,500 vpd Length = 26,485 feet = 5.02 miles Nspf-rs = (AADTn) (L) (365) (10-6) e-0.312 Nspf-rs = (3,500) (5.02) (365) (10-6) e-0.312 Example calculation using the SPF base model for 2-lane rural highway segments without effect of lane width, shoulder width and type of shoulder, hazard rating, access, or grade. Does not include crashes for curves nor for intersections The model estimates crashes per mile per year Note the effect of AADT (non linear) = (3,500) (5.02) (365) (10-6) (0.7320) = 4.69 crashes per year Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Applying CMF’s for Conditions other than “Base”
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Next Step is: Npredicted-rs = Nspf-rs x (CMF1r … CMFxr) Cr Where: Npredicted-rs = predicted average crash frequency for an individual roadway for a specific year (crashes per year) Nspf-rs = predicted average crash frequency for base conditions for an individual roadway segment (crashes per year) CMF1r ... CMFxr = Crash Modification Factors for individual design elements Cr = calibration factor Instructor: Key Message: The NEXT STEP is to apply the appropriate Crash Modification Factors to the predicted crash frequency to account for conditions that differ from the base conditions. Predictive models for rural two-lane, two-way roadway Segments The predictive models can be used to estimate total predicted average crash frequency (i.e., all crash severities and collision types) or can be used to predict average crash frequency of specific crash severity types or specific collision types. The predictive model for an individual roadway segment or intersection combines a SPF with CMFs and a calibration factor. For rural two-lane, two-way undivided roadway segments the predictive model is shown in Equation 10-2: Npredicted rs = Nspf rs × Cr × (CMF1r × CMF2r × … × CMF12r) (Equation 10-2) Where: Npredicted rs = predicted average crash frequency for an individual roadway segment for a specific year; Nspf rs = predicted average crash frequency for base conditions for an individual roadway segment; Cr = calibration factor for roadway segments of a specific type developed for a particular jurisdiction or geographical area; and CMF1r …CMF12r = crash modification factors for rural two-lane, two-way roadway segments. This model estimates the predicted average crash frequency of non-intersection related crashes (i.e., crashes that would occur regardless of the presence of an intersection). Session 4 – Predicting Highway Safety for Multilane Urban Streets

Crash Modification Factors (CMFs)
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop May 2009 Crash Modification Factors (CMFs) CMFs quantify the expected change in crashes at a site caused by implementing a particular treatment, countermeasure, intervention, action, or alternative. CMFs are used to adjust the SPF estimated predicted average crash frequency for the effect of individual geometric design and traffic control features. Crash Modification Factors In Step 10 of the predictive method shown in Section 10.4, crash modification factors (CMFs) are applied to account for the effects of site-specific geometric design and traffic control features. CMFs are used in the predictive method in Equations 10-2 and A general overview of crash modification factors (CMFs) is presented in Chapter 3, Section The Part C—Introduction and Applications Guidance provides further discussion on the relationship of CMFs to the predictive method. This section provides details of the specific CMFs applicable to the safety performance functions presented in Section 10.6. Crash modification factors (CMFs) are used to adjust the SPF estimate of predicted average crash frequency for the effect of individual geometric design and traffic control features, as shown in the general predictive model for Chapter 10 shown in Equation The CMF for the SPF base condition of each geometric design or traffic control feature has a value of Any feature associated with higher crash frequency than the base condition has a CMF with a value greater than Any feature associated with lower crash frequency than the base condition has a CMF with a value less than 1.00. The CMFs used in Chapter 10 are consistent with the CMFs in Part D, although they have, in some cases, been expressed in a different form to be applicable to the base conditions. The CMFs presented in Chapter 10 and the specific site types to which they apply are summarized in Table 10-7. Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

August 2010 Crash Modification Factors (CMFs) Applying CMFs for Lane Width, Shoulder Width & Type, Driveway Density, TWTLs, and Roadside Design Nspf-rs = (AADTn) (L) (365) (10-6) e-0.312 Npredicted-rs = Nspf-rs(CMF1r x CMF2r x CMF6r x CMF9r x CMF10r) Where: CMF1r is for Lane Width CMF2r is for Shoulder Width and Type CMF6r is for Driveway Density CMF9r is for Two-Way Left-Turn Lanes CMF10r is for Roadside Design After computing the predicted number of crashes for base conditions, the next step is to apply the applicable CMFs for existing or proposed conditions. In this case, CMFs for Lane Width, Shoulder Width & Type, Two-Way Left-Turn Lanes, and Roadside Design (Roadside Hazard Rating) are being considered. Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Rural Two-Lane Highway Segment CMFs
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Rural Two-Lane Highway Segment CMFs Here are some of the major CMFs that are applicable to two-lane rural roads. This is not an all inclusive list, there are other CMFs in Part D of the HSM that are not covered here. The CMFs used in Chapter 10 are consistent with the CMFs in Part D, although they have, in some cases, been expressed in a different form to be applicable to the base conditions. The CMFs presented in Chapter 10 and the specific site types to which they apply are summarized in Table 10-7. crash modification factors for roadway Segments The CMFs for geometric design and traffic control features of rural two-lane, two-way roadway segments are presented below. These CMFs are applied in Step 10 of the predictive method and used in Equation 10-2 to adjust the SPF for rural two-lane, two-way roadway segments presented in Equation 10-6, to account for differences between the base conditions and the local site conditions. Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Crash Modification Factor for Lane Width (CMF1r)
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop Crash Modification Factor for Lane Width (CMF1r) August 2010 CMF1r = (CMFra – 1.0)pra + 1.0 CMFra for lane width ‘Base condition’ is 12-ft lanes CMFs for ADT >2000 based on Zegeer CMFs for ADT <400 based on studies by Griffin and Mak Expert panel developed transition lines, referencing other research The current consensus in the research community is shown here. Note that the effect of lane width increases with traffic volume (conversely, for very low volume roads, the incremental safety contribution of 1 foot for lane width is negligible.) Also note that there is very little additional safety benefit of 12 foot versus 11 foot lanes. The CMFs were developed using 12-foot as the “base condition” CMF1r—Lane Width The CMF for lane width on two-lane highway segments is presented in Table 10-8 and illustrated by the graph in Figure This CMF was developed from the work of Zegeer et al. (16) and Griffin and Mak (4). The base value for the lane width CMF is 12 ft. In other words, the roadway segment SPF will predict safety performance of a roadway segment with 12-ft lanes. To predict the safety performance of the actual segment in question (e.g., one with lane widths different than 12 ft), CMFs are used to account for differences between base and actual conditions. Thus, 12-ft lanes are assigned a CMF of CMF1r is determined from Table 10-8 based on the applicable lane width and traffic volume range. The relationships shown in Table 10-8 are illustrated in Figure Lanes with widths greater than 12 ft are assigned a CMF equal to that for 12-ft lanes. For lane widths with 0.5-ft increments that are not depicted specifically in Table 10-8 or Figure 10-7, a CMF value can be interpolated using either of these exhibits since there is a linear transition between the various AADT effects. If the lane widths for the two directions of travel on a roadway segment differ, the CMF are determined separately for the lane width in each direction of travel and the resulting CMFs are then be averaged. 1.23 Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Crash Modification Factor for Lane Width (CMF1r)
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop Crash Modification Factor for Lane Width (CMF1r) August 2010 CMF1r = (CMFra – 1.0)pra + 1.0 From example on previous Slide with AADT =1,600 vpd, and Lane Width =10 ft CMFra = x10^-4(1, ) = 1.23 The CMFs shown in Table 10-8 and Figure 10-7 apply only to the crash types that are most likely to be affected by lane width: single-vehicle run-off-the-road and multiple-vehicle head-on, opposite-direction sideswipe, and same-direction sideswipe crashes. These are the only crash types assumed to be affected by variation in lane width, and other crash types are assumed to remain unchanged due to the lane width variation. The CMFs expressed on this basis are, therefore, adjusted to total crashes within the predictive method. This is accomplished using Equation 10-11: CMF1r = (CMFra − 1.0) × pra (Equation 10-11) Where: CMF1r = crash modification factor for the effect of lane width on total crashes; CMFra = crash modification factor for the effect of lane width on related crashes (i.e., single-vehicle run-off-theroad 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 10-8; and pra = proportion of total crashes constituted by related crashes. The proportion of related crashes, pra, (i.e., single-vehicle run-off-the-road, and multiple-vehicle head-on, oppositedirection sideswipe, and same-direction sideswipes crashes) is estimated as (i.e., 57.4 percent) based on the default distribution of crash types presented in Table This default crash type distribution, and therefore the value of pra, may be updated from local data as part of the calibration process. Note equations for ADT’s between 400 and 2000 Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

CMF - Lane and Shoulder Width Adjustment for Related Crashes
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop CMF - Lane and Shoulder Width Adjustment for Related Crashes August 2010 Table 10-4 - Adjust for (Run off Road + Head-on + Sideswipes) to total crashes pra = 0.574 The CMFs shown in Table 10-8 and Figure 10-7 apply only to the crash types that are most likely to be affected by lane width: single-vehicle run-off-the-road and multiple-vehicle head-on, opposite-direction sideswipe, and same-direction sideswipe crashes. These are the only crash types assumed to be affected by variation in lane width, and other crash types are assumed to remain unchanged due to the lane width variation. The CMFs expressed on this basis are, therefore, adjusted to total crashes within the predictive method. This is accomplished using Equation 10-11: CMF1r = (CMFra − 1.0) × pra (Equation 10-11) Where: CMF1r = crash modification factor for the effect of lane width on total crashes; CMFra = crash modification factor for the effect of lane width on related crashes (i.e., single-vehicle run-off-theroad 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 10-8; and pra = proportion of total crashes constituted by related crashes. The proportion of related crashes, pra, (i.e., single-vehicle run-off-the-road, and multiple-vehicle head-on, oppositedirection sideswipe, and same-direction sideswipes crashes) is estimated as (i.e., 57.4 percent) based on the default distribution of crash types presented in Table This default crash type distribution, and therefore the value of pra, may be updated from local data as part of the calibration process. CMF1r = (CMFra – 1.0)pra + 1.0 Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Calculation for Lane Width (CMF1r): Example
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Calculation for Lane Width (CMF1r): Example For 3,500 AADT for a 10 foot wide lane: From Table 10-8: CMFra = 1.30 Adjustment for lane width and shoulder width related crashes (Run off Road + Head-on + Sideswipes) to obtain total crashes using default value for pra = 0.574 CMF1r = (CMFra ) pra + 1.0 Example calculation on determining an CMF1r for Lane Width for total crashes when the CMFra is given for a particular crash type or severity level. Here CMFra is for crash types relating to ROR, Head-On and Sideswipe Crashes. = ( ) * = (0.30) (0.574) + 1.0 = Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Calculation for Shoulder Width and Type (CMF2r)
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop Calculation for Shoulder Width and Type (CMF2r) August 2010 CMF2r = (CMFwra CMFtra– 1.0)pra + 1.0 CMFwra for shoulder width: Base condition is 6-ft shoulders CMFs for ADT >2000 based on Zegeer (FHWA) CMFs for ADT <400 based on other studies by Zegeer (NCHRP 362) Expert panel developed transition lines, referencing other research The current consensus of the research on the effect of shoulder width is shown here, with a 6-foot shoulder considered the base condition. Note the rather marked sensitivity, particularly as volume increases. The instructor may want to lead a discussion of what the sensitivity is and why. May 2004 – Iowa DOT - All state highways over 3000 present-day AADT receive 4 feet. All others receive 2 feet. CMF2r—Shoulder Width and Type The CMF for shoulders has a CMF for shoulder width (CMFwra) and a CMF for shoulder type (CMFtra). The CMFs for both shoulder width and shoulder type are based on the results of Zegeer et al. (16, 17). The base value of shoulder width and type is a 6-foot paved shoulder, which is assigned a CMF value of 1.00. CMFwra for shoulder width on two-lane highway segments is determined from Table 10-9 based on the applicable shoulder width and traffic volume range. The relationships shown in Table 10-9 are illustrated in Figure 10-8. Shoulders over 8-ft wide are assigned a CMFwra equal to that for 8-ft shoulders. The CMFs shown in Table 10-9 and Figure 10-8 apply only to single-vehicle run-off the-road and multiple-vehicle head-on, opposite-direction sideswipe, and same-direction sideswipe crashes. Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Crash Modification Factor for Shoulder Width (CMFwra)
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 CMF2r = (CMFwra CMFtra– 1.0)pra + 1.0 From example on previous Slide with AADT =1,600 vpd, and Shoulder Width =0 ft CMFrra = x10^-4(1, ) = 1.40 CMF2r—Shoulder Width and Type The CMF for shoulders has a CMF for shoulder width (CMFwra) and a CMF for shoulder type (CMFtra). The CMFs for both shoulder width and shoulder type are based on the results of Zegeer et al. (16, 17). The base value of shoulder width and type is a 6-foot paved shoulder, which is assigned a CMF value of 1.00. CMFwra for shoulder width on two-lane highway segments is determined from Table 10-9 based on the applicable shoulder width and traffic volume range. The relationships shown in Table 10-9 are illustrated in Figure 10-8. Shoulders over 8-ft wide are assigned a CMFwra equal to that for 8-ft shoulders. The CMFs shown in Table 10-9 and Figure 10-8 apply only to single-vehicle run-off the-road and multiple-vehicle head-on, opposite-direction sideswipe, and same-direction sideswipe crashes. Note equations for ADT’s between 400 and 2000 Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Crash Modification Factor for Shoulder Type (CMFtra)
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop Crash Modification Factor for Shoulder Type (CMFtra) August 2010 The base condition for shoulder type is paved. Table presents values for CMFtra which adjusts for the safety effects of gravel, turf, and composite shoulders as a function of shoulder width. If the shoulder types and/or widths for the two directions of a roadway segment differ, the CMF are determined separately for the shoulder type and width in each direction of travel and the resulting CMFs are then be averaged. The CMFs for shoulder width and type shown in Table 9, Figure 8, and Table 10 apply only to the collision types that are most likely to be affected by shoulder width and type: single-vehicle run-off the-road and multiple-vehicle head-on, opposite-direction sideswipe, and same-direction sideswipe crashes. The CMFs expressed on this basis are, therefore, adjusted to total crashes using Equation CMF2r = (CMFwra × CMFtra − 1.0) × pra (Equation 10-12) Where: CMF2r = crash modification factor for the effect of shoulder width and type on total crashes; CMFwra = crash modification factor for related crashes (i.e., single-vehicle run-off-the-road and multiple-vehicle head-on, opposite-direction sideswipe, and same-direction sideswipe crashes), based on shoulder width (from Table 10-9); CMFtra = crash modification factor for related crashes based on shoulder type (from Table 10-10); and pra = proportion of total crashes constituted by related crashes. The proportion of related crashes, pra, (i.e., single-vehicle run-off-the-road, and multiple-vehicle head-on, oppositedirection sideswipe, and same-direction sideswipes crashes) is estimated as (i.e., 57.4 percent) based on the default distribution of crash types presented in Table This default crash type distribution, and therefore the value of pra, may be updated from local data by a highway agency as part of the calibration process. Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

CMF – Lane and Shoulder Width Adjustment for Related Crashes
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Table 10-4 - Adjust for (Run off Road + Head-on + Sideswipes) to total crashes pra = 0.574 The CMFs shown in Table 10-8 and Figure 10-7 apply only to the crash types that are most likely to be affected by lane width: single-vehicle run-off-the-road and multiple-vehicle head-on, opposite-direction sideswipe, and same-direction sideswipe crashes. These are the only crash types assumed to be affected by variation in lane width, and other crash types are assumed to remain unchanged due to the lane width variation. The CMFs expressed on this basis are, therefore, adjusted to total crashes within the predictive method. This is accomplished using Equation 10-11: CMF1r = (CMFra − 1.0) × pra (Equation 10-11) Where: CMF1r = crash modification factor for the effect of lane width on total crashes; CMFra = crash modification factor for the effect of lane width on related crashes (i.e., single-vehicle run-off-theroad 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 10-8; and pra = proportion of total crashes constituted by related crashes. The proportion of related crashes, pra, (i.e., single-vehicle run-off-the-road, and multiple-vehicle head-on, oppositedirection sideswipe, and same-direction sideswipes crashes) is estimated as (i.e., 57.4 percent) based on the default distribution of crash types presented in Table This default crash type distribution, and therefore the value of pra, may be updated from local data as part of the calibration process. CMF2r = (CMFwraCMFtra – 1.0)pra + 1.0 Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Calculation for Shoulder Width and Type (CMF2r): Example
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop Calculation for Shoulder Width and Type (CMF2r): Example August 2010 For 3,500 AADT with a 2 ft wide aggregate shoulder: CMFwra = 1.30 (Table 10-9) and CMFtra = 1.01 (Table 10-10) Adjustment from crashes related to lane and shoulder width (Run off Road + Head-on + Sideswipes) to total crashes using default value for pra = 0.574 CMF2r = (CMFwra CMFtra ) pra + 1.0 = ((1.30)(1.01) - 1.0) * = (0.313) (0.574) + 1.0 = Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Crash Modification Factors Lane and Shoulder Width – Example:
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Crash Modification Factors Lane and Shoulder Width – Example: For 1,500 AADT 10’ lane and no shoulder: What is CMF1r&2r? Lane Width = 10’ (From Table 10-8) CMFra = 1.213 CMF1r = ( )x = 1.122 Shoulder Width = 0’ (From Table 10-9) CMFwra = 1.375 CMF2r= (((1.375 x 1.00) -1.0)x 0.574) = 1.215 Photo shows a shoulder, but further down at the horizontal curve there are no shoulders, this photo was take from a safe location out of the way of traffic. LANE WIDTH CMF For 1,500 AADT, use Table 10-8, CMFra = x10-4(1, ) = SHOULDER WIDTH CMF for 1,500 AADT, use Table 10-9, CMFwra = x10-4(1, ) = 1.375 Combined CMF for Lane Width and Shoulder Width & Type = x = 1.363 Combined CMF: CMF1r&2r = 1.122x = 1.363 Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Crash Modification Factors Example: Combination Shoulder Type
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 6 ft Shoulder, AADT > 2,000 vpd From Table 10-10: CMF6’ paved = 1.00 CMF6’ gravel = 1.02 6 ft 4’ 2’ Combination Shoulder Type CMFtra Calculation: Example Calculation for a composite shoulder other than the 50% paved and 50% turf values. Proportion the width of each type to the total width then multiply the proportion value to the CMF value and add together. CMFtra = (4’/6’)x (2’/6’) x 1.02 = 1.007 Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Some Insights Review of CMFs for Lane Width and Shoulders
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Some Insights Review of CMFs for Lane Width and Shoulders Not much difference between 11- and 12-ft lanes Lane width is less important for very low volume roads Incremental width for shoulders is much more sensitive than for lanes Shoulder width effectiveness increases significantly as AADT increases Let’s review -- here are the major insights from the research and in looking at the CMFs for lane and shoulder width. Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

CMF for % Grade for Roadway Segments (CMF5r)
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop May 2009 CMF5r—Grades The base condition for grade is a generally level roadway. Table presents the CMF for grades based on an analysis of rural two-lane, two-way highway grades in Utah conducted by Miaou (8). The CMFs in Table are applied to each individual grade segment on the roadway being evaluated without respect to the sign of the grade. The sign of the grade is irrelevant because each grade on a rural two-lane, two-way highway is an upgrade for one direction of travel and a downgrade for the other. The grade factors are applied to the entire grade from one point of vertical intersection (PVI) to the next (i.e., there is no special account taken of vertical curves). The CMFs in Table apply to total roadway segment crashes.. IMPORTANT: Separate crash predictions can be performed for grades grouped as: 3% > Grades < 6%, where CMF5r = 1.10, and Grades > 6%, where CMF5r = 1.16 Or a single crash prediction can be performed for all graded segments using (calculating) a weighted CMF5r for all grades combined (see following slides). For Roadway Segment on 4% Grade: CMF5r = ? 1.10 Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

CMF for Driveway Density (Access) (CMF6r)
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 ( Ln(AADT))*DD _____________________ CMF6r = ( *Ln(AADT))*5 Where: CMF6r = effect of driveway density on total crashes DD = Driveway Density (driveways per mile) AADT = Average Annual Daily Traffic Per Chapter 10 of the HSM CMF6r—Driveway Density The base condition for driveway density is five driveways per mile. As with the other CMFs, the model for the base condition was established for roadways with this driveway density. The CMF for driveway density is determined using Equation 10-17, derived from the work of Muskaug (9). CMF6r = [ ln(AADT)]DD (Equation 10-17) [ ln(AADT)](5) Where, CMF6r= crash modification factor for the effect of driveway density on total accidents; AADT= average annual daily traffic volume of the roadway being evaluated (vehicles/day); and DD= driveway density considering driveways on both sides of the highway (driveways/mile) If driveway density is less than 5 driveways per mile, CMF6r is Equation can be applied to total roadway crashes of all severity levels. Driveways serving all types of land use are considered in deter mining the driveway density. All driveways that are used by traffic on at least a dail y basis for entering or leaving the highway are considered. Driveways that receive only occasional use (less than daily), such as field entrances are not considered. Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Calculation for Driveway Density (CMF6r): Example
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Where: AADT = 3,500 Access is 31 driveways in 5.02 miles DD = 31/5.02 = driveways per mile ( *Ln(AADT))*DD _____________________ CMF6r = ( *Ln(AADT))*5 Example calculation of the CMF for access Does not include crashes for intersections ( *Ln(3,500))*6.17 = ( *Ln(3,500))*5 = Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

CMF for Installing Centerline Rumble Strips (CMF7r)
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop CMF for Installing Centerline Rumble Strips (CMF7r) May 2009 From Chapter 13, Final HSM CMF7r—Centerline Rumble Strips Centerline rumble strips are installed on undivided highways along the centerline of the roadway which divides opposing directions of traffic flow. Centerline rumble strips are incorporated in the roadway surface to alert drivers who unintentionally cross, or begin to cross, the roadway centerline. The base condition for centerline rumble strips is the absence of rumble strips. The value of CMF7r for the effect of centerline rumble strips for total crashes on rural two-lane, two-way highways is derived as 0.94 from the CMF value presented in Chapter 13 and crash type percentages found in Chapter 10. Details of this derivation are not provided. The CMF for centerline rumble strips applies only to two-lane undivided highways with no separation other than a centerline marking between the lanes in opposite directions of travel. Otherwise the value of this CMF is 1.00. Rural two-lane roads The crash effects of installing centerline rumble strips on rural two-lane roads are shown in Table 13‑46 (8). The crash effects for head-on and opposing-direction sideswipe crashes are also shown in Table 13‑46. The CMFs are applicable to a range of centerline rumble strip designs (e.g., milled-in, rolled-in, formed, raised) and placements (e.g., continuous, intermittent) (26). The CMFs are also applicable to horizontal curves and tangent sections, and passing and no-passing zones (26). The base condition of the CMFs (i.e., the condition in which the CMF = 1.00) is the absence of centerline rumble strips. Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

*CRFrumble = 13% reduction in total crashes
Safety Effects of Installing Shoulder Rumble Strips: Two-Lane Rural Roads Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop May 2009 * Not in HSM *CRFrumble = 13% reduction in total crashes CMF = 1 – *CRF CMFrumble = = 0.87 The HSM final draft does not include an AMF for longitudinal shoulder rumble which is a significant shortcoming in light of the widespread deployment of longitudinal shoulder rumble throughout the states since 2002 …so for the time being until at such point in time that an AMF for shoulder rumble is added to the HSM, from Jeff Lindley’s July 10, 2008 memo cites an Overall crash reduction of 13% and injury reduction of 18% on rural two-lane highways Applying the AMF-CRF relationship results in: AMFsh-rumble = 0.87 *From FHWA CMF Clearinghouse Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

CMF for Passing Lane/Climbing Lane (CMF8r)
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop CMF for Passing Lane/Climbing Lane (CMF8r) August 2010 Final HSM: Provide a Passing Lane/Climbing Lane or a Short Four-Lane Section Passing lanes allow vehicles to pass, and may have the potential to reduce crashes such as head-on, same-direction sideswipe, and opposite-direction sideswipe crashes at some locations. Passing-related head-on crashes are a relatively low percentage of all head-on crashes.(12) Passing lanes may affect traffic operations 3 to 8 mi downstream of the passing lane due to the segregation they permit between faster and slower vehicles.(7,12) Climbing lanes allow vehicles to pass on grades, and may have the potential to reduce rear-end and same-direction sideswipe crashes at some locations that may result from speed differentials and conflicts between slow-moving and passing vehicles. Climbing lanes allow traffic platoons which have formed behind slower vehicles to dissipate without using an oncoming traffic lane to complete a passing maneuver. CMF8r—Passing Lanes The base condition for passing lanes is the absence of a lane (i.e., the nor mal two-lane cross section). The CMF for a conventional passing or climbing lane added in one direction of travel on a rural two-lane, two-way highway is 0.75 for total crashes in both directions of travel over the length of the passing lane from the upstream end of the lane addition taper to the downstream end of the lane drop taper. This value assumes that the passing lane is operationally warranted and that the length of the passing lane is appropriate for the operational conditions on the roadway. There may also be some safety benefit on the roadway downstream of a passing lane, but this effect has not been quantified. The CMF for short four-lane sections (i.e., side-by-side passing lanes provided in opposite directions on the same section of roadway) is 0.65 for total crashes over the length of the shor t four-lane section. This CMF applies to any portion of roadway where the cross section has four lanes and w here both added lanes have been provided over a limited distance to increase passing oppor tunities. This CMF does not apply to extended fourlane highway sections. The CMF for passing lanes is based primaril y on the work of Harwood and St.John (6), with consideration also given to the results of Rinde (11) and Nettelbl ad (10). The CMF for short four-lane sections is based on the work of Harwood and St. John (6). Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

CMF for Rural Two-Way Left Turn Lanes (CMF9r)
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 From Chapter 10 Final HSM: CMF9r—Two-Way Left-Turn Lanes The installation of a center two-way left-turn lane (TWLTL) on a rural two-lane, two-way highway to create a three-lane cross-section can reduce crashes related to turning maneuvers at driveways. The base condition for two-way left-turn lanes is the absence of a TWLTL. Rural two-way left turn lanes have been shown to be effective in some rural applications and for short segments of local development, through small towns with multiple driveways. * “Safety Evaluation of Installing Center Two-Way Left-Turn Lanes on Two-Lane Roads”, Sept 2007, Persaud, Lyon, Eccles, Lefler, Carter, and Amjadi, FHWA Low Cost Safety Pooled Fund Study for 78 sites in North Carolina, 10 sites in Illinois, 31 sites in California and 25 sites in Arkansas using Imperical Bayes methodology This is a safety improvement; note that the effectiveness is greater when flow rates exceed 300 vph. Most effective where one direction flow rate > 300 vph and in rural areas Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

CMF for TWLTL Lanes (CMF9r)
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop CMF for TWLTL Lanes (CMF9r) August 2010 CMF9r = 1.0 – 0.7 PD * PLT/D Where: Pdwy = Driveway-related crashes as a proportion of total crashes Pdwy = ( DD DD2) ( DD DD2) DD = drive density (driveways/mi > 5/mile) PLT/D = Left turn crashes susceptible to correction by a TWLTL as a proportion of driveway related crashes (estimated as 0.50) CMF9r—Two-Way Left-Turn Lanes - The installation of a center two-way left-turn lane (TWLTL) on a rural two-lane, two-way highway to create a three-lane cross-section can reduce crashes related to tur ning maneuvers at driveways. The base condition for two-way left-turn lanes is the absence of a TWLTL. The CMF for installation of a TWLTL is calculated using Equation : CMF9r = 1.0 − (0.7 × pdwy × pLT/D) ( Equation 10-18) Where: CMF9r = crash modification factor for the effect of two-way left-turn lanes on total crashes; pdwy = driveway-related crashes as a proportion of total crashes; and pLT/D = left-turn crashes susceptible to correction by a TWLTL as a proportion of driveway-related crashes. The value of pdwy can be estimated using Equation shown in the above slide. (Equation 10-19) Pdwy = driveway-related crashes as a proportion of total crashes; and DD = driveway density considering driveways on both sides of the highway (driveways/mile). The value of pLT/D is estimated as 0.5 (6). Equation provides the best estimate of the CMF for TWLTL installation that can be made without data on the left-turn volumes within the TWLTL. Realistically, such volumes are seldom available for use in such analyses though Section A.1. of Appendix A to Part C describes how to appropriately calibrate this value. This CMF applies to total roadway segment crashes. The CMF for TWLTL installation is not applied unless the driveway density is greater than or equal to five driveways per mile. If the driveway density is less than five driveways per mile, the CMF for TWLTL installation is 1.00. Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Example Calculation for TWLTL (CMF9r)
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop Example Calculation for TWLTL (CMF9r) August 2010 CMF9r = 1.0 – (0.7 Pdwy x PLT/D) For 35 driveways in 0.8 mile long segment DD = 35/0.8 = driveways/mi Pdwy = (0.0047(43.47)) (43.472) ( (43.47) (43.472) = 0.80 Example calculation for TWLTLs. CMF9r = 1.0 – (0.7 x 0.80 x 0.50) = 0.72 Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

August 2010 Roadside Quality is Strongly Linked to Related Crashes Roadside Design (CMF10r) Roadside Design is based on Roadside Hazard Ratings that are dependent on the roadside environment Ratings range from 1 to 7: 1 = forgiving roadside environment 7 = unforgiving roadside environment We will address roadside factors later; but the very important point here is to note that one can not look at cross section safety by considering lanes and shoulders in isolation from the roadside. The roadside plays a very strong effect. The instructor may want to point out or discuss what is happening. The alignment and/or lane and shoulder widths may influence the risk or probability of a vehicle leaving the road; but once it does so, the quality of the roadside determines whether that encroachment will become a reported crash and resulting severity. AMF10r - Roadside Design For purposes of the HSM predictive method, the level of roadside design is represented by the roadside hazard rating (1-7 scale) developed by Zegeer et al.(15). The AMF for roadside design was developed in research by Harwood et al(4). Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Roadside Hazard Ratings
Base condition is Hazard Rating = 3

Roadside Hazard Rating of 1
Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop Roadside Hazard Rating of 1 May 2009 Wide clear zones greater than or equal to 30 ft from the pavement edgeline. Sideslopes flatter than 1:4 Recoverable sideslope Photos taken from report -- self explanatory Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop Roadside Hazard Rating of 2 May 2009 4 1 Clear zone between 20 and 25 ft from pavement edgeline Sideslope about 1:4 Recoverable sideslope From NCHRP 486, 2003, page 34 and 35 Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop Roadside Hazard Rating of 3 May 2009 3 1 Clear zone about 10 ft from pavement edgeline Sideslope about 1:3 or 1:4 Rough roadside surface Marginally recoverable Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop Roadside Hazard Rating of 4 May 2009 Clear zone between 5 and 10 ft from pavement edgeline Sideslopes about 1:3 or 1:4 May have guardrail (5 to 6.5 ft from pavement edgeline). May have exposed trees, poles, or other objects (about 10 ft from pavement edgeline) Marginally forgiving, but increased chance of a reportable roadside crash Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop Roadside Hazard Rating of 5 May 2009 Clear zone between 5 and 10 ft from pavement edgeline Sideslope about 1:3 Virtually non-recoverable May have guardrail (0 to 5 ft from edgeline) May have exposed trees, poles, or other objects (about 10 ft from pavement edgeline) Rating of 5 to 7 is indicative of rolling to mountainous Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop Roadside Hazard Rating of 6 May 2009 2 1 Clear zone less than or equal to 5 ft Sideslope about 1:2, Non-recoverable No guardrail Exposed rigid obstacles within 0 to 6.5 ft of the pavement edgeline Rating of 5 to 7 is indicative of rolling to mountainous Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Safety and Operational Effects of Geometric Design Features for Two-Lane Rural Highways Workshop May 2009 Roadside Hazard Rating of 7 Clear zone less than or equal to 5 ft Sideslope 1:2 or steeper, Non-recoverable Cliff or vertical rock cut No guardrail High likelihood of severe injuries from a roadside crash Rating of 7 is indicative of mountainous terrain Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

CMF for Roadside Hazard Rating for Roadside Design (CMF10r)
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop CMF for Roadside Hazard Rating for Roadside Design (CMF10r) August 2010 e( ( x RHR)) e( ) CMF10r = Where: CMF10r = CMF for the effect of roadside design RHR = Roadside Hazard Rating From IHSDM model. Also this is Equation 25 in NCHRP Report 486, page 35. From HSM Chapter 10: CMF10r—Roadside Design For purposes of the HSM predictive method, the level of roadside design is represented by the roadside hazard rating (1–7 scale) developed by Zegeer et al. (16). The CMF for roadside design was developed in research by Harwood et al. (5). The base value of roadside hazard rating for roadway segments is 3. The CMF is: (Equation 10-20) Where: CMF10r = crash modification factor for the effect of roadside design; and RHR = roadside hazard rating. This CMF applies to total roadway segment crashes. Photographic examples and quantitative definitions for each roadside hazard rating (1–7) as a function of roadside design features such as sideslope and clear zone width are presented in Chapter 13, Appendix 13A. Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Calculation for Roadside Design (CMF10r): Example
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop Calculation for Roadside Design (CMF10r): Example August 2010 Where: RHR = 5 CMF10r = e( (0.0668xHR)) /e = e( (0.0668x5)) /e Example calculation of the CMF for Roadside Design with a Roadside Hazard Rating=5. = 1.143 Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Calculated CMFs for Roadside Hazard Ratings
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 HAZARD RATING CMF'S (calculated) Clear zone width Roadside Obstacles (ft) Roadside Slope Hazard Rating CMF10r None within clear Zone 30 or more Flatter than 1:4 1 0.875 1:4 1.5 0.905 20 to 30 2 0.935 1:3 2.5 0.967 10 to 20 3 1.000 1:2 or steeper 3.5 1.034 5 to 10 4 1.069 5 1.143 5.5 1.182 0 to 5 N/A 6 1.222 Barrier ft fro edge of travel way None Barrier 0-5 ft from edge of travel way Rock cut or cliff with no barrier 7 1.306 Table of Roadside Design CMFs for various Roadside Hazard Ratings. Base condition is HR = 3. Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

CMF for Lighting (CMF11r)
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop CMF for Lighting (CMF11r) August 2010 CMF11r = 1.0 – [(1.0 – 0.72pinr – 0.83 ppnr ) pnr] Where: CMF11r = effect of lighting on total crashes pinr = proportion of total nighttime crashes for unlighted roadway segments that involve a fatality or injury ppnr = proportion of total nighttime crashes for unlighted roadway segments that involve PDO crashes only pnr = proportion of total crashes for unlighted roadway segments that occur at night Instructor: from Chapter 10 of the Final HSM CMF11r—Lighting The base condition for lighting is the absence of roadway segment lighting. The CMF for lighted roadway segments is determined, based on the work of Elvik and Vaa (2), as: CMF11r = 1.0 − [(1.0 − 0.72 × pinr − 0.83 × ppnr) × pnr] (Equation 10-21) Where: CMF11r = crash modification factor for the effect of lighting on total crashes; pinr = proportion of total nighttime crashes for unlighted roadway segments that involve a fatality or injury; ppnr = proportion of total nighttime crashes for unlighted roadway segments that involve property damage only; and pnr = proportion of total crashes for unlighted roadway segments that occur at night. This CMF applies to total roadway segment crashes. Table presents default values for the nighttime crash proportions pinr, ppnr, and pnr. HSM users are encouraged to replace the estimates in Table with locally derived values. If lighting installation increases the density of roadside fixed objects, the value of CMF10r is adjusted accordingly. Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

August 2010 CMF for Lighting CMF11r = 1.0- [(1.0 – 0.72 pinr – 0.83 ppnr ) pnr ] Instructor: Chapter 10 Final HSM These are default values that could be used if local data is not available for proportional crashes. These are default values for nighttime crash proportions; replace with local values if available Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Calculation for Lighting (CMF11r) for Total Crashes
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop Calculation for Lighting (CMF11r) for Total Crashes August 2010 Example: Two-Lane Undivided: Obtaining coefficients from Table 10-12 CMF11r = 1.0- [(1.0 – 0.72 pinr – 0.83 ppnr ) pnr ] CMF11r = 1.0 – [(1.0 – 0.72(0.382) – 0.83(0.618))(0.370)] = 1.0 – [(1.0 – – 0.513)(0.370)] Instructor: from Chapter 10 of Final HSM Example calculation for CMF for lighting No lighting is CMF=1.00 = (0.212)(0.370) = 0.922 Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

CMF for Lighting for Nighttime Crashes
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop CMF for Lighting for Nighttime Crashes August 2010 Instructor: Chapter 13 Final HSM. The above CMFs are to be used to determine the safety effect for nighttime injury and non-injury (PDO) crashes. Provide Highway Lighting Rural two-lane roads, rural multilane highways, freeways, expressways, and urban and suburban arterials The crash effects of providing highway lighting on roadway segments that previously had no lighting are shown in Table 13‑56. The base condition of the CMFs (i.e., the condition in which the CMF = 1.00) is the absence of lighting. The CMFs for nighttime injury crashes and nighttime crashes for all severity levels were derived by Harkey et al (15). using the results from Elvik and Vaa (8) along with information on the distribution of crashes by injury severity and time of day from Minnesota and Michigan. Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Safety Effects of Automated Speed Enforcement (CMF12r)
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Use of video or photographic identification in conjunction with radar or lasers to detect speeding drivers: CMF12r—Automated Speed Enforcement Automated speed enforcement systems use video or photographic identification in conjunction with radar or lasers to detect speeding drivers. These systems automatically record vehicle identification information without the need for police officers at the scene. The base condition for automated speed enforcement is that it is absent. The value of CMF12r for the effect of automated speed enforcement for total crashes on rural two-lane, two-way highways is derived as 0.93 from the CMF value presented in Chapter 17 and crash type percentages found in Chapter 10. Details of this derivation are not provided. From Chapter 17, Final HSM: Install Automated Speed Enforcement Automated enforcement systems use video or photographic identification in conjunction with radar or lasers to detect speeding drivers. The systems automatically record vehicle registrations without needing police officers at the scene. The crash effects of installing automated speed enforcement in urban or rural areas on all road types are shown in Table 17-5 (1,3,5,7,9,12). The base condition for this CMF (i.e., the condition in which the CMF = 1.00) is the absence of automated speed enforcement. Multiyear programs indicate operating speeds dropped substantially at sites with fixed cameras compared with sites with mobile cameras (8). However, the magnitude of the crash effect of mobile- versus fixed-camera sites is not certain at this time. Some speed enforcement approaches are known to have spillover effects across the network. For example, speed cameras may affect behavior at locations not equipped with the cameras. The publicity and public interest accompanying installation of the cameras may lead to a generalized change in driver behavior at locations with and without cameras (10). Some enforcement approaches may also have “time halo” effects. For example, the effect of operating speeds being enforced for a specific period may remain after the enforcement is withdrawn. CMF12r = 0.93 for Total Crashes Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

CMFs of Other Roadway Elements
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 CMFs of Other Roadway Elements *Horizontal Curves & Superelevation Flattening Side Slopes Centerline Pavement Markings Edgeline Pavement Markings Post Mounted Delineators Installing Combination Horizontal/Advisory Speed Signs Raised Pavement Markers Here are some of the major CMFs that are applicable to two-lane rural roads. This is not an all inclusive list, there are other CMFs in Chapters 10 and 13 of the HSM that are not covered here. * Horizontal Curves & Superelevation are covered in Session 3 Note to Instructor: For the remaining roadway elements it is not essential, or the intention, to cover each slide in depth. At the discretion of the instructor(s) (i.e., if time is short) some or all of these slides can be hidden. If this is done, refer the participants to their manual or workbook for CMF values for the conditions above. Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

CMF for Flattening Sideslopes for Total Crashes
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 CMF for Total Crahes Flatten Sideslopes Rural two-lane roads The effect on total crashes of flattening the roadside slope of a rural two-lane road is shown in Table 13‑18 (15). The effect on single-vehicle crashes of flattening side slopes is shown in Table 13‑19 (15). The base conditions of the CMFs (i.e., the condition in which the CMF = 1.00) is the sideslope in the before condition. Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

CMF for Flattening Sideslopes for Single Vehicle Related Crashes
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 CMFs for Single Vehicle Related Crashes: Flatten Sideslopes Rural two-lane roads The effect on total crashes of flattening the roadside slope of a rural two-lane road is shown in Table 13‑18 (15). The effect on single-vehicle crashes of flattening side slopes is shown in Table 13‑19 (15). The base conditions of the CMFs (i.e., the condition in which the CMF = 1.00) is the sideslope in the before condition Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Safety Effects of Placing Standard Edgeline Markings
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Final HSM Chapter 13 Place Standard Edgeline MarkingsPlace Standard Edgeline Markings (4 to 6 inches wide) The MUTCD contains guidance on installing edgeline pavement markings (9). Rural two-lane roads The crash effects of installing standard edgeline markings, 4 to 6 inches wide, on rural two-lane roads that currently have centerline markings are shown in Table 13‑36. The base condition of the CMFs (i.e., the condition in which the CMF = 1.00) is the absence of standard edgeline markings. combining Injury (32.1%)and Non-Injury (67.9%) into an CMF for total crashes: CMFedgeline = 0.97* *0.679 = 0.97 Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

CMF for Placing Standard Centerline Markings
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop CMF for Placing Standard Centerline Markings August 2010 HSM Chapter 13 Place Centerline Markings The MUTCD provides guidelines and warrants for installing centerline markings (9). Rural two-lane roads The crash effects of placing centerline markings on rural two-lane roads that currently do not have centerline markings are shown in Table 13‑38 (8). The base condition of the CMFs (i.e., the condition in which the CMF = 1.00) is the absence of centerline markings. combining Injury (32.1%)and Non-Injury (67.9%) into an CMF for total crashes: CMFcenterline = 0.99* *0.679 = 1.004 Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

CMF for Placing Standard Centerline and Edgeline Markings
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Final HSM Chapter 13 Place Edgeline and Centerline Markings The MUTCD provides guidelines and warrants for applying edgeline and centerline markings (9). Rural two-lane roads and rural multilane highways Placing edgeline and centerline markings where no markings exist decreases injury crashes of all types, as shown in Table 13‑39. The base condition of the CMF (i.e., the condition in which the CMF = 1.00) is the absence of markings. using Injury (32.1%)and Non-Injury (67.9%) into an CMF for total crashes: CMFcenterline+Edge = (CMF-1.0) x = ( ) x = 0.923 Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

CMF for Placing Standard Centerline and Edgeline Markings and Post Mounted Delineators HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Final HSM Chapter 13 Install Edgelines, Centerlines, and PMDs Edgeline markings, centerline markings, and PMDs are often combined on roadway segments. Rural two-lane roads, and rural multilane highways The crash effects of installing edgelines, centerlines, and PMDs where no markings exist are shown in Table 13‑40. The base condition of the CMF (i.e., the condition in which the CMF = 1.00) is the absence of markings. using Injury (32.1%)and Non-Injury (67.9%) into an CMF for total crashes: CMFcenterline+Edge+Del = (CMF-1.0) x = ( ) x = 0.856 Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Applying CMFs to the SPF Base Prediction Model
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Applying CMFs to the SPF Base Prediction Model Npredicted-rs = Nspf-rs x (CMF1r x…CMFxr) Cr Where: Npredicted-rs = predicted average crash frequency for an individual roadway for a specific year (crashes/year) Nspf-rs = predicted average crash frequency for base conditions for an individual roadway segment (crashes per year) CMF1r ... CMFxr = Crash Modification Factors for individual design elements Cr = calibration factor CMFs are multiplied together and to the SPF Base Model values to determine the total safety effects (crash prediction) for individual geometric and traffic control features: Nspf-rs is the expected crash frequency from the base model, i.e., for the highway segment’s particular traffic volume and length. The CMFs adjust for the effect of the actual geometry of the segment. There are CMFs for 12 individual highway segment geometric or design elements. The calibration factor adjusts for differences between crash experience in the particular state and in the state for which the base model was fit. predictive models for rural two-lane, two-way roadway Segments The predictive models can be used to estimate total predicted average crash frequency (i.e., all crash severities and collision types) or can be used to predict average crash frequency of specific crash severity types or specific collision types. The predictive model for an individual roadway segment or intersection combines a SPF with CMFs and a calibration factor. For rural two-lane, two-way undivided roadway segments the predictive model is shown in Equation 10-2: This model estimates the predicted average crash frequency of non-intersection related crashes (i.e., crashes that would occur regardless of the presence of an intersection). Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Applying CMFs to the SPF Base Prediction Model: Example
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop Applying CMFs to the SPF Base Prediction Model: Example August 2010 Lane Width = 10 ft CMF1r = 1.172 Shoulder Width = 2 ft gravel CMF2r = 1.180 Segments on Grade (none) CMF5r = 1.000 Driveway Density (6.17/mi) CMF6r = 1.029 Centerline Rumble, None CMF7r = 1.000 Edgeline Rumble CMF7re = 0.870 Passing/Climbing Lanes, None CMF8r = 1.000 TWLTLs, None CMF9r = 1.000 Roadside Design, RHR = 5 CMF10r = 1.143 Lighting, None CMF11r = 1.000 Automated Enforcement, None CMF12r = 1.000 From Example Calculations: Rural Two-Lane Road: AADT = 3,500 vpd, Length = 5.02 mi, 31 Driveways, RHR = 5, Nspf-rs = 4.69 From the example calculations previously performed we can look at the resulting CMFs and tell which geometric elements or traffic control features are contributing to the increase and decrease of crash potential. Remember: CMFs > 1.0 have the potential for higher crash frequency CMFs < 1.0 have the potential for lower crash frequency Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Applying CMFs to the SPF Base Prediction Model: Example
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop Applying CMFs to the SPF Base Prediction Model: Example August 2010 Npredicted-rs = Nspf-rs x (CMF1rx … CMFxr) Cr From example calculations (letting Cr = 1.0): Npredicted-rs = 4.69 x (1.172 x x x x 1.00 x x x x x x 1.000) x 1.000 Applying CMFs to the SPF Base Prediction Model CMFs are multiplied together and to the SPF Base Model values to determine the total safety effects (crash prediction) for individual geometric and traffic control features: Nspf-rs is the expected crash frequency from the base model, i.e., for the highway segment’s particular traffic volume and length. The CMFs adjust for the effect of the actual geometry of the segment. There are CMFs for 12 individual highway segment geometric or design elements. The calibration factor adjusts for differences between crash experience in the particular state and in the state for which the base model was fit. = 4.69 x1.415 = 6.64 crashes per year Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Predicting Highway Safety for Two-Lane Rural Highway Segments
HSM Practitioner's Guide for Two-Lane Rural Highways Workshop August 2010 Learning Outcomes: Described the Safety Performance Function (Base) equation for prediction of Crash Frequency Described the Quantitative Safety Effects of Crash Modification Factors (CMFs) Applied CMFs to the Base Equation Learning Objectives for Session #1 Introduction and Background for Intersection Safety Session 2 – Predicting Highway Safety for 2-Lane Rural Highway Segments

Questions and Discussion:
HSM Practitioner's Guide for Two-Lane Rural Highways HSM Practitioner’s Guide for Two-Lane Rural Highways Workshop August 2010 Questions and Discussion:

HSM Practitioner's Guide for Two-Lane Rural Highways Workshop

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HSM Practitioner's Guide for Two-Lane Rural Highways Workshop

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