Scale and Scope of Safety 

Assessment Methods in the 

Project Development Process 

FHWA Safety Program
 

http://safety.fhwa.dot.gov 
2

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Cover image sources: Left photo: Thinkstock, top right photo: Thinkstock, bottom right photo: Missouri Department of Transportation 
 

 

 

 

 

Form DOT F 1700.7 (8-72) 

1. Report No. 

FHWA-SA-16-106 

2. Government Accession 

No. 

3. Recipient’s Catalog No. 

4.Title and Subtitle 

Scale and Scope of Safety Assessment Methods in the Project 

Development Process 

5. Report Date 

November 2016 

6. Performing Organizations Code 

7. Authors

 Jennifer Atkinson, Karen Dixon, Joseph Jones, 

Kelly Donoughe-Palframan, Mike Colety, Michael Pratt 

8. Performing Organization Report 

No. 

9. Performing Organization Name and Address 

Leidos, Inc. 

11251 Roger Bacon Drive 

Reston,VA 20190 

Subconsultants: 

Texas Transportation Institute 

Kimley-Horn Associates 

10. Work Unit No. (TRAIS) 

11. Contract or Grant No. 

DTFH61-10-D-00024 

12. Sponsoring Agency Name and Address 

United States Department of Transportation 

Federal Highway Administration 

Office of Safety 

1200 New Jersey Avenue, SE 

Washington, DC 20590 

13.Type of Report and Period 

Covered 

Technical Report, 

September 2014-October 2016 

14. Sponsoring Agency Code 

HSA 

15. Supplementary Notes 

Ray Krammes (Ray.Krammes@dot.gov), Office of Safety, served as the Task Manager for the Federal 

Highway Administration (FHWA). Jerry Roche (Jerry.Roche@dot.gov), Office of Safety, served as a 

technical advisor for FHWA.The Highway Safety Manual Implementation Pooled Fund Study funded this 

project. Pooled-fund State representatives served as reviewers. 

16. Abstract 

This document is intended to help users identify and apply suitable methods to assess quantitatively 

the safety performance impacts of project development decisions in terms of crash frequency 

and severity.This Guide suggests safety assessment methods that may be suitable for answering 

safety performance related questions that typically arise during each phase of the development 

process and for projects of various types. It also provides examples that illustrate the thought process 

for selecting a safety assessment method.This information on safety performance can then be 

considered in concert with other project criteria to make more informed highway investment decisions. 

17. Key Words 

Highway Safety Manual, project development, safety assessment 

method, safety performance 

18. Distribution Statement 

 No restrictions. 

19. Security Classif. (of this report) 

Unclassified 

20. Security Classif. (of this 

page) Unclassified 

21. No of Pages

 152 

22. Price 

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized. 
SI* (MODERN METRIC) CONVERSION FACTORS
 

APPROXIMATE CONVERSIONS TO SI UNITS 

SYMBOL WHEN YOU KNOW MULTIPLY BY TO FIND SYMBOL 

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g 

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oz 

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oC Celsuis 1.8C+32 Fahrenheit oF 

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poundforce per square inch 

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lbf/in2 

*SI is the symbol for the International System of Units. Appropriate rounding should be made to comply with Section 4 of 

American Society for Testing and Materials E380. 
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Table of Contents
 

CHAPTER 1. Introduction.......................................................................................................................................... 1
 

1.1 Overview of the Project Development Process..................................................................................................................................... 1
 

1.2 Safety Assessment Methods for Varying Project Applications..................................................................................................... 3
 

1.3 Selecting Suitable Safety Assessment Methods....................................................................................................................................8
 

1.4 How to Navigate This Guide ............................................................................................................................................................................10
 

1.5 Chapter References.................................................................................................................................................................................................12
 

CHAPTER 2. Planning and Scoping Applications .............................................................................................. 13
 

2.1 Priority Ranking Urban Signalized Intersections based onPedestrian Crashes................................................................17
 

2.2 Conducting Site Evaluations Supplemented by Collision Diagrams for an Urban Arterial Intersection.........21
 

2.3 Justifying the Need for Potential Cost-effective Safety Countermeasures for a Rural Two-lane Highway....26
 

2.4 Predicting Crashes to Evaluate Level of Service of Safety for a Rural Two-lane Highway.........................................32
 

2.5 Predicting Incremental Benefits Associated with Increased Lane Width for a Rural Two-lane Highway...... 40
 

CHAPTER 3. Alternatives Analysis Applications ................................................................................................ 49
 

3.1 Comparing Design Alternatives for Seven-Lane Urban Arterial ................................................................................................52
 

3.2 Predicting Crash Frequency for Alternative Intersection Turn Lane Options at
 

Four-lane Rural Undivided Highways..........................................................................................................................................................56
 

3.3 Calculating Expected Crashes for Rural Two-Lane Highways.................................................................................................... 66
 

3.4 Calculating Expected Crashes for Urban Freeway Ramps ...........................................................................................................73
 

CHAPTER 4. Preliminary and Final Design Applications.................................................................................. 83
 

4.1 Predicting Crashes for a New Urban Multilane Arterial.................................................................................................................. 86
 

4.2 Calculating Predicted Crash Frequency for Proposed Two-Lane Roundabout ............................................................ 90
 

4.3 Documenting a Design Decision for a Sharp Horizontal Curve on a Rural Two-Lane Highway..........................95
 

CHAPTER 5. Urban Street Continuous Case Study..........................................................................................103
 

5.1 Urban Street Continuous Case Study – Planning and Scoping..............................................................................................103
 

5.2 Urban Street Continuous Case Study – Alternatives Analysis ...................................................................................................112
 

5.3 Urban Street Continuous Case Study – Preliminary & Final Design ................................................................................117
 

Appendix – Alternative Calculations for Select Example Problems ........................................................... 121
 

A-3.2 Hand Calculated Example — Predicting Crash Frequency for Alternative Intersection
 

Turn-lane Options at Four-Lane, Rural, Undivided Highways................................................................................................... 121
 

A-3.4 Hand Calculated Example — Calculating Expected Crashes for Urban Freeway Ramps ................................ 127
 

A-4.1 Hand Calculated Example — Predicting Crashes for a New Urban Multilane Arterial....................................... 137
 

A-4.3 Hand Calculated Example — Documenting a Design Decision for a Sharp Horizontal Curve
 

on a Rural Two-Lane Highway......................................................................................................................................................................140
 

iii
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

List of Figures
 

Figure 1. Project Development Phases and Corresponding Chapter Organization...............................................................2
 

Figure 2. Example Problem Sample Header...................................................................................................................................................11
 

Figure 3. The Project Development Cycle and Corresponding Planning and Scoping Chapter.................................13
 

Figure 4. The Approach for Selecting a Safety Assessment Method for Planning and Scoping .................................14
 

Figure 5. The Project Development Cycle and Corresponding Alternatives Analysis Chapter .....................................49
 

Figure 6. The Project Development Cycle and Corresponding Design Chapters..................................................................83
 

List of Tables 

Table 1. Primary Analysis Application for Safety Assessment Methods.......................................................................................... 5
 

Table 2. Safety Assessment Methods for Project Phase, Task, and Type......................................................................................... 8
 

Table 3. Example Project Type Descriptions for Safety Assessment Method Identification ............................................. 9
 

Table 4. Data Needs for Safety Assessment Methods..............................................................................................................................10
 

Table 5. Planning and Scoping Safety Assessment Objective............................................................................................................15
 

Table 6. Safety Assessment Performance Measures and Data Needs ...........................................................................................16
 

Table 7. Example Summary of Available Data................................................................................................................................................17
 

Table 8. Alternatives Evaluation and Identification Safety Assessment Objective.................................................................51
 

Table 9. Preliminary and Final Design Safety Assessment Objective..............................................................................................85
 

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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

List of Abbreviations and Acronyms
 

AADT Annual Average Daily Traffic 

AASHTO American Association of State Highway and Transportation Officials 

ADT average daily traffic 

B/C benefit/cost 

CMF Crash Modification Factor 

DOT department of transportation 

EB Empirical Bayes 

EPDO equivalent property damage only 

FAQ frequently asked questions 

FHWA Federal Highway Administration 

FI fatal and injury 

HSM Highway Safety Manual 

IHSDM Interactive Highway Safety Design Model 

ISATe Enhanced Interchange Safety Analysis Tool 

ITE Institute of Transportation Engineers 

LOSS level of service of safety 

MP mile post 

NCHRP National Cooperative Highway Research Program 

NEPA National Environmental Policy Act 

PDO property damage only 

S.E. standard error 

SPF Safety Performance Function 

vpd vehicles per day 

v
vi
 
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

CHAPTER 1. INTRODUCTION
 

Safety is a key consideration in many project development decisions. Project development 

professionals—who include planners, designers, analysts, safety and operations specialists, 

managers, or others—can use a variety of safety assessment methods to inform, justify, and defend 

these decisions. These professionals are the target audience for this informational guide. 

A relatively new safety resource, the Highway Safety Manual (HSM), is the motivation for this Scale 

and Scope of Safety Assessment Methods in the Project Development Process guide (Guide). The 

American Association of State Highway and Transportation Officials (AASHTO) published the first 

edition of the HSM in 2010. The HSM describes itself as: 

...a resource that provides safety knowledge and tools in a useful form to facilitate improved 

decision making based on safety performance. The focus of the 

HSM is to provide quantitative information for decision making. The 

HSM assembles currently available information and methodologies 

on measuring, estimating and evaluating roadways in terms of 

crash frequency (number of crashes per year) and crash severity 

(level of injuries due to crashes). The HSM presents tools and 

methodologies for consideration of “safety” across the range of 

highway activities: planning, programming, project development, 

construction, operations, and maintenance. The purpose is to 

convey present knowledge regarding highway safety information for 

use by a broad array of transportation professionals.1 

In many States, project development professionals are still on the learning 

curve of when and how they can make effective use of the methods in 

the HSM. The purpose of the Guide is to help transportation professionals 

select safety assessment methods suitable at each step in their project 

development processes. 

This purpose of 

this Guide is to 

help transportation 

professionals select 

suitable safety 

assessment methods 

at each step of the 

project development 

processes. 

1.1 Overview of the Project Development Process 

Transportation agencies differ in how they characterize the project development process, the phases 

in the process, and the functions performed in each phase. This section, therefore, describes the 

generalized process used in this Guide so that professionals can relate it to their own agency’s 

approach. Figure 1 illustrates the process within the broader context of the overall project life cycle. 

1 The American Association of State Highway and Transportation Officials. Highway Safety Manual, 1st Edition. 2010 

1
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

1.1.1 Project Development Activity Organization 

As noted in Figure 1, the overall project life cycle encompasses system planning and programming, 

project development, construction, and maintenance and operation activities. The project 

development process generally includes activities associated with planning and scoping, alternatives 

analysis, environmental analysis, and preliminary and final design of an individual project. Many of 

these activities can directly benefit from safety assessment methods described in the HSM. These are 

discussed in greater detail in chapters 2 through 4. 

Project Life Cycle 

System

Planning and

Programming 

Project

Development Construction Maintenance and Operations 

Planning and

Scoping 

Environmental Analysis 

Alternatives 

Identification/Analysis 

Preliminary

Design Final Design 

Chapter 2 

Chapter 3 

Chapter 4 

NOTE: Elements of ongoing life cycle activities are incorporated into Chapter 2 and 4 as appropriate. 

Figure 1. Project Development Phases and Corresponding Chapter Organization 

1.1.2 Integrating Safety into the Project Development 

Process 

Several existing documents provide concepts for integrating safety into 

the project development process. For example: 

•	 The Federal Highway Administration’s (FHWA’s) Integrating Road 

Safety into NEPA Analysis: A Practitioner’s Primer2 provides basic 

information to help practitioners get started in understanding how 

to improve consideration of safety in NEPA [National Environmental 

Policy Act] analysis. 

•	 FHWA’s Integrating the HSM into the Highway Project Development 

Process3 provides general concepts and a few examples of how the 

HSM can be used in the process. 

This Guide focuses on 

selection of suitable safety 

assessment methods 

for the following project 

development phases: 

• Planning and scoping, 

• Alternatives identification 

and analysis, 

• Preliminary design, and 

• Final design. 

2 Federal Highway Administration, Integrating Road Safety into NEPA Analysis: A Practitioner’s Primer. 

Publication No. FHWA-SA-11-37. June 2011. Available at: http://safety.fhwa.dot.gov/p/fhwasa1137/. 

3 Federal Highway Administration, Integrating the HSM into the Highway Project Development Process. Publication No. FHWA-SA-11-50. May 

2012. Available at: http://safety.fhwa.dot.gov/hsm/hsm_integration/. 

2
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

• 	 ITE’s Integration of Safety in the Project Development Process and Beyond: A Context Sensitive 

Approach4 provides additional details on how to integrate safety into the project development 

process (using tools like the HSM) and presents 12 case studies illustrating how these concepts 

and tools have been applied to real-world projects. 

As noted above, this document is an information guide; that is, it provides information intended to 

help users identify and apply suitable methods for qualitatively assessing the safety performance 

impacts of project development decisions in terms of crash frequency and severity. This Guide 

suggests safety assessment methods that may be suitable for answering questions related to safety 

performance that typically arise during each phase of the development process and for projects 

of various types. It also provides examples that illustrate the thought process for selecting a safety 

assessment method. This information on safety performance can then be considered in concert with 

other project criteria to make more informed highway investment decisions. 

1.2 Safety Assessment Methods for Varying Project Applications 

Recently developed methods included in the HSM can estimate safety 

performance based upon road characteristic and traffic volume 

information in combination with or in lieu of observed crash information. 

These methods may provide a more reliable basis for estimating an existing 

or proposed facility’s safety performance than assessments that consider 

only crash history. 

1.2.1 Foundational Elements for Safety Assessment Methods 

The safety assessment methods described in the HSM and presented in this

Guide use one or more of the following basic “foundational elements”: 

•	 Observed crashes, 

•	 Crash modification factors/functions, and 

•	 Safety performance functions. 

The alternative safety 

assessment methods 

illustrated in this Guide can 

provide more statistically 

reliable estimates of a 

facility’s future safety 

performance as compared 

to crash history alone. 

Observed crashes refer to one or more years of crash history for a location. Safety assessments that 

focus on observed crashes can provide meaningful information for existing facilities. 

A crash modification factor (CMF) is a measure of the safety effectiveness for a particular roadway 

treatment or design element. For example, a CMF value of 0.85 would suggest that the presence 

of that treatment or element would result in a 15 percent decrease in crashes. A CMF value of 1.0 

suggests that a particular feature would have no effect on the number of crashes. 

There are CMFs for a wide variety of roadway treatments and alternative design element dimensions. 

These CMFs are available in Part D (Volume 3) of the HSM, at the Crash Modification Factors 

Clearinghouse (www.cmfclearinghouse.org), or in State-specific guidelines in which some State 

departments of transportation have customized CMFs for their regional conditions. Each CMF is 

uniquely defined by associated base conditions, road type, and crash type. 

Institute of Transportation Engineers, Integration of Safety in the Project Development Process and Beyond: A Context Sensitive Approach. May 

2015. Available at: http://library.ite.org/pub/e4edb88b-bafd-b6c9-6a19-22e98fedc8a9. 

3

4 
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

A safety performance function (SPF) is a statistically derived equation that estimates (or predicts) 

the average number of crashes per year likely to occur on a roadway of a particular type (e.g., twoway two-lane roadways or urban arterials) with a particular traffic volume. Using SPFs can enhance 

a safety assessment method’s predictive reliability by taking advantage of crash information for other 

similar roadways and not relying solely on recent crash history for the specific roadway to be treated. 

When site-specific geometric conditions are known, CMFs can be used with SPFs to provide more 

refined insights into the predicted safety performance (resulting in a calculated predicted number of 

crashes for roadways with similar conditions). Similar to CMFs, States may also customize SPFs to reflect 

local conditions. 

Combining observed crash data with predicted crash values (calculated using the CMF and SPF 

combination) can further improve the predictive reliability of crash prediction methods for a specific 

location (resulting in a calculated expected number of crashes). 

In summary, the three levels of analysis presented in the HSM are observed, predicted, and expected: 

Observed: Historical crash data for a location will tend to fluctuate over time, but an average (or 

mean) value can be calculated. These average crash values are referred to as observed crashes. 

Three Common 

Levels of Analysis 

• Observed Crashes 

• Predicted Crashes 

• Expected Crashes 

Predicted: Additional information from similar facilities and for similar volumes 

is likely to strengthen the estimated prediction by considering more crashes 

and to result in a more reliable estimate of the average number of crashes. This 

additional information can also include crash trends for varying traffic volumes 

and road geometry (presented in the format of SPFs and CMFs). This type of 

data strengthens the estimate for typical roads with the varying volumes and 

geometry and so is referred to as predicted crashes. 

Expected: Weighting the site-specific crashes with the crash estimates for similar roads further 

improves the reliability for predicted crashes. The HSM refers to these estimates as expected crashes. 

1.2.2 Candidate Safety Assessment Methods 

Safety assessment methods that use the three foundational elements, identified in Section 1.2.1, can 

be generally categorized as basic, intermediate, and advanced. 

•	 The basic safety assessment methods evaluate observed crashes and/or use CMFs related to 

the observed crashes. The basic methods introduced in this Guide include: 

- Site Evaluation or Audit 

- Historical Crash Data Evaluation 

- CMF Applied to Observed Crashes
 

- CMF Relative Comparison
 

•	 Intermediate safety assessment methods include the use of SPFs and generally result in more 

reliable predictions of the average number of crashes. The intermediate methods introduced in 

this Guide include: 

- AADT-only SPF 


- SPF with CMF Adjustment
 

4
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

• 	 Advanced safety assessment methods include all three foundational elements and generally 

result in the most reliable predictions for estimates of the expected average number of crashes. 

The advanced safety assessment method introduced in this Guide is: 

-	 SPF with CMF Weighted with Observed Crashes. 

Table 1. Primary Analysis Application for Safety Assessment Methods 

Basic Intermediate Advanced 

Data 

Applied to 

ashes

AADT-Only SPF

eighted with 

ashes 

Application 

Site Evaluation

or Audit

Historical Crash 

Evaluation

CMF

Observed Cr

Relative Comparison

SPF with CMF

Adjustment

SPF with CMF

W 

vObser ed Cr 

Predicted 

CMF

Crashes 

Expected 

Crashes Observed Crashes 

Performance of an Existing Road 1 1, 2 1, 2, 3 1, 3 1, 4 1, 3, 4 1, 2, 3, 4 

Future Impact of Minor Geometric 

Changes to Existing Road 

1, 2, 3 1, 3 1, 3, 4 1, 2, 3, 4 

Future Impact of Major Geometric 

Changes to Existing Road 

1, 3, 4 

Future Performance for a New Facility 1, 4 1, 3, 4 

Note: AADT = average annual daily traffic. CMF = crash modification factor. SPF = safety performance function. 

Basis for Analysis: 1 = site characteristics, 2 = crash history, 3 = CMF values, and 4 = AADT. 

Table 1 shows, at a glance, the typical analysis application for which these safety assessment 

methods are best suited. For many years, transportation professionals evaluated safety performance 

based on observed (i.e., historical) crash frequencies or crash rates. Although observed crashes 

can be very relevant and useful in evaluating the recent safety performance on existing facilities, 

they become less relevant and useful in estimating the future safety performance of existing facilities 

when traffic conditions on those facilities change significantly and/or when projects make substantial 

design changes to those facilities. Observed crashes may be of limited or no relevance for project 

alternatives that substantially change the type and character of the roadway or for facilities on new 

locations. There is a need, therefore, to select the appropriate safety assessment method or methods 

for the unique project development task. The following descriptions briefly introduce these individual 

methods. 

Basic Methods (Observed Crashes) 

The four basic safety assessment methods presented in this Guide can be used for evaluating 

observed crash conditions or for comparing prospective roadway features. Often, practitioners use 

basic methods for smaller-scale projects at existing locations. These four methods are: 

5
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Site evaluation or audit – Safety assessment and diagnosis for existing facilities may include 

a field review of site conditions. A typical site evaluation or audit focuses on (1) identifying site 

characteristics, (2) observing traffic operations and user interactions, and (3) evaluating potential site 

features that may contribute to a crash. Example information that may be documented during the 

evaluation includes site geometric characteristics; traffic control devices; heavy truck, motor vehicle, 

pedestrian, and bicycle volumes; unusual site features; and any potential elements of the road that 

may suggest a safety concern. The subsequent evaluation includes a diagnostic component to 

identify opportunities to eliminate or mitigate potential safety concerns at the site. The use of historic 

crash data, when available, further enhances the evaluation. 

HSM Reference: Sections 5.3 and 5.4 

Additional Resources: FHWA Road Safety Audit website, http://safety.fhwa.dot.gov/rsa 

Historical crash data evaluation – The evaluation of the crashes, typically for a period of 3 to 5 years, 

can provide meaningful information about crashes with specific information regarding crash trends 

over time, including those related to crash types and severity. While this evaluation period is typical, 

if conditions (e.g., roadway, traffic volumes/patterns, adjacent development, and access) have not 

changed considerably, evaluating additional years of data can more clearly reveal locations with 

potential geometric issues. This method applies to existing sites and requires observed crash data. 

Knowledge of the road type and road characteristics can provide additional valuable information for 

practitioners using this method. 

HSM Reference: Section 5.2 

Additional Resources: ITE Traffic Engineering Handbook, 7th Edition, pp. 131 - 145 

CMF applied to observed crashes – One of the simplest safety assessment methods is to adjust the 

observed number of crashes for a given site/corridor by a percent increase or decrease based on 

proposed changes to roadway characteristics. The number of observed crashes multiplied by a CMF 

that represents a potential change in a road characteristic can provide information about how the 

change may help to reduce the number of crashes. This method applies to existing sites that are 

candidates for roadway improvement projects and requires observed crash data and CMFs that 

represent the recommended change for the specific road type and road characteristics. 

HSM Reference: Sections C.6.3 and/or C.7, Section D.4, Method 4
 

Additional Resources: CMFs in Practice Series (FHWA-SA-13-010 through 016), http://safety.
 

fhwa.dot.gov/tools/crf/resources/cmfs/
 

CMF relative comparison – In some cases, the historic crash data is not always available for a site. If a 

potential improvement project is being considered, one option is to compare CMFs with similar base 

conditions in order to help determine the appropriate roadway characteristics. This CMF comparison 

approach can be accomplished without the use of observed crash data. 

HSM Reference: Section D.4
 

Additional Resources: CMFs in Practice Series (FHWA-SA-13-010 through 016), http://safety.
 

fhwa.dot.gov/tools/crf/resources/cmfs/
 

6
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

7

Intermediate Methods (Predicted Crashes) 

Two intermediate safety assessment methods also incorporate traffic volume into the analysis 

and, therefore, can be used to predict current and future crashes for a road type with specific 

characteristics. This procedure also incorporates a calibration factor that allows the SPFs to be further 

adjusted for local conditions. These two methods are: 

AADT-only SPF – An SPF that is based only on traffic volume can be used for larger-scale system-wide 

evaluations or for locations with similar base condition road characteristics. This method applies to 

existing or proposed facility types and requires traffic volume information for a specific road type. 

HSM Reference: Part C, Volume 2 

Additional Resources: CMFs in Practice Series (FHWA-SA-13-010 through 016), 

http://safety.fhwa.dot.gov/tools/crf/resources/cmfs/ 

SPF with CMF adjustment – An SPF combined with CMF adjustments can be used to evaluate 

unique roadway configurations that differ from common (base) conditions. This method applies to 

existing or proposed facility types and requires traffic volume information as well as the varying road 

characteristic information for the specific road type. For some States, multivariate models can be an 

alternative to SPFs with CMF adjustments. 

HSM Reference: Section C.7, Methods 1, 2, and 3 

Additional Resources: CMFs in Practice Series (FHWA-SA-13-010 through 016), 

http://safety.fhwa.dot.gov/tools/crf/resources/cmfs/ 

Advanced Method (Expected Crashes) 

The following advanced method can be used for projects with traffic volume information and 

observed crash data for a specific existing location. 

SPF with CMF weighted with observed crashes – The predicted number of crashes identified using 

the SPF with CMF adjustment method for a facility type can be weighted with observed crashes to 

provide a more statistically reliable method for estimating expected crashes at a particular location. 

This technique, referred to as the Empirical Bayes or EB method, is simply a weighting of observed 

and predicted crashes. This method is considered the most statistically reliable of the seven safety 

assessment methods because it takes advantage of both information about observed crashes at the 

location in question and information on predicted crashes based upon crash experience at other 

similar sites. 

HSM Reference: Section A.2, Part C, Volume 3, pp. A-15 to A-23 

Additional Resources: Observational Before-After Studies in Road Safety by Ezra Hauer, 

Emerald Group Publishing Ltd, UK, 1997 
Table 2. Safety Assessment Methods for Project Phase,Task, and Type 

 

SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

1.3 Selecting Suitable Safety Assessment Methods 

The goal of this Guide is to provide information that helps project development professionals 

select a safety assessment method for their project task. This section introduces the range of safety 

assessment methods that may be suitable for various project development phases and project types. 

“Suitable”, in this context, means that a method has the capability to answer most of the questions 

that generally arise using data typically available during that particular project development phase 

and task for that particular project type. 

Table 2 summarizes the safety assessment methods generally suitable for each project development 

phase. Chapters 2, 3, and 4 provide more detailed companion tables for planning and scoping, 

alternatives identification and evaluation, and preliminary and final design, respectively. 

Project Phase Related Task Project 

Type1 

Safety Assessment Method to Consider 

Basic Intermediate Advanced 

Planning 

and Scoping 

(Chapter 2) 

Preliminary Planning and 

Needs Assessment 

1R, 2R, 3R, 

4R, NL 

Establish Project Purpose 

and Need 

2R 

3R, 4R   

NL 

Establish Project Scope 

2R 

3R  

4R   

NL 

Alternatives 

Identification 

and Evaluation 

(Chapter 3) 

Alternative Selection 

2R 

3R, 4R   

NL  

Interchange Access 

Justification and 

Documentation 

3R, 4R   

NL 

Preliminary and 

Final Design 

(Chapter 4) 

Selecting specific design 

elements and their 

dimensions 

2R 

3R, 4R   

NL  

Design Exception 

3R, 4R   

NL 

Value Engineering 

4R   

NL 

Establishing the Work 

Zone Transportation 

Management Plan 

2R 

3R, 4R 

NL 

Note: = suitable safety assessment method. R1 = routine maintenance. R2 = resurfacing existing facilities. 

R3 = major rehabilitation of an existing facility. R4 = major retrofit construction efforts. NL = highway construction 

at a new location. 

8
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Within each project development phase, several related tasks may benefit from targeted safety 

assessments. These related tasks, and the safety performance related questions that arise during the 

execution of the tasks, are the first important considerations in selecting a suitable safety assessment 

method. 

The type of project is a second important consideration in selecting a suitable safety assessment 

method. The project types shown in Table 2 represent a wide range of construction activities. The 

project type abbreviations 1R, 2R, 3R, 4R, and NL represent different types of pavement work. Of 

primary importance to this Guide are the companion design and operational changes typically 

included in these projects. Table 3 summarizes these project types and the associated design and 

operational changes that would be the focus of a safety assessment. 

Table 3. Example Project Type Descriptions for Safety Assessment Method Identification 

Project Type Example Description 

1R 

The 1R project type designation is often associated with routine maintenance 

activities. This type of project could include a pavement overlay, roadside 

maintenance, or a minor upgrade to existing roadside hardware. For 1R projects, 

there are very few, if any, new improvements. 

2R 

The 2R project type designation is generally associated with resurfacing existing 

facilities or restoring road characteristics that are in need of an upgrade. As part 

of the 2R project, a limited number of new design or operational changes may 

be incorporated. These enhancements are minor and do not change the overall 

character of the facility. 

3R 

The 3R project type is often associated with major rehabilitation of an existing 

facility. This could include pavement improvements for the existing road, minor 

roadway widening, roadside shoulder improvement projects, and construction of 

select low-cost safety improvements at the site or system-wide level. 

4R 

The 4R project type includes major retrofit construction efforts including 

modification of the design to meet geometric criteria standards. This type of project 

generally includes substantial changes to the character of the road (significant 

widening, realignment, major operational modifications). 

NL The NL project type indicates constructing a highway at a new location. This type of 

project has all new construction for the majority of the alignment. 

9
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

A third important consideration in selecting a suitable safety assessment method is the project data 

typically available during the project development phase in relation to the data required by the 

safety assessment method. Table 4 summarizes the general types of data needed for the seven safety 

assessment methods identified in this Guide. 

Table 4. Data Needs for Safety Assessment Methods 

Safety Assessment 

Method 

Data Needs 

Road Type1 

Road 

Characteristics2 Traffic Volume3 

Observed 

Crash Data4 

Site Evaluation or Audit   

Historical Crash Data 

Evaluation   

CMF Applied to Observed 

Crashes   

CMF Relative Comparison  

AADT-Only SPF  

SPF with CMF Adjustment   

SPF with CMF Weighted with 

Observed Crashes    

1 Road Type refers to rural two-lane highway, rural multi-lane highway, urban freeway, etc. 

2 Road Characteristics includes physical features such as lane widths, access density, etc. 

3 Traffic Volume is the average daily traffic (ADT) or annual average daily traffic (AADT) in vehicles per day. 

4 Observed Crash Data represents the historic crash data at the study site. 

Note:  = required data.  = recommended data 

1.4 How to Navigate This Guide 

This Guide is organized to align with the individual phases of the project development process. The 

safety assessment methods described in this Guide can be used for a variety of analyses, but are 

organized in a format intended to help an analyst easily locate methods most suitable for a given 

project type, phase, and task. The tables included in this introductory chapter provide an initial 

introduction to the seven potential safety assessment methods and their common applications, 

including the range of safety assessment methods generally suitable for project development phases, 

related tasks, and project types. 

Chapters 2, 3, and 4 provide additional information to help make a selection among the range of 

suitable safety assessment methods for a particular project development phase, related task, and 

project type. Each chapter begins with an overview of the specific project development phase, 

including an associated safety assessment method option table (see Table 5, Table 8, and Table 9) 

that narrows down prospective candidate methods for a related task and project type. 

Chapters 2, 3, and 4 also provide examples that illustrate the selection and application of the various 

methods. Each example begins with a summary header similar to Figure 2. The header identifies the 

safety assessment method, the project development phase, the related task, and the project type 

(Table 3). All seven of the safety assessment methods can be hand calculated, but computerized 

tools are available for the intermediate and advanced methods. Example problems developed using 

10
 

PROBLEM OVERVIEW 

Safety Assessment Method: Historical Crash Data Evaluation 

Project Phase: Planning & Scoping Project Type: 2R 

Related Task: Establish Project Scope 

Calculation Method: 

 Hand Calculated  Tool Based 

Comments: 

This example problem demonstrates how to prioritize potential intersection improvements based on 

crash data. 

LEVEL OF ANALYSIS

 Basic Intermediate Advanced 

SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

11

the available computerized tools may, in some cases, have a companion hand-calculated version 

included in the appendix of the Guide. A note is shown in the calculation method example problem 

header section that indicates when an alternative hand-calculated version is available. Finally, the 

example problem header includes a section for comments and level of analysis. 

Figure 2. Example Problem Sample Header 

Each example reviews the scope of the problem, notes the data available for the analysis, 

summarizes how to select the appropriate safety assessment method, identifies linkage to the AASHTO 

HSM, and provides a detailed analysis. The examples conclude with a summary of findings and 

interpretation of results, possible errors to avoid, and alternative analysis approaches. 

To use this Guide, a project development professional with a question related to safety performance 

can begin by reviewing Table 1 and Table 2 to determine the applicable project development phase 

and related task. The next step is to go to the Chapter corresponding to that phase: 

• Planning and Scoping: Chapter 2, 

• Alternatives Identification and Evaluation: Chapter 3, and 

• Preliminary and Final Design: Chapter 4. 

Within the appropriate chapter, the professional would review the introductory content and 

associated navigation table (see Table 5, Table 8, and Table 9) to determine candidate safety 

assessment methods suitable to their task. In many cases, several different assessment methods may 

be available for a specific project type and phase, but the selection of the analysis method should 

be based on the practitioner’s specific question about safety performance. For example, a basic 

and an intermediate method may both be candidates under consideration. The analyst should 

determine the type of analysis appropriate for answering the specific question. Data requirements 

and availability also often play a major role in narrowing down suitable assessment methods. 

The examples presented in this Guide are not intended to cover all project development task phases 

or potential questions related to safety performance analysis, but rather to demonstrate how an 

analyst can select a suitable assessment method based on the required level of effort, phase of the 

project development process, associated type of project, and available data. 
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

1.5 Chapter References 

•	 American Association of State Highway and Transportation Officials (AASHTO). Highway Safety 

Manual, 1st Edition, Washington, D.C., 2010. 

•	 Federal Highway Administration. Integrating Road Safety into NEPA Analysis, A Practitioner’s 

Primer. Publication FHWA-SA-11-36, Washington, D.C., 2011. Available online at: http://safety.fhwa. 

dot.gov/tsp/fhwasa1136/fhwasa1136.pdf. 

•	 Federal Highway Administration. Integrating the HSM into the Highway Project Development 

Process. Publication FHWA-SA-11-50, Washington, D.C., 2012. Available online at: http://safety.fhwa. 

dot.gov/hsm/hsm_integration/hsm_integration.pdf. 

•	 Hauer, Ezra. Observational Before-After Studies in Road Safety, Emerald Group Publishing Limited, 

UK, 1997. 

•	 Institute of Transportation Engineers. Integration of Safety in the Project Development Process 

and Beyond: A Context Sensitive Approach. Publication IR-140, Washington, D.C., 2015. Available 

online at: http://library.ite.org/pub/e4edb88b-bafd-b6c9-6a19-22e98fedc8a9. 

•	 Institute of Transportation Engineers. Traffic Engineering Handbook, 7th Edition. Washington, D.C., 

2016. 

12
NOTE: Elements of ongoing life cycle activities are incorporated into 

Chapters 2 and 4 as appropriate.

SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

CHAPTER 2. Planning and Scoping Applications
 

NOTE: Elements of ongoing life cycle activities are 

incorporated into Chapter 2 and 4 as appropriate 

Planning and

Scoping 

Environmental Analysis 

Alternatives 

Identification/Analysis 

Preliminary

Design Final Design 

Chapter 2 

Chapter 3 

Chapter 4 

Project

Development 

Figure 3.The Project Development Cycle and 

Corresponding Planning and Scoping Chapter 

Planning and scoping activities occur 

early in the project development process 

and involve identifying the needs and 

range of actions, alternatives, and 

impacts to be addressed as part of 

the specific project scope. This Guide 

specifically focuses on project-level (rather 

than system-level) planning activities. 

Common considerations in this project 

development phase vary based on 

the type of project and may include 

operational efficiency, construction cost, 

right-of-way needs, effects on the human 

and natural environment, and safety. 

This chapter provides information to help 

practitioners select safety assessment 

methods suitable for addressing 

questions about safety performance 

that arise during planning and scoping based upon the related task and project type. This Guide 

describes planning- and scoping-related tasks in three general categories: 

• Conduct preliminary planning and needs assessment, 

• Establish project purpose and need, and 

• Establish project scope. 

Preliminary planning and needs assessment occurs early in project development and may be part 

of a corridor or project planning study. The goal of this task is to assess the current and future needs 

of a transportation facility. As the planning process evolves, the transportation agency will establish 

a project purpose and need where the term “purpose” can generally be defined as what will be 

addressed and the “need” provides data to support that purpose. Following some level of project 

planning, the transportation agency can then establish the project scope, which often includes 

identifying and diagnosing opportunities to reduce crashes and then determining potential limits and 

types of treatments or mitigation strategies. 

Table 5 identifies the safety assessment methods generally suitable for tasks related to planning and 

scoping and the objective of their safety performance analysis. The check marks in Table 5 suggest 

suitable safety assessment methods for each related task and objective and are, in some cases, 

distinguished by project type. In this context, the term “suitable” means that the method generally has 

the capability to address the safety performance related analysis objective with the data typically 

available for the related task and project type. 

13
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

Figure 4.The Approach for Selecting a Safety Assessment Method for Planning and Scoping 

The following example questions demonstrate the type of questions the analyst may develop at the 

beginning of the safety assessment. These questions are based on the example problems included in 

this chapter. 

1.	 How does the analyst assess where the limited funding could be most effectively spent? 

2.	 What can the analyst do to assess if there is actually a need for safety treatments at this 

location? 

3.	 How can the analyst estimate the safety performance of previously identified candidate 

low-cost countermeasures? 

4.	 How can the analyst estimate which curves are functioning as anticipated and which ones 

could benefit from low-cost treatments? 

5.	 How does the analyst estimate the reduction in the number of fatal and injury crashes due 

to these potential incremental improvements? 

Table 5 shows that the level of predictive reliability generally increases along the spectrum of methods 

from basic to advanced. At the same time, the required resources for the analysis also will increase. In 

some cases, it may not be feasible to implement the preferred safety assessment method fully due to 

limitations in site information, crash data, traffic volume, or similar information. For example, the Basic 

safety assessment method for a CMF Applied to Observed Crashes cannot be executed if historic 

crash data is not available. 

The approach for selecting a safety assessment method for planning and scoping looks like this: 

Determine Planning 

and Scoping 

Related Task 

Identify Project 

Type 

Select Safety 

Assessment Method 

(Confirm Data Requirements) 

A second safety assessment decision is the selection of the appropriate performance measure for the 

specific study question. In some cases, the performance measure may simply be based on average 

crash frequency or crash rate for an existing facility. Often, however, the performance measure is 

used to estimate some future performance (referred to as estimated, predicted, or expected crashes). 

Table 6 demonstrates several of these potential performance measures and their companion needs. 

14
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS -

Table 5. Planning and Scoping Safety Assessment Objective 

Related Task Objective Project Type 

Basic Intermediate Advanced 

Site 

Evaluation 

or Audit 

Historical 

Crash 

Data 

Evaluation 

CMF

Applied to 

Observed 

Crashes 

CMF

Relative 

Comparison 

AADT

Only

SPF 

SPF with

CMF

Adjustment 

SPF with CMF

Weighted with 

Observed 

Crashes 

Observed Crashes Predicted Crashes Expected Crashes 

Safety Assessments: Increasing Level of Predictive Reliability

PreliminaryPlanning and Needs Assessment 

Characterize Existing Safety Performance 

All  6  6 

Establish ProjectPurpose and Need 

Diagnose Safety Issues the ProjectShould Address 

2R  3R, 4R     9  10 

NL  

Establish Project Scope 

Refine Extent of Project and Safety AssessmentNeeds 

2R   5  7  7 

3R       8 4R       NL  

Data Requirements: Increasing Level of Resources Needed (Staff,Analysis,Time, etc.) 

Road Type 1      Road Characteristics 2     Traffi c Volume (vpd) 3   Observed Crash Data 4    1Road Type refers to rural two-lane highway, rural multi-lane highway, urban freeway, etc. 2 Road Characteristics includes physical features such as 

lane widths, access density, etc. 3 Traffic Volume is the ADT or AADT in vehicles per day. 4 Observed Crash Data represents the historic crash data at 

the study site for a period of more than 1 year (preferably 3 to 5 years). 5 See Example Problem 2.1. 6 See Example Problem 2.2.

7 See Example Problem 2.3. 8 See Example Problem 2.4. 9 See Example Problem 2.5. 10 See Example Problem 5.1.

Note: R2 = resurfacing existing facilities. R3 = major rehabilitation of an existing facility. R4 = major retrofit construction efforts. NL = highway construction at a new location.

 = suitable safety assessment method. ADT = average daily traffi c. AADT = annual average daily traffi c.

15
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

Table 6. Safety Assessment Performance Measures and Data Needs 

Performance Measure 

Data Requirements 

Road Type / 

Characteristic 

Traffic 

Volume 

Observed 

Crash 

Data Other Inputs 

Average Crash Frequency  

Crash Rate   

Equivalent Property Damage 

Only (EPDO) Average Crash 

Frequency 

  EPDO Weighting 

Factors 

Relative Severity Index   Relative Severity 

Indices 

Critical Rate   

Excess Predicted Average 

Crash Frequency Using 

Method of Moments 

  

Level of Service of Safety   

Calibrated SPF with 

Overdispersion 

Parameter 

Excess Predicted Average 

Crash Frequency Using SPFs    Calibrated SPF 

Probability of Specific Crash 

Types Exceeding Threshold 

Proportion 

 

Excess Proportion of Specific 

Crash Types  

Expected Average Crash 

Frequency with EB Adjustment   

Calibrated SPF with 

Overdispersion 

Parameter 

EPDO Average Crash 

Frequency with EB Adjustment   

Calibrated SPF with 

Overdispersion 

Parameter & EPDO 

Weighting Factors 

Excess Expected Average 

Crash Frequency with EB 

Adjustment 

  

Calibrated SPF with 

Overdispersion 

Parameter 

Note: SPF = Safety Performance Function, EB = Empirical Bayes 

Source: Adapted from the AASHTO Highway Safety Manual,Table 4-1, p. 4-8. 

This chapter provides examples that demonstrate the selection process for the planning and scoping 

safety assessment methods. These examples are simplified hypothetical problems intended to illustrate 

the thought process for selecting a method and demonstrate how to apply the method to answer the 

associated safety question. 

16
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

2.1 Priority Ranking Urban Signalized Intersections based on 

Pedestrian Crashes 

PROBLEM OVERVIEW 

Safety Assessment Method: Historical Crash Data Evaluation 

Project Phase: Planning & Scoping Project Type: 2R 

Related Task: Establish Project Scope 

Calculation Method: 

 Hand Calculated  Tool Based 

Comments: 

This example problem demonstrates how to prioritize potential intersection improvements based on 

crash data. 

LEVEL OF ANALYSIS

 Basic Intermediate Advanced 

Note: See table 3 for a full definition of Project Type designations. 

PROBLEM DESCRIPTION 

As part of planning and scoping activities, a city has identified six candidate intersections for rehabilitation 

of pedestrian facilities; however, the city needs to narrow the list to only four of the sites. The associated 

Project Type is 2R and the Related Task is to Establish Project Scope. The expected improvements will 

include replacing/widening the sidewalks and installing/updating crosswalks. How does the analyst 

assess where the limited funding could be most effectively spent? 

Summary of Available Data: 

Table 7 presents a 3-year summary of observed crashes for the six signalized intersections. The 

sidewalks and crosswalks currently located at the intersections are of similar age and design. 

Pedestrian and vehicle volumes are unknown. Additional site information can be obtained, if needed, 

by reviewing aerial photographs or by visiting the six intersection locations. 

Table 7. Example Summary of Available Data 

Intersection Number 

Number of Crashes 

(Three Years) 

K+A* Pedestrian 

Crashes 

Total 

Crashes 

Number of Crashes (Average per Year) 

K+A* Pedestrian 

Crashes Total Crashes 

1 12 144 4 48 

2 6 141 2 47 

3 12 99 4 33 

4 18 99 6 33 

5 9 150 3 50 

6 12 96 4 32 

Average 11.5 121.5 3.8 40.5 

*K+A refers to fatal and serious injury crashes. 

17
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

ANALYSIS 

Selecting Appropriate Safety Assessment Method(s): 

The analyst can first review the potential safety assessment methods shown in Table 5. The Establish 

Project Scope task and the 2R project type are associated with one of the four Basic safety 

assessment methods shown in Table 5. Table 1 (see Chapter 1) indicates that an evaluation of 

existing performance can be accomplished with the Site Evaluation or Audit or Historical Crash Data 

Evaluation safety assessment methods. The CMF-based methods require the use of CMF values as 

key elements of the analysis. Recall that a CMF commonly represents the change in the number of 

crashes due to varying a road characteristic. The analyst plans to use consistent improvements for the 

four selected intersections, and so the CMF assessment methods are not informative for this analysis. 

Site Evaluation or Audit 

Historical Crash Data Evaluation 

CMF Applied to Observed Crashes 

CMF Relative Comparison 

Establish Project 

Scope 

2R 

The analyst can narrow down the prospective analysis approach to the remaining two basic safety 

assessment methods of Site Evaluation or Audit or Historical Crash Data Evaluation. A review of 

the data requirements for the safety assessment methods shown in Table 5 confirms that observed 

crash data is required or recommended for both assessment methods. In addition, the road type 

and road characteristics can be considered if a site evaluation is the selected assessment method. 

The requirements for each of the two assessments are comparable, and the analyst may choose to 

perform one or both. 

Because every intersection is unique and site evaluations or audits can help isolate locationspecific issues but may not help to establish priorities, the analyst selected the Historical Crash Data 

Evaluation method for the initial ranking of sites. The Site Evaluation or Audit method could then be 

conducted to reinforce the recommendations resulting from this Historical Crash Data Evaluation 

effort. 

Linkage to the Highway Safety Manual (HSM): 

The HSM provides a list of potential ranking methods commonly used for network screening purposes. 

These are summarized in Table 6 (based on HSM Table 4-1, p. 4-8). Several potential performance 

measures shown in Table 6 may be suitable for this screening, but may also require additional data 

not available for these sites. Two potentially suitable performance measures are (1) Average Crash 

Frequency (HSM p. 4-24), and (2) Excess Proportions of Specific Crash Types (HSM p. 4-52). In addition, 

the HSM includes suggestions for site evaluations if the analyst elects to pursue the additional Site 

Evaluation or Audit assessment method (HSM Chapter 5, pp. 5-1 to 5-24). 

18
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Detailed Analysis: 

The average crash frequency can be ranked for K+A pedestrian crashes 

or for total crashes. Because the focus of this analysis is on improved 

pedestrian facilities and the expectation is to reduce the number of fatal 

or serious injury pedestrian crashes, evaluating K+A pedestrian crashes 

is important. In some locations, crash report information for this type of 

pedestrian crash may be limited. Similarly, a review of total crashes may 

help further clarify prevailing conditions at the intersection that are not 

clearly indicated when evaluating K+A pedestrian crashes alone. For these 

reasons, evaluation of these crashes can be complimented with a safety 

assessment of total crashes to confirm overall issues that may contribute to 

the number of crashes at the intersection. 

Where possible, the crash frequency method can be applied to locations with similar volumes. The 

excess proportions of specific crash types method ranks sites based on the proportion of a target 

crash type—in this case, K+A pedestrian crashes. The following steps summarize these calculations. 

Perform High 

Proportion Test 

Rank Candidate 

Intersections 

STEP 1: Summarize the crash data. 

Develop a summary table that includes average K+A pedestrian crashes per year, average total 

intersection crashes per year, and associated K+A pedestrian proportion of total crashes. A threshold 

to assess the proportion of the K+A pedestrian crashes at each site would be the total proportion of all 

K+A pedestrian crashes for the six potential locations. 

The threshold proportion is calculated as 23 ÷ 243 = 0.09. Locations with K+A pedestrian proportion of 

total crash values greater 0.09 merit consideration, based on this performance measure, during the 

ranking process. 

Intersection 

Number 

Average 

K+A 

Pedestrian 

Crashes 

per Year 

Total 

Intersection 

Crashes 

per Year 

K+A 

Pedestrian 

Proportion 

of Total 

Crashes1 

Ranking 

by K+A 

Pedestrian 

Crash 

Frequency 

Ranking 

by Total 

Crash 

Frequency 

Ranking 

by 

Proportion 

1 4 48 0.08 2 2 4 

2 2 47 0.04 6 3 6 

3 4 33 0.12 2 4 3 

4 6 33 0.18 1 4 1 

5 3 50 0.06 5 1 5 

6 4 32 0.13 2 6 2 

Total: 23 243 N/A N/A N/A N/A 

1

The proportion of total crashes is calculated by dividing K+A pedestrian crashes by total intersection crashes. 

Note: N/A = Not applicable.The threshold proportion of K+A pedestrian crashes (for these 6 sites) is 

0.09. Shaded cells represent the top ranked intersections for the specific performance measure. 

19
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

STEP 2: Assess calculation of variance and probabilities. 

The excess proportions ranking method calculations can be expanded to calculate the probability 

that the K+A pedestrian crashes exceed the threshold proportion. This process requires an additional 

calculation of a sample variance and the development of a probability to rank the sites that 

exceeded the threshold proportion. For this analysis, the analyst intends to select four intersections for 

improvement. Upon inspection, it is notable that only three of the intersections exceed the proportion 

and so all three will be considered. As a result, the analyst does not elect to compute the additional 

assessment values that are based on the simple variance. 

STEP 3: Select and treat the highest ranked overrepresented sites. 

Ranking based on the crash proportion test can be expected to produce different results than when 

the intersections are ranked by the average frequency of pedestrian crashes only or by total crashes. 

The shading in the summary table represents the top ranked intersections for each column. The four 

most critical intersections, based on the K+A Pedestrian Crash Frequency ranking, are Intersections 

#1, 3, 4, and 6. For intersections based solely on total crashes, Intersections #1, 2, and 5 are ranked the 

highest. Intersections #3 and 4 tied for fourth place. The ranking based on proportion includes sites 

where the proportion exceeds the overall threshold proportion value of 0.09 (or 9 percent) for all six 

intersections. This resulted in identification of Intersections #3, 4, and 6. Though the proportion value 

did not exceed 0.09, Intersection #1 did rank fourth using this proportion ranking method. 

Intersections #1, 3, and 4 may be the clear priorities since they were ranked in the top four for all three 

ranking methods. The selection of the 4th priority among the remaining intersections boils down to a 

judgment call between intersections with more total crashes (#1 and 5) versus the intersection with 

more pedestrian K&A crashes (#6). 

FINDINGS 

Interpreting the Results: 

This simple analysis identified three intersections that have an overrepresented proportion of 

K+A pedestrian crashes. The intersection rankings based on total crashes provide very different 

recommendations than rankings based on average K+A pedestrian crashes or their associated 

proportion. Indications are that the analyst may want to recommend improving Intersections #1, 

3, 4, and 6; however, the agency may want to evaluate additional site features (i.e. traffic volume, 

pedestrian crossing distance, etc.) to further confirm the sites with the greatest potential for K+A 

pedestrian crash reduction. 

Possible Errors to Avoid: 

Analysts should guard against making decisions about intersections involving a small number of 

crashes or using only 1 or 2 years of crash data. For the purposes of an intersection ranking analysis, 

a small number of crashes can be assumed to be less than 10 total crashes per year. Analysts should 

use a minimum of 3 years of crash data. 

This demonstration included three common ranking methods. All three methods produce different 

results; therefore, using multiple ranking methods and selecting locations that are highly ranked 

by more than one ranking procedure will enable practitioners to consider varying dimensions of 

pedestrian and intersection safety. 

20
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

Alternative Analysis Approaches: 

The focus on evaluating observed K+A pedestrian crashes resulted in the analyst evaluating three 

performance measures associated with the Historical Crash Data Evaluation safety assessment 

method. If traffic volume information can be acquired, this analysis could be expanded to consider 

additional performance measures shown in Table 6, such as the crash rate or critical rate measures. 

As the project shifts from the planning to the design phase, the analyst will assemble more detailed 

site-specific information. At that time, additional analysis procedures that incorporate CMFs and/or 

SPFs can strengthen the overall analysis. 

2.2	 Conducting Site Evaluations Supplemented by Collision Diagrams 

for an Urban Arterial Intersection 

PROBLEM OVERVIEW 

Safety Assessment Method: Site Evaluation or Audit 

Project Phase: Planning & Scoping Project Type: 2R 

Related Task: Preliminary Planning & Needs 

Assessment 

Calculation Method: 

 Hand Calculated  Tool Based 

Comments: 

This example problem demonstrates early project evaluation diagnostics for determining if a need 

exists. 

LEVEL OF ANALYSIS

 Basic Intermediate Advanced 

Note: See table 3 for a full definition of Project Type designations. 

PROBLEM DESCRIPTION 

The city has been asked to determine whether the entrance from an urban arterial to an 

unsignalized commercial driveway is in need of safety enhancements. In recent years, this location 

has experienced a number of minor crashes near the driveway entrance. The associated Project 

Type is 2R and the Related Task can be classified as Preliminary Planning and Needs Assessment. 

What can the analyst do to find out whether there is actually a need for safety treatments at this 

location? 

Summary of Available Data: 

The urban arterial corridor has two-way traffic with a total of six through lanes in the region of the 

unsignalized driveway, as shown on the following aerial photograph. 

21
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Driveway of Interest 

Source: ©Google Earth 

The annual average daily traffic (AADT) for the study corridor (at the driveway location) is 8,400 vpd. 

The 3-year crash information for this location includes three angle crashes and five rear end crashes 

for a 3-year total of eight crashes. This information is summarized in the following table. 

Mile 

point 

Time/ 

Date 

Direction 

of Travel for 

V1, V2, V3 

KABCO 

Severity 

Level 

Maneuver 

Type 

Light 

Condition 

Road 

Surface 

Condition 

Weather 

Condition 

Angle Crashes: 

19.16 18:18 / 

1-17-13 North, West C V1: Turning Left V2: Straight Dusk Dry Clear / 

Cloudy 

19.13 07:25 / 

3-2-14 North, West C V1: Turning Left V2: Straight Daylight Wet Rain 

19.11 14:15 / 

4-28-12 North, West C V1: Turning Left V2: Straight Daylight Dry Clear / 

Cloudy 

Rear End Crashes: 

19.25 12:15 / 

8-6-13 West, West O V1: Straight V2: Slowing Daylight Wet Rain 

19.22 08:06 / 

11-18-12

 West, West, 

West O 

V1: Slowing 

V2: Stopped 

V3: Stopped 

Daylight Dry Clear / 

Cloudy 

19.17 00:38 / 

10-31-14 East, East C V1: Straight V2: Straight 

Dark 

(Lighted) Dry Clear / 

Cloudy 

19.18 08:00 / 

10-3-13 

West, West, 

West O 

V1: Slowing 

V2: Stopped 

V3: Stopped 

Daylight Dry Clear / 

Cloudy 

19.33 18:50 / 

1-30-14 East, East O V1: Slowing V2: Stopped 

Dark 

(Lighted) Dry Clear / 

Cloudy 

Note: C = Possible Injury. O = Property Damage Only. V1 = Vehicle 1.V2 = Vehicle 2.V3 = Vehicle 3. 

22
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

ANALYSIS 

Selecting Appropriate Safety Assessment Method(s): 

The analyst should review the prospective safety assessment methods shown in Table 5. Based 

on the related task and project scope, the analyst can narrow the focus to two prospective safety 

assessment methods to consider: Site Evaluation or Audit or Historical Crash Data Evaluation. Based 

on Table 1 (see Chapter 1), both of these options are viable analysis techniques for evaluating existing 

performance. 

Preliminary 

Planning & Needs 

Assessment 

2R 

Site Evaluation or Audit 

Historical Crash Data Evaluation 

The analyst notes that a Site Evaluation or Audit will allow an inspection of visual crash trends or 

vehicle conflicts that could provide useful information. The Historical Crash Data Evaluation can also 

provide valuable insights. Both safety assessment methods are applicable and can provide useful 

information using similar data requirements. The analyst elects to conduct the Historical Crash Data 

Evaluation by developing a collision diagram prior to the site visit. The analyst also selects the Site 

Evaluation or Audit method. 

Linkage to the HSM: 

Chapter 5 of the HSM addresses options for summarizing crashes by 

location (HSM Section 5.2.2, pp. 5-4 to 5-7). An example diagram is shown 

in HSM Figure 5-3 (p. 5-5). If the site evaluation highlights an issue that may 

be contributing to crashes at the site, the analyst can refer to Chapter 6 of 

the HSM (pp. 6-3 to 6-9) for help identify specific contributing factors. During 

subsequent stages of the project development process, the personnel may 

have a need to explore and select potential countermeasures presented 

in the HSM Part D (Volume 3) or available on the FHWA-sponsored CMF 

Clearinghouse (www.cmfclearinghouse.org). 

In addition, the HSM provides a list of potential performance measures 

and their associated data needs that can be considered if the analyst 

ultimately conducts the Historical Crash Data Evaluation method. These 

are summarized in Table 6 (based on HSM Table 4-1, p. 4-8). A suitable 

performance measure for this study is the Critical Rate method (HSM p. 4-11). 

Identify Key Site 

Features 

Develop Collision 

Diagram 

Determine 

Candidate 

Countermeasures 

23
24

SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Detailed Analysis: 

A first step in conducting a site evaluation is to use collision diagrams to diagnose potential safety 

issues and prevailing crash types at a location. 

A diagram that shows the intersection/roadway alignment with the crashes superimposed is known 

as a collision diagram. This diagram typically includes relevant information including type and 

severity of crash, date and time of crash, weather, and lighting conditions. Plotting the general crash 

location and associated information can help to highlight crash trends, if present. The following steps 

depict this process. 

STEP 1: Identify key site features. 

An aerial photo or a condition diagram (refer to HSM Figure 5-5, p. 5-7) can be used to identify and 

document important site characteristics. By inspecting the aerial imagery for this location, one sees 

that there is a directional median opening that allows left-turns into the driveway, but restricts vehicles 

exiting the driveways to a right-turn only. 

STEP 2: Develop the collision diagram. 

The historical (observed) crash data can be used as the basis for developing the collision diagram. 

The following collision diagram shows the study site crashes for the three-year period. 
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Interpreting the Results: 

Possible Errors to Avoid: 

This location predominantly experienced angle and rear-end crashes. The angle crashes appear 

to be due to the conflict between the left-turning vehicles and the opposing through vehicles at the 

unsignalized entrance on the north side of the road. These crashes could be a result of restricted 

sight distance, sun glare issues, or high total intersection volumes with few gaps in traffic. Two of these 

crashes occurred during rainy conditions. The rear end crashes could be associated with large traffic 

volumes (and potentially queues from a downstream signalized intersection) or similar issues. 

STEP 3: Identify potential countermeasures. 

Prior to the field inspection and study by the analyst, it is helpful to explore potential treatments for 

future mitigation of issues. For the crashes observed at this site, example candidate countermeasures 

may include: 

• Installation of traffic signals (refer to the Manual on Uniform Traffic Control Devices). 

• Adjustment of upstream or downstream signal timing to allow progression or to create longer gaps. 

FINDINGS 

This simple analysis method used collision and condition diagram techniques to identify specific 

crash types, prevailing conditions, and potential contributing factors. At this location, eight crashes 

occurred over a 3-year period. Four of the eight crashes included at least one injury. Based on the 

collision diagram, it appears that suitable countermeasures will target rear-end and angle crashes; 

however, due to the small number of crashes the analyst may want to assess how crashes at this 

location are comparable to crashes at similar locations in an effort to determine whether this location 

merits improvement at this time. 

The crash data for this site spanned a period of 3 years. It is recommended that analysts gather data 

from at least 3 to 5 years of crashes to avoid drawing conclusions that do not accurately reflect the 

crash history. On a cautionary note, for locations with a limited number of observed crashes, the 

analyst should not attempt to draw definitive conclusions without extending the analysis to similar 

sites or by comparing the number of crashes to how many would be predicted for the specific facility 

using intermediate or advanced safety assessment methods. 

Alternative Analysis Approaches: 

The Historical Crash Data Evaluation safety assessment methods can also be used to supplement 

this analysis. If the analyst determines that there is sufficient justification to extend the assessment 

to a detailed evaluation of the candidate location, the next step would be to acquire specific site 

characteristic information. This added information would then enable the analyst to predict crashes 

(using the SPF with CMF Adjustment procedure). The predicted number of crashes represents an 

estimate of how many crashes are typically observed for facilities with similar traffic volumes and 

roadway characteristics. This additional comparison will strengthen the analysis and help clarify if the 

location has more crashes than similar locations. 

25
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

 

2.3 Justifying the Need for Potential Cost-effective Safety 

Countermeasures for a Rural Two-lane Highway 

PROBLEM OVERVIEW 

Safety Assessment Method: CMF Applied to Observed Crashes and CMF Relative Comparison 

Project Phase: Planning & Scoping Project Type: 2R 

Related Task: Establish Project Scope 

Calculation Method: 

 Hand Calculated  Tool Based 

Comments: 

This example problem shows how a basic analysis can be used to identify potential countermeasures 

that will reduce crashes. 

LEVEL OF ANALYSIS

 Basic Intermediate Advanced 

Note: See table 3 for a full definition of Project Type designations. 

PROBLEM DESCRIPTION 

As part of the planning and scoping activities, a roadway agency has identified a 10-mile section of 

rural two-way, two-lane highway targeted for safety improvements. Many of the crashes appear to be 

due to vehicles running off of the road. In the most recent 3 years, almost 40 crashes have occurred 

within this 10-mile section of highway. During the diagnosis process, the roadway agency identified 

potential treatments that included removal or relocation of fixed objects, installation of center line 

rumble strips, and delineation of obstacles. The associated Project Type is 2R and the Related Task 

is to Establish Project Scope. How can the analyst estimate the safety performance of previously 

identified candidate low-cost countermeasures? 

Summary of Available Data: 

The following data describes the current conditions and associated crash data. 

• Two-lane, two-way operation with lane widths of 11 ft. 

• No paved or graded shoulders. 

• Average annual daily traffic (AADT) 1,723 vehicles per day (vpd). 

• Rolling terrain with numerous horizontal curves. 

• Average side slope 1V:3H (1 ft. vertical for every 3 ft. horizontal). 

•	 Three-year crash count = 38 total. 

- Two fatal. 

- Nine injury crashes. 

- Twenty-seven PDO crashes. 

26
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

The crash data currently does not include extensive detail about the individual vehicle maneuvers for 

each crash, and so the analyst will focus on total crashes and crash severity for this evaluation. 

ANALYSIS 

Selecting Appropriate Safety Assessment Method(s): 

The roadway agency can evaluate the candidate safety assessment methods shown in Table 5. 

The purpose for this analysis is to use the crash information to assess what types of improvements 

can be implemented to help reduce future crashes along the corridor. This task and project type 

is associated with one of the four Basic safety assessment methods. Since a CMF-based method 

considers a change in road characteristics, the two candidate safety assessment methods that 

use CMFs are applicable for this analysis. Both methods can be considered. The CMF Applied to 

Observed Crashes method can be directly applied to the observed crashes. The estimated future 

crashes can be compared to historical crash data to evaluate the potential reduction in crashes 

based on the individual improvements. The cost of the reduced number of crashes can then be 

quantified by applying the equivalent property damage method of calculating a benefit/cost 

(B/C) ratio. 

Establish Project 

Scope 2R 

Site Evaluation or Audit 

Historical Crash Data Evaluation 

CMF Applied to Observed Crashes 

CMF Relative Comparison 

The CMF Relative Comparison approach does not explicitly consider historical crashes at the site and 

so is simply used to compare two candidate improvements. 

Linkage to the HSM: 

Chapter 6 of the HSM (Section 6.2.2, pp. 6-3 to 6-9) summarizes contributing factors to consider when 

selecting appropriate countermeasures. Section 6.3 of the HSM (p. 6-9 to 6-10) provides guidance for 

selecting potential countermeasures. Finally, Part D (Volume 3) of the HSM and the FHWA sponsored 

CMF Clearinghouse (www.cmfclearinghouse.org) include a wide variety of potential improvements. 

These CMF resources identify the base conditions and applicable site applications for the individual 

countermeasure of interest. 

Detailed Analysis: 

Method 1: CMF Applied to Observed Crashes 

The analyst will evaluate the candidate treatments recommended as a result of the site diagnosis. 

There are many potential improvements designed to help mitigate run-off-the-road crashes, but this 

detailed analysis focused on the three low-cost options. 

27
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

 

Calculate the Equivalent 

Property Damage Only 

(EPDO) 

Develop 

Benefit-to-Cost Ratio for 

Candidate 

Countermeasures 

The following steps review the use of the equivalent property damage 

only (EPDO) performance measure to first identify the value of the 

crashes and then the CMF Applied to Observed Crashes safety 

assessment method to further evaluate how the low-cost improvements 

may help to reduce crashes. A benefit/cost (B/C) ratio can then be used 

to assess which treatments would be the most cost effective. 

STEP 1: Calculate the EPDO value for existing crashes. 

The HSM provides sample weighting factors to calculate the EPDO (HSM 

Table 4-7, p. 4-29). The roadway agency performing this analysis uses 

the following cost values and resulting weighting factors; however, the 

values can vary by agency and so it is recommended that they be 

confirmed prior to analysis. 

Severity Comprehensive 

Crash Costs (f(cost)) 

Weighting Factor 

Calculation 

Weighting Factor 

(f(weight)) 

Fatal (K) $4,008,900 f

k (weight) = 4,008,900

 7,400 542 

Injury (A/B/C) $82,600 f

inj (weight) = 82,600

 7,400 11 

PDO (O) $7,400 f

PDO (weight) = 7,400

 7,400 1 

Note: A=incapacitating injury. B=serious injury. C=possible injury. PDO = property damage only. 

The EPDO weighting factors are then applied to the individual crash severity frequencies to calculate 

the equivalent number of property damage only (PDO) crashes. 

Total EPDO Calculation Notes 

=fk(weight) (N(observed,k))+finj(weight) (Nobserved,inj) 

+fPDO(weight) (Nobserved,PDO) 

None. 

=(542)(2)+(11)(9)+(1)(27)=1210 The makeup of the existing crashes (all severity 

levels) is equivalent to having 1210 PDO crashes. 

Note: EPDO = equivalent property damage only. PDO = property damage only. 

STEP 2: Calculate the B/C ratio of the candidate treatments. 

For this evaluation, the analyst is considering improving the clear zone (removing roadside obstacles 

and trees), adding center line rumble strips, or delineating obstacles where appropriate. The following 

summary presents these three low-cost treatment options and information related to the cost and 

effectiveness of each. 

28
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

Proposed 

Treatment 

Estimated 

Cost 

(for 10-mile 

segment) 

CMF 

(S.E.)1 

Crash 

Type (Base 

Condition) 

Crash 

Severity Source 

Remove or 

relocate 

fixed objects 

outside of 

clear zone 

$200,000 0.62 

(0.103) 



All crash 

types and 

roadway 

types 

All http://www.cmfclearinghouse. 

org/detail.cfm?facid=1024 

Install center 

line rumble 

strips 

$30,000 0.91 

(0.02) 



All crash 

types and 

rural roadway 

types 

All http://www.cmfclearinghouse. 

org/detail.cfm?facid=3361 

Delineate 

obstacles $10,000 1.0 (0.1) 

All crash 

types and 

roadway 

types 

All 

The HSM CMF value is 1.0. 

Because CMFs are multiplicative, 

a value of 1.0 has no effect on 

crash reduction for these crash 

types. 

1 S.E. refers to the standard error 

 CMF Clearinghouse Star Rating. 

Note: CMF = crash modification factor. HSM = Highway Safety Manual. 

For these potential improvements, the crash type is the “all” or total crash category. Many of the 

available CMF values focus on run-off-road crashes. Since this level of information is not available for 

the study site, the analyst must check to be sure that the correct CMF base conditions are applicable 

to the study site. For these three specific proposed treatments, the CMFs do apply to “All” crashes and 

not just to run-off-road collisions. Whenever the data is available, a preferred CMF comparison would 

be to evaluate the target crashes by type for each CMF. This requires information about the individual 

crash types at the site as well as CMFs that have the crash type base condition of interest. For this 

calculation, the target crash level of detail is limited (thus the “all crash type” approach). 

Using the Total EPDO in Step 1 as a baseline, apply the CMF to calculate the estimated EPDO for each 

potential treatment. 

29
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

 

 

 

Calculations Notes 

Remove or relocate fixed objects outside of clear zone: 

Estimated EPDO= Total EPDO×CMF=1210×0.62=750.2 – 

Cost of Existing EPDO= 1210×$7,400=$8,954,000 This is the estimated cost of the existing 

crashes on the roadway. 

Cost of Estimated EPDO= 750.2×$7,400=$5,551,480 

This is the estimated cost of the future 

crashes for a similar time period if this 

treatment is selected 

B/C Ratio = Cost of Existing EPDO-Cost of Estimated EPDO 

Cost of Treatment 

B/C Ratio = $8,954,000-$5,551,480 = 17.0 

$200,000 

This indicates that the benefits outweigh the 

cost of the countermeasure (for every $1 

the agency spends there is an equivalent 

benefit of $17). 

Install center line rumble strips: 

Estimated EPDO = 1210×0.91=1101.1 -­

Cost of Existing EPDO = 1210×$7400=$8,954,000 This is the estimated cost of the existing 

crashes on the roadway. 

Cost of Estimated EPDO = 1101.1×$7400=$8,148,140 

This is the estimated cost of the future 

crashes on the roadway if this treatment is 

selected 

B/C Ratio = $8,954,000-$8,148,140 = 26.9 

$30,000 

This indicates that the benefits outweigh the 

cost of the countermeasure (for every $1 

the agency spends there is an equivalent 

benefit of $27). 

Delineate obstacles: 

Estimated EPDO= Total EPDO×CMF=1210×1.0=1210 

Cost of Existing EPDO= 1210×$7400=$8,954,000 

This is the estimated cost of the existing 

crashes on the roadway. (same as the 

other treatments) 

Cost of Estimated EPDO= 1,210×$7400=$8,954,000 

This is the estimated cost of the future 

crashes on the roadway if this treatment is 

selected. 

B/C Ratio = $8,954,000-$8,954,000 = 0.0 

$10,000 

This indicates that there is no real financial 

benefit for choosing this countermeasure if 

the objective is to target all crash types. 

Note: B/C = benefit/cost. CMF = crash modification factor. EPDO = equivalent property damage only. 

Based on the resulting B/C ratios, the analyst concludes that the recommended treatments, in order 

of priority, should be: 

1.	 Install center line rumble strips (B/C ratio of 26.9), and then 

2.	 Remove or relocate fixed objects outside of the clear zone (associated with a B/C ratio 

of 17.0). 

30
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

STEP 1: Review the three CMFs previously identified and compare their relative values. 

The analyst eliminates the delineate obstacles option because it has a B/C ratio of 0.0, indicting no 

real financial benefit. 

Method 2: CMF Relative Comparison 

The CMF Relative Comparison safety assessment method can be used to compare potential CMFs to 

evaluate which have the greatest impact on reducing crashes. Whereas in the previous CMF Applied 

to Observed Crashes calculations, the analyst used observed crash data as a key input into the 

analysis, only the CMF information is required for the relative comparison approach. 

The information for the three previously reviewed CMFs is summarized in the following table. Recall 

that a CMF value less than 1.0 is associated with a larger reduction in future crashes when compared 

to a CMF with a value of one (assumed to have no real effect on reducing crashes). Based on this 

simple comparison, the analyst concludes that the recommended treatments, in order of priority, 

should be: 

1.	 Remove or relocate fixed objects outside of the clear zone (associated with an estimated 38 

percent reduction in crashes), and then 

2.	 Install center line rumble strips (associated with a 9 percent estimated crash reduction). 

For the purposes of reducing crashes, the analyst removes delineating obstacles from consideration. 

Proposed Treatment CMF (S.E.)1 Crash Type 

(Base Condition) Crash Severity 

Remove or relocate 

fixed objects outside of 

clear zone 

0.62 (0.103) 



All crash types and roadway types 

All 

Install center line 

rumble strips 0.91 (0.02) 



All crash types and rural roadway types 

All 

Delineate obstacles 1.0 (0.1) All crash types and roadway types All 

1 S.E. refers to the standard error. 

 CMF Clearinghouse Star Rating. 

Note: CMF = crash modification factor. HSM = Highway Safety Manual. 

31
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

FINDINGS 

Interpreting the Results: 

Based on the low-cost improvements considered using Method 1, the analyst concluded that the 

center line rumble strips and clear zone improvements provide evidence that the societal benefit 

of installing the treatments will outweigh the cost of installation. Other considerations may include 

whether there are any ordinances that limit where rumble strips can be installed. Because this 

evaluation targeted all crash severities and all crash types, the effect of countermeasures specifically 

expected to reduce roadside crashes may not be clear. 

The Method 2 approach similarly identified the same two CMFs, but did not directly consider crash 

data, weighting of severity levels, or benefit/cost analysis. This relative comparison approach, based 

simply on anticipated treatment effectiveness, prioritized the clear zone improvements above the 

center line rumble strips. This more basic approach provided useful information, but did not include 

site-specific information required for Method 1. 

Possible Errors to Avoid: 

It is recommended that analysts utilize crash cost estimations and weighting based on their State or 

local crash cost databases. Appropriate CMFs used for this type of analysis will have crash type and 

base condition characteristics that match those of the project highway. 

Alternative Analysis Approaches: 

The two methods may be effective approaches to initially narrow down the large list of potential 

safety treatments. If additional data that includes individual road geometry and crash type could 

be acquired for this location, a more comprehensive evaluation based on predicted or expected 

crashes could be performed during the preliminary and final project development phases. 

2.4	 Predicting Crashes to Evaluate Level of Service of Safety for a 

Rural Two-lane Highway 

PROBLEM OVERVIEW 

Safety Assessment Method: SPF with CMF Adjustment 

Project Phase: Planning & Scoping Project Type: 3R 

Related Task: Establish Project Scope 

Calculation Method: 

 Hand Calculated  Tool Based 

Comments: 

This example problem calculates the predicted number of crashes for alternative curve geometry 

options along a rural two-lane highway and then applies a level of service of safety ranking. 

LEVEL OF ANALYSIS

 Basic Intermediate Advanced 

Note: See table 3 for a full definition of Project Type designations. 

32
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

PROBLEM DESCRIPTION 

As part of planning and scoping activities, a DOT is developing plans to resurface a seven-mile 

section of a rural two-lane highway that contains 13 horizontal curves. The analyst wants to ensure 

that any low-cost curve countermeasures deployed as part of this project will have a positive 

impact on reducing crashes. The DOT does not have information about the roadside hazard rating 

or the driveway access density for each curve, and so the analyst visually inspected several of the 

curve locations and developed an average value to use for these road characteristics during this 

preliminary analysis phase. The associated Project Type is 3R and the Related Task is to Establish 

Project Scope. How can the analyst estimate which curves are functioning as anticipated and 

which ones could benefit from low-cost treatments? 

Summary of Available Data: 

The relevant curve characteristics include the following for all 13 curves: 

• AADT = 10,250 vpd (Curves #1 through #11), AADT = 9,700 (Curves #12 and #13). 

• Lane width = 11 ft. 

• Paved shoulder width = 6 ft. (right and left shoulders). 

• No spiral transitions. 

• Superelevation at curves compatible with design values. 

• Level grade. 

• No center line rumble strips, passing lanes, or two-way left-turn lanes. 

• Driveway density = 5 driveways/mi (estimated average). 

• Roadside hazard rating = 3 (estimated average).
 

The individual curve characteristics are further described as follows:
 

Curve 

Number 

Delineation 

Devices 

Present 

Length (mi) Radius (ft) Superelevation 

rate (percent) 

3-Year Total 

Crash Count 

1 None 0.25 1229 4.7 7 

2 None 0.26 1269 5.5 3 

3 Chevrons 0.12 384 7.8 1 

4 Delineators 0.08 588 3.1 2 

5 None 0.09 629 3.1 1 

6 None 0.19 750 6.2 1 

7 None 0.08 1124 6.2 2 

8 Delineators 0.06 309 7.0 5 

9 None 0.05 818 6.2 0 

10 Delineators 0.15 794 9.4 1 

11 Chevrons 0.17 678 3.9 2 

12 Chevrons 0.22 706 3.9 0 

13 None 0.15 800 7.8 1 

Note that total crash count information is currently the only available crash data. 

33
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

ANALYSIS 

Selecting Appropriate Safety Assessment Method(s): 

The analyst must first review the potential safety assessment methods shown in Table 5 for 3R project 

types and the task of establishing the project scope. This task and project type is associated with 

the four Basic and the two Intermediate safety assessment methods. For this analysis, the DOT only 

has limited funds available for select curve improvements. The analysis needs to include enough 

detail to evaluate the individual curves so that the limited funds are targeted effectively. Note that an 

intersection located between curves #11 and #12 results in different (lower) traffic volumes for the last 

two curves. 

Establish Project 

Scope 3R 

Site Evaluation or Audit 

Historical Crash Data Evaluation 

CMF Applied to Observed Crashes 

CMF Relative Comparison 

AADT Only SPF 

SPF with CMF Adjustment 

Of the four Basic methods, only the CMF-related safety assessment methods can directly consider 

horizontal curve radius, but the two basic CMF-related methods do not consider the varying traffic 

volume. For the two Intermediate safety assessment methods, the SPF with CMF Adjustment safety 

assessment method can also evaluate the horizontal curve geometry. The AADT-Only SPF safety 

assessment method does not consider varying geometric characteristics and can be eliminated as a 

candidate for this analysis. 

The SPF with CMF Adjustment will allow an evaluation of the different curve radii as well as varying 

traffic volume for the last two curves. This method also results in predicted crash information as noted 

in Table 1. To most effectively use this approach, an agency should calibrate the SPF for their local 

jurisdiction. A calibration factor of 1.0 can be used if this information is not available, but the results will 

not be refined to local conditions. 

Once predicted crashes are calculated, a variety of analysis approaches can be used to then 

determine which curves merit additional treatment. For this assessment, the analyst will calculate the 

predicted crash frequencies and then compare these frequencies to the observed crash frequencies 

by applying the measure of Level of Service of Safety (LOSS). 

34
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

STEP 1: Input the data describing each curve into the spreadsheet tool. 

Detailed Analysis: 

Predict Segment 

Average Crash 

Frequency for Base 

Conditions 

Nspf ru 

Predict Segment 

Specific Average 

Crash Frequency for 

Lane Width Options 

Npredicted ru 

Linkage to the HSM: 

The HSM can be used to estimate the number of predicted crashes for a 

rural, two-lane highway by applying the procedures introduced in HSM 

Chapter 10 (pp. 10-1 to 10-74). By determining predicted crashes, the 

analyst can estimate how many crashes may be expected for a specific 

road type with varying road conditions (in this case the curve radii and 

traffic volumes). The HSM provides manual calculations, but a spreadsheet 

tool is available and can be used to simplify this analysis. 

Once the number of predicted crashes has been calculated, the predicted 

crashes can be compared to the historical (observed) average crash 

frequency to identify which curves have diminished levels of service of 

safety (LOSS) that merit additional consideration. This LOSS procedure is 

reviewed on p. 4-12 of the HSM. An example problem is included on pp. 

4-44 to 4-48 of the HSM. 

The following steps provide the calculations for predicting rural two-lane highway crashes based on 

using the HSM “Smart Spreadsheets” available for download at: http://www.highwaysafetymanual. 

org/Pages/tools_sub.aspx#4. For this example problem, the analyst can use the “HSM prediction rural 

two-lane roads” spreadsheet tool. 

Note that no lighting or automated speed enforcement is present. The following graphic shows a 

representation of Worksheet 1A. This roadway segment worksheet includes input information similar to 

that shown in the HSM worksheet (see HSM p. 10-68). The input data shown in the graphic represents 

data cells for the first curve on the highway section (radius of 1229 ft). 

35
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Worksheet 1A -- General Information and Input Data for

 Rural Two-Lane Two-Way Roadway Segments 

General Information Location Information 

Analyst SD Roadway Section 3 

Agency or Company TTI Roadway Section Segment 1 

Date Performed 09/07/15 Jurisdiction 

Analysis Year 2015 

Input Data 

Base 

Conditions 

Site Conditions 

Length of segment, L (mi) -­ 0.25 

AADT (veh/day) AADTMAX = 17,800 

(veh/day) -­ 10,250 

Lane width (ft) 12 11 

Shoulder width (ft) 6 Right Shld: 6 Left Shld: 6 

Shoulder type Paved Right Shld: Paved Left Shld: Paved 

Length of horizontal curve (mi) 0 0.25 

Radius of curvature (ft) 0 1229 

Spiral transition curve (present/not present) Not Present Not Present 

Superelevation variance (ft/ft) < 0.01 0 

Grade (%) 0 0 

Driveway density (driveways/mile) 5 5 

Centerline rumble strips (present/not present) Not Present Not Present 

Passing lanes [present (1 lane) / 

present (2 lane) / not present] Not Present Not Present 

Two-way left-turn lane (present/not present) Not Present Not Present 

Roadside hazard rating (1-7 scale) 3 3 

Segment lighting (present/not present) Not Present Not Present 

Auto speed enforcement (present/not present) Not Present Not Present 

Calibration Factor, Cr 1 1.00 

Note: AADT = annual average daily traffic,TTI = Texas Transportation Institute. 

STEP 2: Tabulate the predicted crash frequency for each curve and convert the frequencies to 

predicted 3-year crash counts. 

The following graphic shows a representation of Worksheet 1C. This roadway segment worksheet 

summarizes the predicted crash frequency for each year. The values shown represent segment #1. 

This worksheet includes input information similar to that shown in the HSM worksheet (see HSM p. 10-69). 

36
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

STEP 3: Use the crash counts to compute the level of service of safety procedure described in HSM 

Section 4.4.2.7 (HSM pp. 4-44 to 4-48). 

 

 

Worksheet 1C -- Roadway Segment Crashes for Rural Two-Lane Two-Way Roadway Segments 

(1) (2) (3) (4) (5) (6) (7) (8) 

Crash 

Severity 

Level 

Nspf rs Overdispersion 

Parameter, k 

Crash Severity 

Distribution 

Nspf rs by 

Severity 

Distribution 

Combined 

CMFs 

Calibration 

Factor, Cr 

Predicted 

average crash 

frequency, 

Npredicted rs 

(crashes/year) 

from 

Equation 

10-6

 from 

Equation 

10-7 

from 

Table 10-3 

(proportion) 

(2) TOTAL x (4) 

(13) from 

Worksheet 

1B 

(5) x (6) x (7) 

Total 0.685 0.94 1.000 0.685 1.20 1.00 0.823 

Fatal and 

Injury (FI) -­ -­ 0.321 0.220 1.20 1.00 0.264 

Property 

Damage 

Only (PDO) -­ -­ 0.679 0.465 1.20 1.00 0.559 

Note: CMF = crash modification factor. 

The total predicted numbers of crashes per year, the equivalent total predicted crashes for 3 years, 

and the observed 3-year crash frequency are shown in the following table. The 3-year predicted crash 

count for Curve #1 is calculated as 2.47 (= 0.823 crashes/year x 3 years). Upon inspection of the crash 

values, the design team notes that Curves #1, 2, 4, 7, and 8 all have more observed crashes than were 

predicted for a road of this type with similar volume and curve radii characteristics, though when 

compared to rounded predicted crashes, only Curves #1 and #8 have observed crashes that are 

noticeably higher. Additional evaluations can help determine if these five curve locations should be 

the final improvement curves. This evaluation is included in Step 3. 

The level of service of safety is based on assessing the amount of deviation between the predicted 

and the observed crashes. To calculate the LOSS, the curves are separated into four categories (I to 

IV). Sites with a moderate to high potential for crash reduction (or LOSS category rankings of III or IV) 

are then identified for future study. 

Curve 

Number 

Predicted Crashes 

Calculated Total 

for 1 year 

Total for 3 years 

Calculated Rounded 

Observed Crash Total for 3 

years 

1 0.823 2.47 3 7 

2 0.847 2.54 3 3 

3 0.718 2.15 3 1 

4 0.473 1.42 2 2 

5 0.485 1.46 2 1 

6 0.730 2.19 3 1 

7 0.355 1.07 2 2 

8 0.641 1.92 2 5 

9 0.319 0.96 1 0 

10 0.606 1.82 2 1 

11 0.694 2.08 3 2 

12 0.782 2.35 3 0 

13 0.572 1.72 2 1 

37
38

SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

The LOSS category criteria are summarized as follows:
 

Category I: σ < N < (N – (1.5 x σ)) [Low potential for crash reduction].
 observed

Category II: (N – (1.5 x σ))≤ N ≤ N [Low to moderate potential for crash reduction]. observed

Category III: N ≤ N ≤ (N + (1.5 x σ)) [Moderate to high potential for crash reduction]. observed

Category IV: N ≥ (N + (1.5 x σ)) [High potential for crash reduction]. observed

For these classifications, the standard deviation, σ, is calculated as follows (Equation 4-16 of the HSM): 

σ=√ k×N2 

Where: 

•	 k represents the overdispersion parameter of the SPF (this specific SPF has a value of 0.236 ÷ L as 

noted in the HSM equation 10-7, p. 10-16). 

•	 N represents the predicted crash frequency. 

•	 L represents the length of roadway segment in miles. 

The following summary table identifies the individual LOSS rankings for the 13 curve locations. The 

shaded curves are the same five curves identified during the Step 2 evaluation. All five curves have 

LOSS category classifications of III and IV, suggesting that they are the curves that merit attention. 

As an example, the standard deviation (σ) for Curve Number 1 is calculated as follows: 

 

σ=√ k×N2 =√ 0.236 × 2.472 = 2.40 0.25 

 

 

 

 

The four potential LOSS category classifications can then be evaluated. As an example, the LOSS 

assessment for Curve Number 1 is calculated as follows: 

Category I Assessment – Curve #1: 

Is σ < N < (N – (1.5 x σ))? observed

Is 2.40 < 7 < [(2.47 – (1.5 x 2.4)) = -3.6]?
 

No, 7 is not less than -3.6, so proceed to evaluate Category II.
 

Category II Assessment – Curve #1:
 

Is (N – (1.5 x σ)≤ N ≤ N?
 observed

Is [(2.47 – (1.5 x 2.4)) = -3.6] ≤ 7 ≤ 2.47?
 

No, 7 is not less than 2.47, so proceed to evaluate Category III.
 

Category III Assessment – Curve #1:
 

Is N ≤ N ≤ (N + (1.5 x σ))?
 observed

Is 2.47 ≤ 7 ≤ [(2.47 + (1.5 x 2.40)) = 6.07]?
 

No, 7 is not less than 6.07, so proceed to evaluate Category IV.
 

Category IV Assessment – Curve #1:
 

Is N ≥ (N + (1.5 x σ))?
 observed

Is 7 ≥ [(2.47 + (1.5 x 2.40)) = 6.07]?
 

Yes, so Curve #1 has a Category IV LOSS.
 
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

Curve Number 

3-Year Crash Count 

Predicted Observed 

σ

Level of Service 

of Safety 

1 2.47 7 2.40 IV 

2 2.54 3 2.42 III 

3 2.15 1 3.02 II 

4 1.42 2 2.44 III 

5 1.46 1 2.36 II 

6 2.19 1 2.44 II 

7 1.07 2 1.84 III 

8 1.92 5 3.81 III 

9 0.96 0 2.09 II 

10 1.82 1 2.28 II 

11 2.08 2 2.45 II 

12 2.48 0 2.57 II 

13 1.82 1 2.28 II 

FINDINGS 

Interpreting the Results: 

The results of the level of service of safety analysis show that there is a high potential for crash 

reduction (i.e. LOSS Category of IV) on Curve #1 and a moderate-to-high potential for crash reduction 

(i.e. LOSS Category III) for Curves #2, #4, #7, and #8. These curves are candidates for additional crash 

reduction treatments. The simple comparison between rounded predicted values and observed 

crashes, however, further indicates that Curves #1 and #8 merit priority attention. 

Possible Errors to Avoid: 

When using the HSM predictive method, the input value for superelevation differential can be 

confusing. The value used in the procedure is for a difference (in ft. per ft) of how the existing 

superelevation slope deviates from the design slope. As an example, if the proposed superelevation 

design value for a curve is 4.80 percent but instead the actual slope is 2.20 percent, the deviation 

would be calculated as: 0.048 –0.022 = 0.026. For this evaluation, there were not any superelevations 

variances of significance, and so this value would be 0.00. 

Alternative Analysis Approaches: 

The analyst could have used the CMF Applied to Observed Crashes safety assessment method 

to compare Curves #1 through #11 since they all had the same traffic volume. A separate, similar 

analysis for Curves #12 and #13 could also be performed. The result of this procedure, however, 

provides estimated crashes (instead of predicted crashes that are calculated when using an SPF). 

LOSS is one of several measures that are presented in Chapter 4 of the HSM for use in planning and 

programming highway improvements that can reduce crash frequency or severity. As shown 

in Table 6, an alternative performance measure that also uses SPF information as it applies to 

contrasting predicted crashes is the Excess Predicted Average Crash Frequency Using SPFs. 

39
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Expected crashes can also be considered for comparing site-specific conditions prior to making 

substantial changes. As the project shifts from the planning stage to the design stage, these more 

detail-oriented procedures may be appropriate as they can prove to be particularly useful as input 

into benefit/cost analyses during the final stages of the project. 

2.5	 Predicting Incremental Benefits Associated with Increased Lane 

Width for a Rural Two-lane Highway 

 

PROBLEM OVERVIEW 

Safety Assessment Method: SPF with CMF Adjustment 

Project Phase: Planning & Scoping Project Type: 3R 

Related Task: Establish Project Scope 

Calculation Method: 

 Hand Calculated  Tool Based 

Comments: 

This example problem will evaluate the effect lane widening can have on reducing fatal and injury 

crashes for a two-lane undivided rural highway at a site where crash data is not available. 

LEVEL OF ANALYSIS

 Basic Intermediate Advanced 

Note: See table 3 for a full definition of Project Type designations. 

PROBLEM DESCRIPTION 

A two-lane county highway serving a newly developed residential community is in need of some 

upgrades. Currently, the two-lane highway has 10-ft. lanes and the road does not have any graded 

or paved shoulders (a typical rural low-volume design at the time it was constructed). The county 

has funding limitations but intends to widen the travel lanes as a short-term solution to help improve 

corridor operations. The analyst would also like to evaluate safety estimates and compare the added 

incremental benefits of widening the lanes from 10-ft. to 11-ft. wide and from 10-ft. to 12-ft. wide. A 

specific goal would be to evaluate how these improvements would reduce the number of fatal or 

injury crashes annually along this corridor. A future project could also consider adding graded 

shoulders, but due to limited funding, this option must be deferred to a later date. The affected 

corridor is approximately 20 miles long with a current AADT of 10,000 vpd. The Project Type is 3R and 

the Related Task is to Establish Project Scope. How does the analyst estimate the reduction in the 

number of fatal and injury crashes due to these potential incremental improvements? 

Summary of Available Data: 

The County does not have any available crash data for this corridor. To simplify this initial assessment, 

the analyst has made the following assumptions: 

• No horizontal curves. 

• No vertical grades steeper than 2 percent. 

• Driveway density of five driveways per mile. 

40
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

• No center line rumble strips, passing lanes, or two-way left turn lanes. 

• Roadside hazard rating of three. 

• No street lights. 

• No automated speed enforcement. 

ANALYSIS 

Selecting Appropriate Safety Assessment Method(s): 

Based on Table 5, the Establish Project Scope task and the 3R project type are associated with the 

four Basic and the two Intermediate safety assessment methods. There is limited data available for 

the corridor and the goal is to estimate the reduction in fatal and injury crashes due to incremental 

widening. Because this condition eliminates methods that only evaluate existing conditions (see Table 

1 in Chapter 1), the Site Evaluation or Audit and the Historical Crash Data Evaluation safety assessment 

methods are not applicable. In addition, the CMF Applied to Observed Crashes is also not feasible 

due to the lack of available crash data. The AADT-Only SPF method does not explicitly allow evaluation 

of unique road features and so this method also does not apply. 

Site Evaluation or Audit 

Historical Crash Data Evaluation 

CMF Applied to Observed Crashes 

CMF Relative Comparison 

AADT Only SPF 

SPF with CMF Adjustment 

Establish Project 

Scope 3R 

The remaining potential safety assessment methods include CMF Relative Comparison and SPF with 

CMF Adjustment. The relative comparison method can be used if CMFs are available for the proposed 

lane widening. 

The analyst conducts a quick search on the FHWA CMF Clearinghouse (http://www.cmfclearinghouse. 

org/) to determine if CMFs are available for the widening scenario (i.e., widen lane from 10 ft. to 11 

ft., widen lane from 10 ft. to 12 ft.). Note that the base condition would be 10 ft. lane widths for the two 

options. In addition, the lane width CMFs that the analyst does locate do not provide information 

about the number of fatal or injury crashes. Based on this assessment, the analyst concludes that 

the SPF with CMF Adjustment method appears to be the preferred safety assessment method for this 

evaluation. 

41
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

STEP 1: Input the data describing each lane width option into the spreadsheet tool. 

Detailed Analysis: 

Predict Segment 

Average Crash 

Frequency for Base 

Conditions 

Nspf ru 

Predict Segment 

Specific Average 

Crash Frequency for 

Lane Width Options 

Npredicted ru 

Linkage to the HSM: 

The HSM can be used to estimate the number of predicted crashes for a 

rural, two-lane highway by applying the procedures introduced in HSM 

Chapter 10 (pp. 10-1 to 10-74). By determining the predicted total, fatal and 

injury, and property damage only crashes for the 10 ft, 11 ft, and 12 ft. lane 

scenarios, the analyst can assess the incremental benefits associated with 

the lane widening options. The HSM provides manual calculations, but a 

spreadsheet tool is available and can be used to simplify this analysis. 

The analyst reviews the criteria for the SPF with CMF Adjustment safety 

assessment method and notes that the SPF from the HSM has not been 

calibrated for the region. Therefore, the analyst can simply use a calibration 

factor value of 1.0. Because the evaluation compares lane-width options 

for the same facility, the absence of a calibration factor for the region will not 

adversely affect the process of comparing design options for the same road. The predicted number 

of crashes may not be exact, but the procedure can accurately calculate the relationship between 

the predicted numbers of crashes for varying lane widths. Due to the absence of a calibrated SPF, 

the analyst will report the results as a percentage reduction in crashes instead of a specific number 

of reduced crashes. This reporting format is appropriate because reporting the number of crashes 

would imply more precision than the calculations (that do not incorporate a calibration factor) 

represent. 

The following steps provide the calculations based on using the HSM “Smart Spreadsheets” available 

for download at: http://www.highwaysafetymanual.org/Pages/tools_sub.aspx#4. For this example 

problem, the analyst can use the “HSM rural two lane roads” spreadsheet tool. 

As previously indicated, the analyst used a calibration factor of 1.0. She also included the base 

conditions for each feature, a volume of 10,000 vpd, and length of 20 miles. The following graphic 

shows Worksheet 1A. This roadway segment worksheet includes input information similar to that 

shown in the HSM worksheet (see HSM p. 10-68). The input data shown in the graphic represents data 

cells for the 10-ft.-lane-width option (existing conditions). 

42
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Worksheet 1A - General Information and Input Data for

 Rural Two-Lane Two-Way Roadway Segments 

General Information Location Information 

Analyst Analyst Name Roadway Example two-lane rural highway 

Agency or Company -­ Roadway Section Example segment 

Date Performed -­ Jurisdiction Anywhere, USA 

Analysis Year 2015 

Input Data 

Base 

Conditions 

Site Conditions 

Length of segment, L (mi) -­ 20 

AADT (veh/day) AADTMAX = 17,800 

(veh/day) -­ 10,000 

Lane width (ft) 12 10 

Shoulder width (ft) 6 Right Shld: 0 Left Shld: 0 

Shoulder type Paved Right Shld: Paved Left Shld: Paved 

Length of horizontal curve (mi) 0 0.000 

Radius of curvature (ft) 0 0 

Spiral transition curve (present/not present) Not Present Not Present 

Superelevation variance (ft/ft) < 0.01 0 

Grade (%) 0 0 

Driveway density (driveways/mile) 5 5 

Centerline rumble strips (present/not present) Not Present Not Present 

Passing lanes [present (1 lane) / 

present (2 lane) / not present] Not Present Not Present 

Two-way left-turn lane (present/not present) Not Present Not Present 

Roadside hazard rating (1-7 scale) 3 3 

Segment lighting (present/not present) Not Present Not Present 

Auto speed enforcement (present/not present) Not Present Not Present 

Calibration Factor, Cr 1 1.00 

This analysis can be repeated for the 11-ft. and the 12-ft. lanes by changing the lane width field value. 

The input data for the 11-ft. alternative is shown with the applicable field outlined in red. 

43
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Worksheet 1A - General Information and Input Data for

 Rural Two-Lane Two-Way Roadway Segments 

General Information Location Information 

Analyst Analyst Name Roadway Example two-lane rural highway 

Agency or Company -­ Roadway Section Example segment 

Date Performed -­ Jurisdiction Anywhere, USA 

Analysis Year 2015 

Input Data 

Base 

Conditions 

Site Conditions 

Length of segment, L (mi) -­ 20 

AADT (veh/day) AADTMAX = 17,800 

(veh/day) -­ 10,000 

Lane width (ft) 12 11 

Shoulder width (ft) 6 Right Shld: 0 Left Shld: 0 

Shoulder type Paved Right Shld: Paved Left Shld: Paved 

Length of horizontal curve (mi) 0 0.000 

Radius of curvature (ft) 0 0 

Spiral transition curve (present/not present) Not Present Not Present 

Superelevation variance (ft/ft) < 0.01 0 

Grade (%) 0 0 

Driveway density (driveways/mile) 5 5 

Centerline rumble strips (present/not present) Not Present Not Present 

Passing lanes [present (1 lane) / 

present (2 lane) / not present)] Not Present Not Present 

Two-way left-turn lane (present/not present) Not Present Not Present 

Roadside hazard rating (1-7 scale) 3 3 

Segment lighting (present/not present) Not Present Not Present 

Auto speed enforcement (present/not present) Not Present Not Present 

Calibration Factor, Cr 1 1.00 

STEP 2: Tabulate the predicted crash frequency for each lane width option. 

Following input in Step 1, the spreadsheet tool automatically calculates the predicted number of 

crashes. To review the example results for the segment calculations, see the CMF results for the 

Existing Condition (shown in Worksheet 1B with the lane width CMF outlined in red) and predicted 

number of crashes (Worksheet 1C). These roadway segment worksheets are similar to the HSM 

worksheets with the same numbering format (see HSM p. 10-69). 

44
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

 

Worksheet 1B - Crash Modification Factors for Rural Two-Lane Two-Way Roadway Segments 

(1) (2) (3) (4) (5) (6) (7) 

CMF for Lane 

Width 

CMF for Shoulder 

Width 

and Type 

CMF for 

Horizontal Curves 

CMF for Superelevation CMF for Grades CMF for Driveway 

Density 

CMF for 

Centerline 

Rumble Strips 

CMF 1r CMF 2r CMF 3r CMF 4r CMR 5r CMF 6r CMF 7r 

from 

Equation 10-11 from Equation 

10-12 

from Equation 

10-13 

from Equations 

10-14, 10-15, 

or 10-16 

from Table 10-11 from Equation 

10-17 

from Section 

10.7.1 

1.17 1.29 1.00 1.00 1.00 1.00 1.00 

(8) (9) (10) (11) (12) (13) 

CMF for Passing 

Lanes 

CMF for 

Two-Way 

Left-Turn Lane 

CMF for 

Roadside 

Design 

CMF for Lighting 

CMF for 

Automated 

Speed 

Enforcement 

Combined CMF 

CMF 8r CMF 9r CMF 10r CMF 11r CMF 12r CMF comb 

from Section 

10.7.1 

from Equation 

10-18 & 10-19 

from Equation 

10-20 

from Equation 

10-21 

from Section 

10.7.1 (1)x(2)x ... x(11)x(12) 

1.00 1.00 1.00 1.00 1.00 1.509 

Worksheet 1C - Roadway Segment Crashes for Rural Two-Lane Two-Way Roadway Segments 

(1) (2) (3) (4) (5) (6) (7) (8) 

Crash 

Severity 

Level 

Nspf rs Overdispersion 

Parameter, k 

Crash Severity 

Distribution 

Nspf rs by 

Severity 

Distribution 

Combined 

CMFs 

Calibration 

Factor, Cr 

Predicted average 

crash frequency, 

Npredicted rs 

(crashes/year) 

from 

Equation 

10-6

 from 

Equation 

10-7 

from 

Table 10-3 

(proportion) 

(2) TOTAL x (4) 

(13) from 

Worksheet 

1B 

(5) x (6) x (7) 

Total 53.435 0.01 1.000 53.435 1.51 1.00 80.613 

Fatal and 

Injury (FI) -­ -­ 0.321 17.153 1.51 1.00 25.877 

Property 

Damage 

Only 

(PDO) 

-­ -­ 0.679 36.283 1.51 1.00 54.736 

45
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

The summary results are shown in Worksheet 1E (see HSM p. 10-70). The segment analysis results for 

the Existing Condition are shown below. These values are used in Step 3. 

Worksheet 1E -- Summary Results for Rural Two-Lane Two-Way Roadway Segments 

(1) (2) (3) (4) (5) 

Crash Severity Level 

Crash Severity 

Distribution 

(proportion) 

Predicted average 

crash frequency 

(crashes/year) Roadway segment 

length (mi) 

Crash rate 

(crashes/mi/year) 

(4) from Worksheet 1C (8) from Worksheet 1C (3)/(4) 

Total 1.000 80.6 20 4.0 

Fatal and Injury (FI) 0.321 25.9 20 1.3 

Property Damage 

Only (PDO) 0.679 54.7 20 2.7 

The results for the 11 ft. lane width alternative (Worksheet 1E) are shown below. 

Worksheet 1E -- Summary Results for Rural Two-Lane Two-Way Roadway Segments 

(1) (2) (3) (4) (5) 

Crash Severity Level 

Crash Severity 

Distribution 

(proportion) 

Predicted average 

crash frequency 

(crashes/year) Roadway segment 

length (mi) 

Crash rate 

(crashes/mi/year) 

(4) from Worksheet 1C (8) from Worksheet 1C (3)/(4) 

Total 1.000 70.7 20 3.5 

Fatal and Injury (FI) 0.321 22.7 20 1.1 

Property Damage 

Only (PDO) 0.679 48.0 20 2.4 

The results for the 12 ft. lane width alternative (Worksheet 1E) are shown below. 

Worksheet 1E -- Summary Results for Rural Two-Lane Two-Way Roadway Segments 

(1) (2) (3) (4) (5) 

Crash Severity Level 

Crash Severity 

Distribution 

(proportion) 

Predicted average 

crash frequency 

(crashes/year) Roadway segment 

length (mi) 

Crash rate 

(crashes/mi/year) 

(4) from Worksheet 1C (8) from Worksheet 1C (3)/(4) 

Total 1.000 68.8 20 3.4 

Fatal and Injury (FI) 0.321 22.1 20 1.1 

Property Damage 

Only (PDO) 0.679 46.7 20 2.3 

46
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

STEP 3: Summarize the total number of crashes. 

The analyst next created the following table that summarizes the total number of predicted crashes 

per year for the three scenarios (based on the assumed calibration factor of 1.0). 

Alternative Total 

Predicted Crashes per Year (Npredicted ) 

Fatal and Injury Property Damage Only 

10-ft. Existing Lanes 80.6 25.9 54.7 

11-ft. Lane Alternative 70.7 22.7 48.0 

12-ft. Lane Alternative 68.8 22.1 46.7 

The spreadsheet values do not include any rounding errors as would be expected when performing 

calculations by hand. As a result, these numbers will be similar, but may not exactly match predicted 

crashes calculated manually. 

STEP 4: Summarize the total reduction in fatal and injury crashes. 

The analyst’s ultimate goal is to evaluate if the reduction in the number of fatal and injury crashes 

due to widening the lanes from 10 ft. to 11 ft., which would be substantially different from widening the 

lanes from 10 ft. to 12 ft. The following table summarizes this analysis. 

Roadway 

Improvement 

Scenario 

Number of Predicted Fatal and 

Injury Crashes per year 

Existing Width Final Width 

Predicted 

Reduction in the 

Number of Fatal 

and Injury Crashes 

Reduction in the 

Number of Fatal 

and Injury Crashes 

(percent) 

Widening lanes 

from 10 to 11 ft. 

26 23 3 12 

Widening lanes 

from 10 to 12 ft. 

26 22 4 15 

FINDINGS 

Interpreting the Results: 

Based solely on an evaluation of the predicted number of crashes, a road with the characteristics 

described for the existing configuration can be expected to experience approximately 81 crashes per 

year, of which about 26 would involve a fatality or injury. 

11-ft. Lanes: By adding 1 ft. of pavement to each lane (increasing to 11-ft. lanes), the analyst can 

expect to see an approximate 12 percent reduction in fatal and injury crashes per year. 

12-ft. Lanes: Adding 2 ft. of pavement to each lane to create 12-ft. lanes leads to a predicted 

reduction of approximately 15 percent for fatal and injury crashes. 

Based on these findings, the analyst concludes that widening lanes from 10-ft. to 12-ft. lanes will result 

in a 2 to 3 percent additional benefit when compared to widening from 10-ft. to 11-ft. lanes. 

These calculations do not consider any local aspects of the roadway that may impact the benefits or 

costs associated with the increased lane widths. They also do not consider potential improvements in 

operational level of service associated with wider lanes. 

47
48

SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Possible Errors to Avoid: 

The HSM spreadsheet simplifies the effort associated with using the predictive method. This planning 

analysis included several assumptions such as no horizontal curves and vertical grades that are two 

percent or less. The results, therefore, are suitable for comparative purposes but do not represent the 

final roadway geometric conditions. A more detailed analysis during the design stage would provide 

additional useful information about predicted crashes. 

Alternative Analysis Approaches: 

A common method for evaluating design alternatives is to use CMF comparisons that do not directly 

consider traffic volume. This approach is less reliable than the predictive methods, but can be used 

to evaluate alternatives when an SPF is not available for the condition. Although the analyst could 

not locate suitable CMFs for this application, many transportation agencies develop and maintain 

agency-specific CMFs that could be useful for similar analyses. 
NOTE: Elements of ongoing life cycle activities are incorporated into 

Chapters 2 and 4 as appropriate.

SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Chapter 3. Alternatives Analysis Applications
 

Project

Development 

Figure 5.The Project Development Cycle and 

Corresponding Alternative Analysis Chapter 

NOTE: Elements of ongoing life cycle activities are 

incorporated into Chapter 2 and 4 as appropriate 

Planning and

Scoping 

Environmental Analysis 

Alternatives 

Identification/Analysis 

Preliminary Final Design Design 

Chapter 3 

Chapter 2 Chapter 4 

The Alternatives Analysis phase is typically 

conducted after a project need has been 

determined but before a solution has been 

identified. This phase may coincide with 

the Planning and Scoping phase and can 

extend into the early stages of Preliminary 

Design. The purpose of safety assessments 

in the Alternatives Analysis phase is to 

estimate the impact of each alternative on 

safety performance. 

This chapter provides information to help 

select safety assessment methods suitable 

for addressing questions related to safety 

performance that arise during alternatives 

analysis tasks based upon the related task 

and project type. This section describes 

alternatives analysis-related tasks in two 

general categories: 

• Alternative selection, and 

• Interstate access justification and documentation. 

The alternative selection activities generally occur following the preliminary planning activities and 

may be part of a corridor or project alternatives assessment. The objective of the safety assessment 

for this task is to estimate the safety performance of alternatives. This information can then be used to 

help refine the proposed alternatives as the project progresses into the preliminary and final design 

phases. 

New or modified points of access to the Interstate system require Interstate access justification 

and documentation activities. The Federal Highway Administration’s (FHWA) Policy on Access to the 

Interstate System provides the requirements, which include an analysis of the access point’s impact 

on the safety performance of the Interstate facility and the local street system under both the current 

and the planned future traffic projections. 

Table 7 identifies the five (out of a possible seven) safety assessment methods generally suitable for 

alternatives analysis tasks and the objective of the associated safety performance analysis. The check 

marks in Table 7 suggest suitable safety assessment methods for each related task and objective 

and, in some cases, are distinguished by project type. In this context, the term “suitable” means that 

the method generally has the capability to address the objective of the safety performance analysis 

with the data typically available for the related task and project type. 

49
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

The following questions illustrate the types of questions the analyst may develop at the beginning of 

the safety assessment. These questions are based on the example problems included in this chapter. 

1.	 How can the analyst calculate the estimated reduction in injury crashes for a facility’s 

design alternatives? 

2.	 How can the analyst compare the estimated crash frequency for two alternatives and the 

existing configuration? 

3.	 How can the analyst estimate the difference in the average annual crash reduction for a 

5-ft. shoulder compared to that of an 8-ft. shoulder over a 20-year period? 

4.	 How can the analyst predict the estimated number of crashes for an interchange freeway 

loop ramp for a 3-year study period of 2013 to 2015 while directly considering the annual 

average daily traffic (AADT) in the analysis? 

Table 8 shows that the level of predictive reliability generally increases as the methodology used 

becomes more complex. At the same time, the required resources for the analysis also will increase. 

In some cases, it may not be feasible to fully implement the preferred safety assessment method 

due to limitations in site information, crash data, traffic volume, or similar information. For example, 

the Basic safety assessment method for a CMF Applied to Observed Crashes cannot be executed if 

historic crash data is not available. 

The approach for selecting a safety assessment method for alternatives evaluation and identification 

looks like this: 

Determine Alternatives 

Evaluation Related Task 

Identify Project 

Type 

Select Safety Assessment Method 

(Confirm Data Requirements) 

50
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS - Table 8.Alternatives Evaluation and Identification Safety Assessment Objective Related Task Objective Project Type Basic Intermediate Advanced Site Evaluation or Audit Historical Crash Data Evaluation CMF Applied to Observed Crashes CMF Relative Comparison AADT Only SPF SPF with CMF Adjustment SPF with CMF Weighted with Observed Crashes Observed Crashes Predicted Crashes Expected Crashes Safety Assessments: Increasing Level of Predictive Reliability Alternative Selection Estimate the safety performance of alternatives 2R     6 3R, 4R  5    9  7 NL     Interchange Access Justifi cation Estimate the safety performance impact of new or modifi ed points of access 3R, 4R      8 NL   Data Requirements: Increasing Level of Resources Needed (Staff,Analysis,Time, etc.) Road Type 1       Road Characteristics 2      Traffi c Volume (vpd) 3    Observed Crash Data 4     1 Road Type refers to rural two-lane highway, rural multi-lane highway, urban freeway, etc. 2 Road Characteristics includes physical features such as lane widths, access density, etc. 3 Traffic Volume is the ADT or AADT in vehicles per day. 4 Observed Crash Data represents the historic crash data at the study site for a period of more than one year (preferably 3 to 5 years). 5 See Example Problem 3.1. 6 See Example Problem 3.2. 7 See Example Problem 3.3. 8 See Example Problem 3.4. 9 See Example Problem 5.2. Note: Project type definitions are as follows: R2 = resurfacing existing facilities. R3 = major rehabilitation of an existing facility. R4 = major retrofit con­ struction efforts. NL = highway construction at a new location. See Table 3 for a full definition of each Project Type.  = suitable safety assessment method.ADT = average daily traffic.AADT = annual average daily traffic. CMF = crash modification factor. SPF = safety performance function. vpd = vehicles per day.

51
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

This chapter provides examples that demonstrate the selection process for the alternatives analysis 

safety assessment methods. These examples are simplified hypothetical problems intended to 

illustrate the thought process for selecting a method and demonstrate how to apply the method to 

answer the associated safety-based question. 

3.1 Comparing Design Alternatives for Seven-Lane Urban Arterial 

PROBLEM OVERVIEW 

Safety Assessment Method: CMF Applied to Observed Crashes 

Project Phase: Alternative Analysis Project Type: 3R 

Related Task: Alternative Selection 

Calculation Method: 

 Hand Calculated  Tool Based 

Comments: 

This example problem demonstrates how to evaluate the comparative crash reductions due to 

alternative treatment options for urban arterial mid-block locations. 

LEVEL OF ANALYSIS

 Basic Intermediate Advanced 

Note: See table 3 for a full definition of Project Type designations. 

PROBLEM DESCRIPTION 

A city is redesigning a seven-lane arterial urban roadway. As part of the redesign, an analyst for the 

city is considering two design modifications that are also expected to help reduce the number of 

injury crashes. Based on their estimated effectiveness, the City may select one or both alternatives. 

The two alternatives under consideration are: 

1. A raised median, and 

2. Install automated speed enforcement. 

The Project Type is 3R and the Related Task is Alternative Selection for this activity. How can the 

analyst calculate the estimated reduction in injury crashes for design alternatives on this facility? 

Summary of Available Data: 

Currently, there are three travel lanes in each direction with a two-way left-turn lane. Over the past 5 

years there have been a total of 362 injury crashes (72.4 average injury crashes per year) along the 

1-mile segment. Fortunately, there have not been any fatal crashes at this location during the study 

period. The city does not have a current AADT value for this corridor. 

52
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

ANALYSIS 

Selecting Appropriate Safety Assessment Method(s): 

The analyst for the city can evaluate the candidate safety assessment methods shown in Table 8 

and contrast the table information with the 3R project type and the alternative selection task. There 

are five potential safety assessment methods for the analyst to consider: two basic, two intermediate, 

and one advanced. Because this assessment will likely require the use of the injury crash information 

as input into the evaluation, and the analyst’s goal is to calculate the estimated reduction in injury 

crashes following construction of a raised median or implementation of corridor-specific traffic 

calming measures, a CMF-based method that accounts for the a change in road characteristics 

is appropriate. In addition, the city does not have a current AADT value for the corridor, so volumebased methods are not feasible at this time. These constraints result in the elimination of the SPFbased methods. 

Alternative 

Selection 3R 

CMF Applied to Observed Crashes 

CMF Relative Comparison 

AADT Only SPF 

SPF with CMF Adjustment 

SPF with CMF Weighted with 

Observed Crashes 

The two remaining safety assessment methods that use CMFs include CMF Applied to Observed 

Crashes and CMF Relative Comparison. Both methods can be considered. 

The CMF Applied to Observed Crashes method can directly evaluate the proposed road design 

modification and the observed injury crashes. Using the observed crash injury frequency to represent 

the existing conditions and then applying an appropriate CMF, the analyst can estimate the average 

injury crash frequency for the proposed condition. For these reasons, the analyst selected the CMF 

Applied to Observed Crashes safety assessment method. 

The CMF Relative Comparison approach does not explicitly consider the historical (observed) 

crashes along the corridor, but can be used for simple comparison purposes between the two 

proposed treatments. (Refer to Problem 2.2 for an example where the analyst elected to apply both 

methods.) 

Linkage to the Highway Safety Manual (HSM): 

Chapter 6 of the HSM (Section 6.2.2, pp. 6-3 to 6-9) summarizes contributing factors to consider when 

selecting appropriate countermeasures. Section 6.3 of the HSM (p. 6-9 to 6-10) provides guidance for 

selecting potential countermeasures. Finally, Part D (Volume 3) of the HSM and the FHWA sponsored 

CMF Clearinghouse (www.cmfclearinghouse.org) include a wide variety of potential improvements to 

consider. 

53
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Determine CMFs 

for Candidate 

Countermeasures 

(Treatments) 

CMF Treatment 

Calculated Estimated 

Crash Reductions 

Nestimated, Nreduction 

Detailed Analysis: 

The estimated average crash frequency is calculated by multiplying the 

appropriate CMF for the alternative treatment by the number of observed 

crashes that apply to the selected CMF. 

STEP 1: Identify CMFs for each treatment 

The CMF Clearinghouse (www.cmfclearinghouse.org) and the HSM are 

two resources for this activity. It is important to confirm that the CMFs have 

similar crash types and base conditions. The following table shows potential 

CMFs for the two candidate treatments. The CMF Clearinghouse star rating 

is based on a scale (1 to 5), where 5 stars represent the highest or most 

reliable rating. The standard error is one of the rating criteria for the star 

rating (small standard errors are preferred). The analyst should select CMFs 

(from the Clearinghouse) with the highest star rating, lowest standard error, 

and most applicable conditions for the scenario. 

Proposed 

Treatment 

CMF 

(S.E.) 

Setting (Road Type 

and Traffic Volume) 

Crash 

Severity Source 

Treatment –Raised Median 

Provide 

a Raised 

Median 

0.78 

(0.02) 



Urban Arterial Multilane 

(Traffic Volumeunspecified) 

Serious or 

Minor Injury 

HSM Table 13-11, p. 13-14 and 

http://www.cmfclearinghouse. 

org/detail.cfm?facid=22 

Treatment – Traffic Calming Measures 

Implement 

Automated 

Speed 

Enforcement 

Cameras 

0.83 

(0.01) 



All Roads (Traffic 

Volume-unspecified) 

Fatal, Serious 

Injury, Minor 

Injury 

HSM Table 17-5, p. 17-6 and 

http://www.cmfclearinghouse. 

org/detail.cfm?facid=4583 

Note: AADT = average annual daily traffic. CMF = crash modification factor. HSM = Highway Safety 

Manual. S.E. = standard error.  CMF Clearinghouse Star Rating. 

STEP 2: Calculate estimated crash reductions for each alternative. 

The following tables contain the calculations for determining the injury crash reductions based on 

the two individual alternatives. This step can be used to eliminate any treatment combinations that 

increase the total number of crashes or that have very little effect on the number of injury crashes. 

54
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Alternative 1 – Install Raised Median: 

Calculations Notes 

Total Injury Crashes: 

Nestimated(in jury)=CMFRaisedmedian×Nobserved(injury) 

Multiply CMF times observed number of injury crashes at 

location where 

CMFRaised Median=0.78 

Nestimated(injury)=0.78×72.4=56.5 crashes 362 crashes in 5 years = 72.4 crashes/yr. Approximately 

57 injury crashes if this treatment were implemented 

Nreduction (per year)=72.4-56.5=15.9 crashes Approximately 16 fewer injury crashes each year 

CMF = crash modification factor. 

Alternative 2 – Implement Automated Speed Enforcement Cameras: 

Calculations Notes 

Total Injury Crashes: 

Nestimated(injury)=CMFAutomated Speed Enforcement×Nobserved(injury) 

Multiply CMF times observed number of 

injury crashes at location where 

CMFAutomated Speed Enforcement=0.83 

Nestimated(injury)=0.83×72.4=60.0 crashes 

362 crashes in 5 years = 72.4 crashes/yr 

Approximately 60 injury crashes if this 

treatment were implemented 

Nreduction (per year)=72.4-60.0=12.4 crashes Approximately 12 to 13 fewer injury crashes 

each year 

CMF = crash modification factor. 

FINDINGS 

Interpreting the Results: 

For this seven-lane arterial, the installation of a raised median is estimated to reduce the total number 

of injury crashes by approximately 16 per year, equivalent to an approximate 22 percent reduction 

in injury crashes. The application of automated speed enforcement is similarly estimated to reduce 

injury crashes but only by 12 to 13 per year (a 17 percent reduction). These findings demonstrate 

that the raised median will be more effective at reducing injury crashes than the implementation 

of automated speed enforcement cameras. The safety treatment effectiveness would then be 

considered in terms of cost to identify the preferred alternative. Because the two treatments target 

different crash types, the selection of both options may also be preferable for this location. 

55
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Possible Errors to Avoid: 

3.2 Predicting Crash Frequency for Alternative Intersection Turn Lane 

Options at Four-lane Rural Undivided Highways 

 

The selections of CMFs that are not consistent with the conditions of the particular site of interest 

or the specific target crash type(s) will result in inaccurate estimated average crash frequencies. 

It is also important to understand that the application of multiple CMFs will not always have a 

cumulative effect on the estimated average crash frequency. The CMF Clearinghouse FAQ titled 

“How can I apply multiple CMFs” provides additional information and clarification (see http://www. 

cmfclearinghouse.org/faqs.cfm#q4). 

Alternative Analysis Approaches: 

This analysis focused on injury crashes only and did not consider total crashes. Extending the analysis 

to other crash types or to crash severity levels could provide additional insights. As previously noted, 

the use of a relative comparison of CMFs is an additional analysis that can be conducted with the 

available data. In addition, alternative evaluations could include an economic appraisal such as the 

benefit/cost ratio or a prioritization evaluation that uses an incremental benefit/cost ratio. 

For this study, the city did not have current AADT information. It is feasible, however, that a reasonably 

accurate traffic volume estimate could be developed based on a combination of traffic volumes 

from prior years, development trends in the region, and use of comparison sites for volume estimation 

purposes. As the project shifts from the alternative analysis to the design phase, the analyst will 

assemble more detailed site-specific data including additional road characteristics. At that time, 

additional analysis procedures that incorporate CMFs, SPFs, or both can strengthen the overall 

analysis. 

PROBLEM OVERVIEW 

Safety Assessment Method: SPF with CMF Adjustment 

Project Phase: Alternative Analysis Project Type: 2R 

Related Task: Alternative Selection 

Calculation Method: 

 Hand Calculated  Tool Based 

Hand calculated example for Alternative #1 

included in Guide appendix. 

Comments: 

This example problem demonstrates how to conduct a safety assessment for an alternative intersection 

design for a rural, multilane highway and compare the estimated safety improvement to the existing 

configuration. 

LEVEL OF ANALYSIS

 Basic Intermediate Advanced 

Note: See table 3 for a full definition of Project Type designations. 

56
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

PROBLEM DESCRIPTION 

A transportation analyst is finalizing the alternative 

evaluations for a four-lane undivided rural 

highway. The corridor has a two-way, stopcontrolled intersection with a design speed of 

50 mph and relatively little skew. Since about as 

many vehicles turn into the intersection as turn 

out of the intersection, the AADT is approximately 

30,000 vpd on the major road (upstream and 

downstream of the intersection) and 5000 vpd on 

the minor road. The analyst is considering a lowcost redesign option that could be implemented 

within the currently proposed pavement limits. The 

change would apply to the roadway cross section 

that is designed to have 12-ft. inside lane widths, 

13-ft. outside lanes, and 8-ft. paved shoulder 

widths. The analyst wants to identify a prospective 

lane configuration that will contribute to reducing 

the number of crashes at this intersection. He has 

identified two candidate left-turn lane alternatives 

to evaluate. The Project Type is 2R and the 

Related Task is Alternative Selection for this effort. 

How can the analyst compare the estimated 

crash frequency with Alternative #1, Alternative 

#2, and the existing configuration? 

Summary of Available Data: 

The segment length that is influenced by 

the intersection can be assumed to extend 

approximately 1,000 ft. upstream and 1,000 ft. 

downstream of the intersection (measured from 

the intersection center line). This results in a total 

segment length of approximately 0.38 miles (or two approach segments of 0.19 miles each). Site 

features include: 

• No street lights. 

• No automated speed enforcement. 

• Side slopes 1:7 or flatter. 

The schematic depicts the existing and two potential alternative lane configurations. As noted 

previously, the corridor is a four-lane rural, multilane, undivided highway with a two-way stop control 

(on the minor legs). At this time, historical crash data is not available for the existing intersection. 

57
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

ANALYSIS 

Selecting Appropriate Safety Assessment Method(s): 

The analyst can first review the potential safety assessment methods shown in Table 8. The Alternative 

Selection task and the 2R project type are associated with two basic and two intermediate methods. 

For this assessment, the analyst intends to compare the number of estimated crashes for two 

alternative intersection lane configurations and contrast these values to the number of estimated 

crashes for the current intersection approach. Since a CMF-based method enables the specific 

consideration of a change in road characteristics, the safety assessment methods that use CMFs 

are applicable for this analysis. Traffic volume information is also available for the major and minor 

road, and so a safety assessment method that is volume-based can be used. Based on these 

considerations, the AADT-Only SPF method can be eliminated from further consideration since it does 

not directly capture changes in road characteristics. As previously noted, the historical (observed) 

crash data is not currently available for this location, so the CMF Applied to Observed Crashes 

method should also be eliminated. 

Alternative 

Selection 2R 

CMF Applied to Observed Crashes 

CMF Relative Comparison 

AADT Only SPF 

SPF with CMF Adjustment 

The two remaining safety assessment methods include CMF Relative Comparison and SPF with 

CMF Adjustment. The analyst may elect to use one or both of these methods. The ability to directly 

incorporate the traffic volume into the analysis, however, strengthens the evaluation, so the analyst 

elects to use the SPF-based method. Because the data requirements are not substantially different 

for the CMF Relative Comparison option, this method could be an effective way to narrow down 

candidate lane and shoulder width options and to calculate a simple estimate of the effect for 

changing each of the individual road characteristics before conducting the more complicated 

intermediate safety assessments. 

Linkage to the HSM: 

The HSM can be used to estimate the number of predicted crashes for a rural, multilane highway 

by applying the procedures introduced in HSM Chapter 11 (pp. 11-1 to 11-71). By determining the 

predicted total and the fatal and injury crashes for the varying shoulder and lane width scenarios, 

the analyst can calculate the predicted crash reduction percentage for the alternative intersection 

approach configurations. The HSM provides manual calculations, but a spreadsheet tool is available 

and can be used to simplify this analysis. 

58
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Detailed Analysis: 

To use the HSM predictive method, 

the analyst evaluates the intersectionrelated crashes (e.g., angle or turning 

crashes) separately from segment-related 

crashes (e.g., run-off-road) and then 

adds the crashes for each to compute 

the combined number of crashes for 

the roadway segment that overlaps an 

intersection. 

This will provide the number of predicted 

crashes (estimated for a type of facility). 

The predictive equations (SPFs) can be 

locally derived, or the analyst can use 

the HSM equations. If the analyst uses the 

SPFs from the HSM, the SPFs will need to 

be calibrated for local conditions, where 

possible. To perform a relative comparison 

between two options for the same 

location, the procedure outlined in the 

HSM can be used and a calibration value 

of 1.0 assumed. 

The analyst would like to evaluate the 

estimated reduction in crashes associated with two prospective alternative lane and shoulder width 

configurations for an intersection approach. The following analysis provides detailed information for 

the tool-based evaluation for Alternative #1. The analyst conducted a similar analysis for Alternative #2 

(results only shown for this scenario). Alternative #1 hand calculations are included in the Appendix. 

This example uses the rural undivided multilane predictive procedures located in Chapter 11 of the 

HSM. 

The following steps provide the calculations based on using the self-calculating HSM “Smart 

Spreadsheets” available for download at: http://www.highwaysafetymanual.org/Pages/tools_sub. 

aspx#4. For this example problem, the analyst should use the “HSM prediction rural multilane” 

spreadsheet tool. 

STEP 1: Input the known data for each of the alternatives stated. Both intersection and segment data 

should be input as follows: 

Predict Intersection 

Average Crash 

Frequency for Base 

Conditions * 

N spf int 

Predict Segment 

Average Crash 

Frequency for Base 

Conditions * 

N spf ru 

Predict Intersection 

Alternative-Specific 

Average Crash 

Frequency 

N predicted int 

Predict Segment 

Alternative-Specific 

Average Crash 

Frequency 

N predicted ru 

Calculate Total Predicted Crashes 

N predicted int + N predicted ru = N predicted 

*Intersection base condition: 

No turn lanes (HSM p.11-20) 

Segment base conditions: 

6’ shoulders & lanes ≥ 12’ (HSM p. 11-14) 

Intersection Input Data: 

For intersection analysis, input data in the top section (Worksheet 2A) of the tab named “Rural 

Multilane Intersection.” The existing design input data is shown as follows: 

59
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

 

 

 

 

Worksheet 2A -- General Information and Input Data for Rural Multilane Highway Intersections 

General Information Location Information 

Analyst PDP Analyst Roadway Example 4-lane undivided Rural Highway 

Agency or Company -­ Section Example minor road with stop control 

Date Performed -­ Jurisdiction Anywhere, USA 

Analysis Year 2015 

Input Data Base Conditions Site Conditions 

Intersection type (3ST, 4ST, 4SG) -­ 4ST 

AADTmajor 

(veh/day) 

AADTMAX = 78,300 

(veh/day) -­ 30,000 

AADTminor 

(veh/day) 

AADTMAX = 7,400 

(veh/day) -­ 5,000 

Intersection skew angle (degrees) 0 0 

Number of non-STOP-controlled 

approaches with left-turn lanes (0, 1, 2) 0 0 

Number of non-STOP-controlled approaches 

with right-turn lanes (0, 1, 2, 3, or 4) 0 0 

Intersection lighting (present/not present) Not Present Not Present 

Calibration Factor, Ci 1.00 1.00 

The color coding indicates data that must be typed (yellow) and data that can be selected from pulldown lists (blue). For Alternative #1, the number of non-stop-controlled turn-lanes should be changed 

to two as shown in the red circle in the following graphic: 

Worksheet 2A -- General Information and Input Data for Rural Multilane Highway Intersections 

General Information Location Information 

Analyst PDP Analyst Roadway Example 4-lane undivided Rural Highway 

Agency or Company -­ Section Example minor road with stop control 

Date Performed -­ Jurisdiction Anywhere, USA 

Analysis Year 2015 

Input Data Base Conditions Site Conditions 

Intersection type (3ST, 4ST, 4SG) -­ 4ST 

AADTmajor 

(veh/day) 

AADTMAX = 78,300 

(veh/day) -­ 30,000 

AADTminor 

(veh/day) 

AADTMAX = 7,400 

(veh/day) -­ 5,000 

Intersection skew angle (degrees) 0 0 

Number of non-STOP-controlled 

approaches with left-turn lanes (0, 1, 2) 0 2 

Number of non-STOP-controlled approaches 

with right-turn lanes (0, 1, 2, 3, or 4) 0 0 

Intersection lighting (present/not present) Not Present Not Present 

Calibration Factor, Ci 1.00 1.00 

60
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Segment Input Data: 

For segment analysis, input data in the top section (Worksheet 1A) of the “Rural Undivided Multilane 

Seg” tab. The existing design input data is shown as follows: 

Worksheet 1A -- General Information and Input Data for Rural Multilane Roadway Segments 

General Information Location Information 

Analyst PDP Analyst Roadway Example 4-lane undivided Rural Highway 

Agency or Company -­ Section 1.0-1.38 

Date Performed -­ Jurisdiction Anywhere, USA 

Analysis Year 2015 

Input Data Base Conditions Site Conditions 

Roadway type (divided / undivided) Undivided Undivided 

Length of segment, L (mi) – 0.38 

AADT 

(veh/day) 

AADTMAX = 33,200 

(veh/day) – 30,000 

Lane width (ft) 12 12 

Shoulder width (ft) - right 

shoulder width for divided 6 8 

Shoulder type - right shoulder type for divided Paved Paved 

Median width (ft) - for divided only 30 Not Applicable 

Side Slopes - for undivided only 1:7 or flatter 1:7 or Flatter 

Lighting (present/not present) Not Present Not Present 

Auto speed enforcement (present/not present) Not Present Not Present 

Calibration Factor, Cr 1.00 1.00 

Note that the segment length is 0.19 miles. Since the AADT does not change from one side of the 

intersection to the other, this calculation can either be performed as two individual segments 0.19 

miles long (and then the results would be added), or a segment of 0.38 miles can be used that 

extends the entire length of the study section (as demonstrated in this input example). 

61
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

 

The Alternative #1 input data for the segment is demonstrated in the following graphic: 

Worksheet 1A -- General Information and Input Data for Rural Multilane Roadway Segments 

General Information Location Information 

Analyst PDP Analyst Roadway Example 4-lane undivided Rural Highway 

Agency or Company -­ Section 1.0-1.38 

Date Performed -­ Jurisdiction Anywhere, USA 

Analysis Year 2015 

Input Data Base Conditions Site Conditions 

Roadway type (divided / undivided) Undivided Undivided 

Length of segment, L (mi) – 0.38 

AADT 

(veh/day) 

AADTMAX = 33,200 

(veh/day) 30,000 

Lane width (ft) 12 11 

Shoulder width (ft) - right 

shoulder width for divided 6 6 

Shoulder type - right shoulder type for divided Paved Paved 

Median width (ft) - for divided only 30 Not Applicablepplicable 

Side Slopes - for undivided only 1:7 or flatter 1:7 or Flatter 

Lighting (present/not present) Not Present Not Present 

Auto speed enforcement (present/not present) Not Present Not Present 

Calibration Factor, Cr 1.00 1.00 

STEP 2: Following input in Step 1, the spreadsheet tools automatically calculated the predicted 

number of crashes. To review example results for the intersection calculations, see the CMF results for 

Alternative #1 (shown in Worksheet 2B) and predicted number of crashes (Worksheet 2C): 

Worksheet 2B -- Crash Modification Factors for Rural Multilane Highway Intersections 

(1) (2) (3) (4) (5) (6) 

Crash 

Severity 

Level 

CMF for Intersection 

Skew Angle (CMF 1i ) 

CMF for Left-Turn 

Lanes (CMF 2i ) 

CMF for Left-Turn 

Lanes (CMF 3i ) 

CMF for Left-Turn 

Lanes (CMF 4i ) 

Combined CMF 

(CMF COMB ) 

from Equations 11-18 or 

11-20 and 11-19 or 11-21 from Table 11-22 from Table 11-23 from Equation 11-22 (2)*(3)*(4)*(5) 

Total 1.00 0.52 1.00 1.00 0.52 

Fatal and 

Injury (FI) 1.00 0.42 1.00 1.00 0.42 

Note:The 4-leg Signalized Intersection (4SG) models do not have base conditions and so can only be used for 

estimation purposes. As a result, there are not CMFs provided for the 4SG condition. 

Worksheet 2C -- Intersection Crashes for Rural Multilane Highway Intersections 

(1) (2) (3) (4) (5) (6) (7) 

Crash Severity Level 

SPF Coefficients N spf int Overdispersion 

Parameter, k 

Combined 

CMFs 

Calibration 

Factor, Ci 

Predicted average 

crash frequency, 

N i

from Table 11-7 or 11-8 from 

Equation 

11-11 or 11-12 

from Table 

11-7 or 11-8 

from (6) of 

Worksheet 

2B 

(3)*(5)*(6) 

predicted nt 

a b c or d 

(4SG) 

Total -10.008 0.848 0.448 12.803 0.494 0.52 1.00 6.658 

Fatal and Injury (FI) -11.554 0.888 0.525 7.940 0.742 0.42 1.00 3.335 

Fatal and Injurya (FIa) -10.734 0.828 0.412 3.709 0.655 0.42 1.00 1.558 

Property Damage 

Only (PDO) -­ -­ -­ -­ -­ -­ -­ (7)TOTAL - (7)FI 

3.323 

NOTE: a Using the KABCO scale, these include only KAB crashes. Crashes with severity level C (possible injury) are not 

included. 

62
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Summary results are shown in Worksheet 2E. The intersection analysis results for Alternative #1 are 

shown below. The shaded region identifies the summary data used in Step 3. 

Worksheet 2E -- Summary Results for Rural Multilane Highway Intersections 

(1) (2) 

Crash Severity Level Predicted average crash frequency (crashes / year) 

(7) from Worksheet 2C 

Total 6.66 

Fatal and Injury (FI) 3.33 

Fatal and Injurya (FIa) 1.56 

Property Damage Only (PDO) 3.32 

NOTE: a Using the KABCO scale, these include only KAB crashes. Crashes with severity level C (possible injury) are 

not included. 

Similarly, the Alternative #1 spreadsheet results for segment calculations (Worksheets 1B, 1C, and 1E) 

are shown as: 

Worksheet 1B (b) -- Crash Modification Factors for Rural Multilane 

Undivided Roadway Segments 

(1) (2) (3) (4) (5) (6) 

CMF for 

Lane Width 

CMF for 

Shoulder Width 

CMF for 

Side Slopes 

CMF for 

Lighting 

CMF for Automated 

Speed Enforcement Combined CMF 

CMF 1ru CMF 2ru CMF 3ru CMF 4ru CMF 5ru CMF comb 

from Equation 

11-13 

from Equation 

11-14 

from Table 

11-14 

from Equation 

11-15 

from Section 

11.7.1 (1)*(2)*(3)*(4)*(5) 

1.01 1.00 1.00 1.00 1.00 1.01 

Worksheet 1C (b) -- Roadway Segment Crashes for Rural Multilane Undivided Roadway Segments 

(1) (2) (3) (4) (5) (6) (7) 

Crash Severity Level 

SPF Coefficients N spf rs(u) Overdispersion 

Parameter, k 

Combined 

CMFs 

Calibration 

Factor, Cr 

Predicted average 

crash frequency, N 

from Table 11-3 from 

Equation 

11-7 

from Equation 

11-8 

(6) from 

Worksheet 

1B (b) 

(3)*(5)*(6) 

a b c 

Total -9.653 1.176 1.675 4.494 0.493 1.01 1.00 4.543 

Fatal and Injury (FI) -9.410 1.094 1.796 2.461 0.437 1.01 1.00 2.487 

Fatal and Injurya (FIa) -8.577 0.938 2.003 1.133 0.355 1.01 1.00 1.146 

Property Damage 

Only (PDO) -­ -­ -­ -­ -­ -­ -­ (7)TOTAL - (7)FI 

2.055 

NOTE: a Using the KABCO scale, these include only KAB crashes. Crashes with severity level C (possible injury) are 

not included. 

63
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

STEP 3: Summarize the total number of crashes. 

Worksheet 1E -- Summary Results for Rural Multilane Roadway Segments 

(1) (2) (3) (4) 

Crash Severity Level 

Predicted average crash 

frequency (crashes / year) Roadway segment 

length (mi) 

Crash rate (crashes/mi/year) 

(7) from Worksheet 1C (a) or (b) (2)/(3) 

Total 4.54 0.4 12.0 

Fatal and Injury (FI) 2.49 0.4 6.5 

Fatal and Injurya (FIa) 1.15 0.4 3.0 

Property Damage 

Only (PDO) 2.06 0.4 5.4 

NOTE: a Using the KABCO scale, these include only KAB crashes. Crashes with severity level C (possible injury) are 

not included. 

This activity can be performed manually or summarized on the “Rural Multilane Site Total” sheet (note 

that the analyst will need to customize this table to link to individual worksheets as needed). The 

Alternative #2 results are also depicted in the table. The results are summarized as follows: 

Total Crashes per Year FI Crashes per Year 

Alternative 

Intersection 

Crashes 

Segment 

Crashes 

Predicted 

Crashes 

Intersection 

Crashes 

Segment 

Crashes 

Predicted 

Crashes 

Existing 12.80 4.34 

17.14 

(say 18) 

7.94 2.37 

10.31 

(say 11) 

Alternative 

#1 

6.66 4.54 

11.20 

(say 12) 

3.33 2.49 

5.82 

(say 6) 

Alternative 

#2 

6.66 4.68 

11.33 

(say 12) 

3.33 2.56 

5.89 

(say 6) 

Comparison of Existing and Alternative #1 Predicted Crash Reductions 

Difference 

12.80 – 6.66 

= 6.14 

4.34 – 4.54 

= -0.20 

17.14 – 11.20 = 

5.94 

(say 6) 

7.94 – 3.33 = 

4.61 

2.37 – 2.49 

= -0.12 

10.31 – 

5.82 = 4.49 

(say 5) 

Percent 

Reduction 

48.0% -4.6% 35% 58.1% -5.06% 43% 

Comparison of Existing and Alternative #2 Predicted Crash Reductions 

Difference 

12.80 – 6.66 

= 6.14 

4.34 – 4.68 

= -0.34 

17.14 – 11.33 = 

5.81 

(say 6) 

7.94 – 3.33 = 

4.61 

2.37 – 2.56 

= -0.19 

10.31 – 

5.89 = 4.42 

(say 5) 

Percent 

Reduction 

48.0% -7.8% 35% 58.1% -8.02% 43% 

Note: FI = fatality and injury 

The spreadsheet values do not include any rounding errors as would be expected when performing 

hand calculations. As a result, these numbers should be similar but may not exactly match predicted 

crashes calculated manually. 

64
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

FINDINGS 

Interpreting the Results: 

Based solely on an evaluation of the predicted number of crashes, a road with the characteristics 

described for the existing design configuration can be expected to experience approximately 18 

crashes per year, of which about 11 would involve an injury. For locations where a left-turn lane is 

added with standard-use lanes of 11-ft. wide and shoulder widths of 6-ft. wide (Alternative #1), the 

total number of predicted crashes per year reduces to approximately 12 with 6 involving an injury. In 

contrast, an approach with a left-turn lane, standard-use lane widths of 12-ft., but shoulder widths of 

4-ft. (Alternative #2) similarly results in 12 predicted crashes with 6 involving an injury. 

For the conditions outlined in this problem, the addition of a left-turn lane provides added value as it 

can be expected to result in a reduction of approximately 35 percent in the total number of crashes 

and approximately a 43 percent reduction in FI crashes. The two alternatives considered provide 

very similar results, and so the analyst can conclude that either Alternative #1 or Alternative #2, if 

constructed, will result in the predicted crash reductions. 

These calculations are based only on predicted crash performance, but do not consider potential 

operational issues. For example, vehicles positioned in the narrow 10-ft. left-turn lane as shown for 

both alternatives may encroach on the adjacent travel lane. 

While this example focused on left-turn lane options, it is advisable for an analyst to conduct similar 

safety assessments for right-turn treatments. 

Possible Errors to Avoid: 

The HSM procedures require the analyst to understand how the SPFs and CMFs equate to the base 

conditions associated with the procedure. Incorrect use of these values can introduce erroneous 

results. This example compared the crash predictions, and so the use of a calibration factor with 

a value of 1.0 can be used for comparison purposes. If the analyst would like to record findings as 

crash frequency instead of percent reduction in the number of crashes, calibrated SPFs with known 

calibration factors should be used for the analysis. A more detailed analysis during the design phase 

would provide additional useful information about predicted crashes. 

Alternative Analysis Approaches: 

A common method for evaluating design alternatives is to use CMF comparisons that do not directly 

consider traffic volume. This approach is less reliable than the predictive methods, but can be used 

to evaluate alternatives when an SPF is not available for the condition or when a simple comparison 

between two alternatives is all that is needed. 

If the project progresses to the design stage, and should the historical (observed) crash data 

become available at that time, the analyst could use the advanced safety assessment method to 

further evaluate existing conditions and future designs with minor changes. Future configurations 

that include major changes are not suitable for the SPF with CMF Weighted with Observed Crashes 

method as the observed crashes would no longer be representative of the new configuration. 

65
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Alternative A – Widen Existing Roadway to 34-ft. 

Alternative B – Widen Existing Roadway to 40-ft. 

3.3 Calculating Expected Crashes for Rural Two-Lane Highways 

PROBLEM OVERVIEW 

Safety Assessment Method: SPF with CMF Weighted with Observed Crashes 

Project Phase: Alternative Analysis Project Type: 3R 

Calculation Method: 

Related Task: Alternative Selection 

  Hand Calculated  Tool Based 

Comments: 

This example problem demonstrates how to use the IHSDM to calculate the expected number of 

crashes for alternative cross-section geometric options for a rural two-lane highway. 

LEVEL OF ANALYSIS

 Basic Intermediate Advanced 

Note: See table 3 for a full definition of Project Type designations. 

Source: This example problem is based on a case study for Arizona SR 264 Burnside Junction to 

Summit posted at http://www.fhwa.dot.gov/design/pbpd/case_studies.cfm. 

PROBLEM DESCRIPTION 

As part of a 34.5-mile long, two-lane rural highway reconstruction project to enhance a developing 

freight route, a transportation agency would like to evaluate the estimated safety implications of 

two alternative designs. The current two-lane rural highway has intermittent right- and left-turning 

lanes and infrequent passing lanes. The existing horizontal and vertical geometry information has 

been acquired using a survey and was confirmed against the original as-built plans. As part of the 

project, the transportation agency has included the geometric information into a civil design software 

package. The two alternatives currently under consideration are defined as Alternative A and 

Alternative B. 

The purpose of Alternative A is to widen the existing roadway to 34-ft. to provide 5-ft. shoulders by 

widening the existing 1-ft. shoulders to 5-ft. shoulders. The existing travel lane width would remain 12 

ft. The improvements would include adding center line and shoulder rumble strips, flattening side 

slopes, installing guardrail, extending drainage structures, and providing delineators and recessed 

pavement markers. 

The purpose of Alternative B is to widen the existing roadway to 40-ft. to provide full-width shoulders. 

The proposed improvements would widen the existing 1-ft. shoulders to 8-ft. shoulders. The existing 

travel lane width would remain 12-ft. The improvements would include adding center line and 

shoulder rumble strips, flattening side slopes, installing guardrail, extending drainage structures, and 

providing delineators and recessed pavement markers. 

66
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

AADT (vpd) 

Mile Post (MP) 2010 AADT 2036 Projected AADT 

MP 441.02-MP 446.18 5,010 9,900 

MP 446.18-MP 446.91 6,429 12,150 

MP 446.91-MP 448.37 5,199 7,350 

MP 448.37-MP 475.50 4,102 5,400 

Note: AADT = average annual daily traffic, vpd = vehicles per day 

The Project Type is 3R and the Related Task is Alternative Selection. How can the analyst estimate 

the difference in the average annual crash reduction for a 5-ft. shoulder compared to that of an 8-ft. 

shoulder over a 20-year period? 

Summary of Available Data: 

The 2010 AADT, 4-year crash counts, and the manner of collision are provided in the following tables. 

The first table also shows projected AADT values for the design year 2036. 

Crash Data (2007-2010) 

Crash Severity Number of Crashes 

Fatal 6 

Incapacitating Injury 3 

Non-Incapacitating Injury 1 

Possible Injury 24 

No Injury (PDO) 22 

Total Crashes 56 

Note: PDO = property damage only 

Manner of Collision (2007-2010) 

Manner of Collision Number of Crashes 

Head On 2 

Left Turn 3 

Rear End 13 

Angle (Other than 

Left Turn) 

5 

Sideswipe (Opposite 

Direction) 

2 

Sideswipe (Same 

Direction) 

4 

Single Vehicle 27 

Total Crashes 56 

The following table shows the road characteristics for the various configurations. For informational 

purposes, the table shows the base conditions associated with a rural two-lane highway in the HSM. 

Notable variations from the base conditions include the shoulder width, roadside hazard rating, and 

center line rumble strips. 

67
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Road Characteristics (Existing and Proposed) for the Rural Two-Lane,Two Way Road 

Roadway Element Existing Alternative A Alternative B HSM Base 

Condition 

Lane width (ft) 12 12 12 12 

Shoulder width (ft) 1 5 8 6 

Shoulder type Paved Paved Paved Paved 

Roadside hazard 

rating 

Varies (6 or 7 

most frequent) 

Varies (1 or 2 most 

frequent) 

Varies (1 or 2 most 

frequent) 3 

Driveway Density Per survey Per survey Per survey ≤ 5 per mile 

Horizontal curves: 

length, radius, and 

presence or absence 

of spiral transitions 

Per best fit 

alignment 

Per best fit 

alignment (match 

existing) 

Per best fit 

alignment (match 

existing) 

None 

Horizontal curves: 

Superelevation 

Per as-builts & 

survey 

Per as-builts & 

survey (match 

existing) 

Per as-builts & 

survey (match 

existing) 

None 

Grades Per as-builts & 

survey 

Per as-builts & 

survey (match 

existing) 

Per as-builts & 

survey (match 

existing) 

≤ 3% 

Center line rumble 

strips None Present Present None 

Passing lanes Per survey Per survey (match 

existing) 

Per survey (match 

existing) None 

Two-way left-turn 

lanes Per survey Per survey (match existing) 

Per survey (match 

existing) None 

Lighting Present at 

Intersection 

Present at 

Intersection 

(match existing) 

Present at 

Intersection 

(match existing) 

None 

Automated speed 

enforcement None None None None 

HSM = Highway Safety Manual. 

ANALYSIS 

Selecting Appropriate Safety Assessment Method(s): 

The analyst can first inspect Table 8 to identify potential safety assessment methods for a 3R project 

type and the alternative selection task (see Table 3 for a full definition of the 3R project type). Five 

potential methods (two basic, two intermediate, and one advanced) may be considered for this 

analysis. Because the alternatives require comparisons of varying geometric characteristics, an 

assessment method that includes CMFs is appropriate (thus eliminating the AADT-Only SPF). The 

study corridor is lengthy (almost 35 miles long) and the traffic volume and road characteristics vary 

throughout the corridor. Because the analyst would like to verify that the increase in traffic volume is 

considered in the analysis, the two basic methods are eliminated from consideration. 

68
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Alternative 

Selection 3R 

CMF Applied to Observed Crashes 

CMF Relative Comparison 

AADT Only SPF 

SPF with CMF Adjustment 

SPF with CMF Weighted with 

Observed Crashes 

The remaining two potential safety assessment methods use procedures 

specific to the HSM. The SPF with CMF Adjustment can be used to 

evaluate the predicted number of crashes along a corridor with 

similar characteristics to the study site. The SPF with CMF Weighted 

with Observed Crashes includes the crash history so that the safety 

assessment includes consideration of the recent crash history at the 

location. Both of these safety assessment methods can be used. 

To help select the method, the analyst reviews the length of the corridor 

and the varying road characteristic information and notes that the 

alignment information is already included in a civil design software 

package. The proposed analysis is impractical to conduct using hand 

calculations. Both of the methods can be performed, however, using 

the HSM “Smart Spreadsheets” and the FHWA IHSDM analysis software. 

The IHSDM has an added benefit in that it can directly import road 

characteristic data from a civil design software program with little to no 

additional manual entry. The selection of IHSDM, therefore, will enable the 

use of an advanced safety assessment method while also streamlining 

the analysis process. The web address to acquire this free software tool is 

provided in the “Detailed Analysis” section of this problem. 

Calculate Average 

Crash Frequency 

N spf 

Calculated Predicted 

Average Crash 

Frequency 

Npredicted 

Calculated Expected 

Average Crash 

Frequency 

Nexpected 

Linkage to the HSM: 

The HSM can be used to estimate the number of predicted crashes for a rural, two-lane highway by 

applying the procedures introduced in HSM Chapter 10 (pp. 10-1 to 10-74). By determining predicted 

crashes, the analyst can estimate how many crashes may be expected for a specific road type 

with varying road conditions (in this case the curve radii and traffic volumes). Once the predicted 

number of crashes is known, the expected number of crashes for a specific site can be calculated by 

applying the Empirical Bayes Method summarized in HSM, Part C (Volume 2), pp. A-15 to A-23. 

69
70

SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Detailed Analysis: 

The IHSDM analysis software can be used to calculate the expected average crash counts for the 

existing conditions and the given alternatives for the 20-year study period. If a State has calibrated the 

SPF contained in the HSM or has developed its own SPF that has a similar equation format (functional 

form) as the HSM equation, these values can be directly inserted into the IHSDM. 

To calculate the expected average crash frequency for the existing conditions and the proposed 

alternatives, download the IHSDM analysis software. This product can be downloaded free of 

charge at http://www.ihsdm.org. Two training courses are available through the Federal Highway 

Administration’s National Highway Institute for those that would like to learn more about how to use 

the software (http://www.nhi.fhwa.dot.gov/training/course_search.aspx?tab=0&key=IHSDM&res=1). 

STEP 1: Input (or import) the vertical and horizontal roadway alignments into IHSDM. 

The following figures are screen shots of what the alignments should look like following input into 

IHSDM. 
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

STEP 2: Input all remaining crash prediction data for the existing conditions. 

Like the alignment data, all other data is entered using the roadway stationing. The following figure 

shows what the input data for AADT looks like. 

The following figure shows all data that must be entered before running the Crash Prediction Module. 

Items with a green check show that required data was entered. An orange question mark represents 

data for which default values are being used because the analyst did not provide site specific 

information. A red “X” will appear by the data set if the required data is missing. This process of 

creating or importing a new highway must be followed for each alternative. 

71
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Interpreting the Results: 

STEP 3: Run the crash prediction analysis within IHSDM. 

IHSDM has the ability to create a summary, report, raw results, graphs, and spreadsheets of the 

analysis. The following table shows a summary of the results for expected crashes based on output 

from the IHSDM software analysis. 

2016-2036 Expected Total Number of Crashes 

Crash Severity Levels 

Existing Conditions 

1-ft. Shoulder 

Alternative A 

5-ft. Shoulders 

Alternative B 

8-ft. Shoulders 

Total 636.38 531.58 504.16 

Fatal and Injury 283.40 230.45 216.80 

Property Damage Only 352.98 301.13 287.36 

Reduction in Total Crashes NA 104.80 

(say 105) 

132.22 

(say 133) 

2016-2036 Expected Average Annual Crash Frequency 

Crash Severity Levels 

Existing Conditions 

1-ft. Shoulder 

Alternative A 

5-ft. Shoulders 

Alternative B 

8-ft. Shoulders 

Total 31.82 26.58 25.21 

Fatal and Injury 14.17 11.52 10.84 

Property Damage Only 17.65 15.05 14.37 

Reduction in Total Crashes NA 5.24 

(say 6) 

6.61 

(say 7) 

FINDINGS 

The proposed improvements for alternatives A and B respectively reduce the expected number of 

crashes compared to the existing conditions by 105 and 133 crashes, respectively, projected over a 

20-year period. Alternatives A and B have an annual average crash reduction of 6 and 7 crashes per 

year, respectively. Based on the predictive method analysis, Alternative B, with the 8-ft. shoulder, will 

have an average annual reduction of between one and two more crashes per year than Alternative 

A, with a 5-ft. shoulder. This information on safety performance could then be considered alongside 

other project criteria. 

Possible Errors to Avoid: 

The analyst should confirm that data is entered (or imported) and used correctly in the IHDSM analysis 

software. IHSDM only takes into account those SPFs and CMFs associated with the HSM or that have 

been input (as previously indicated). The HSM procedures and the IHSDM are updated periodically 

with the most recent safety knowledge. Before starting an analysis, it is a good idea to confirm 

that you are always using the most recent version of the IHSDM software. As noted in the “Detailed 

Analysis” section of this problem, the IHSDM software and information about using the software can 

be found at the web address http://www.ihsdm.org. 

72
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Alternative Analysis Approaches: 

The expectation to include traffic volume as well as changes in road characteristics narrowed down 

the suitable analysis tools to those that used SPFs with CMFs. This analysis could also have used the 

HSM spreadsheets presented in Problem 3.2. In addition, the findings from this analysis can be used 

as input into an economic appraisal such as a benefit/cost ratio or prioritization method such as 

incremental benefit/cost ratio. 

3.4 Calculating Expected Crashes for Urban Freeway Ramps 

 

PROBLEM OVERVIEW 

Safety Assessment Method: SPF with CMF Weighted with Observed Crashes 

Project Phase: Alternative Analysis Project Type: 4R 

Related Task: Interchange Justification Request 

Calculation Method: 

 Hand Calculated  Tool Based 

Hand calculated example included in Guide 

appendix. 

Comments: 

This example problem demonstrates how to estimate the expected number of crashes for an urban 

freeway loop ramp. 

LEVEL OF ANALYSIS

 Basic Intermediate Advanced 

Note: See table 3 for a full definition of Project Type designations. 

PROBLEM DESCRIPTION 

A State DOT is examining a 0.5-mile long urban freeway connector (loop) ramp to evaluate 

whether changes are needed for possible removal, improvement, or replacement due to recurring 

congestion. As part of this analysis, the DOT is conducting a safety assessment to determine whether 

the loop ramp has more crashes than would be expected for similar ramps. The Project Type is 4R 

and the Related Task is to document analysis for a potential Interchange Justification Request. 

How can the analyst predict the estimated number of crashes for this interchange freeway loop 

ramp for a 3-year study period between 2013 and 2015 while directly considering the AADT in the 

analysis? 

Summary of Available Data: 

In 2013, the ramp had an AADT of 9,800 vpd. Other ramp characteristics are summarized as follows: 

• One through lane. 

• Ramp length = 0.5 miles. 

• Average traffic speed on the freeway of 65 mph. 

73
74

SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

•	 Segment type is a ramp connector. 

•	 Two horizontal curves with the following characteristics: 

- Horizontal curve #1: radius = 11,460 ft, curve length = 0.04 mi, begins 0.26 mi from start of 

ramp (in direction of travel). 

-	 Horizontal curve #2: radius = 150 ft, curve length = 0.12 mi., begins 0.34 mi from start of 

ramp (in direction of travel). 

•	 Lane width = 13 ft. 

•	 Right shoulder width = 10 ft. 

•	 Left shoulder width = 7 ft. 

•	 No lane adds or drops by taper. 

•	 Roadside barrier on right side: 

- Barrier #1: 0.06 mi long positioned 11 ft. from edge of traveled way to barrier face. 

- Barrier #2: 0.05 mi long positioned 11 ft. from edge of traveled way to barrier face. 

•	 Roadside barrier on left side: 

- Barrier #1: 0.02 mi long positioned 8 ft. from edge of traveled way to barrier face. 

- Barrier #2: 0.05 mi long positioned 8 ft. from edge of traveled way to barrier face. 

- Barrier #3: 0.10 mi long positioned 8 ft. from edge of traveled way to barrier face. 

• Ramp entrance and exit not present in the study section.
 

The following aerial photograph depicts the ramp configuration (center line highlighted with yellow).
 

Source: ©2015 Google Earth 

Crash data for the corridor during the study period is summarized in the following table. The small 

number of observed crashes at this location provides information about observed crash frequency. 

For this example, the observed crash information is available for estimating future safety performance. 

The small crash frequencies observed would not be useful for identifying prospective 

countermeasures. 
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Year 

Property Damage Only Crashes 

Multiple-vehicle Single-vehicle 

Fatal and Injury Crashes 

Multiple-vehicle Single-vehicle 

2013 0 1 0 1 

2014 1 0 0 0 

2015 0 1 0 0 

ANALYSIS 

Selecting Appropriate Safety Assessment Method(s): 

The analyst can evaluate the candidate safety assessment methods shown in Table 8. The 

Interchange Justification Request task and the 4R project type are associated with five potential 

safety assessment methods: two basic, two intermediate, and one advanced. Because the goal 

is to comprehensively assess the varying ramp geometric features and calculate the estimated 

number of crashes, an assessment method that includes CMFs is appropriate (thus eliminating the 

AADT-Only SPF). The analyst would like to verify the influence of traffic volume over the three-year 

study period. This requires a volume-based approach (i.e. methods that use an SPF). The two basic 

methods should therefore be eliminated from consideration as they do not include traffic volume in 

the analysis. 

Interchange 

Justification 

Request 

4R 

CMF Applied to Observed Crashes 

CMF Relative Comparison 

AADT Only SPF 

SPF with CMF Adjustment 

SPF with CMF Weighted with 

Observed Crashes 

The remaining two potential safety assessment methods use procedures specific to the HSM. The SPF 

with CMF Adjustment can be used to evaluate the predicted number of crashes along a corridor 

with similar characteristics to the study site. The SPF with CMF Weighted with Observed Crashes 

includes the crash history so that the safety assessment includes consideration of the recent crash 

history at the location. Both of these safety assessment methods can be used, but the site-specific 

emphasis provides additional confidence that the calculated numbers reflect the expected number 

of crashes for the specific study site. For this reason, the analyst selects the SPF with CMF Weighted 

with Observed Crashes method. 

The analyst has the following three available options for performing the analysis for this method: 

• Perform the analysis using hand calculations (see Appendix for this example). 

• Conduct the assessment using the self-calculating “Smart Spreadsheet” tool known as ISATe. 

• Use the FHWA software tool IHSDM. 

75
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

The choice between the two automated tools is simply a preference issue 

as they both correspond to the HSM approach for this analysis. For this 

assessment, the analyst selected ISATe. The web address to acquire this free 

software tool is provided in the “Detailed Analysis” section of this problem. 

Linkage to the HSM: 

The 2014 HSM Supplement can be used to estimate the number of expected 

crashes for a freeway or a ramp. The procedure for ramps is introduced in 

HSM Chapter 19 (pp. 19-1 to 19-152). By determining predicted crashes, the 

analyst can estimate how many crashes may be estimated for a specific 

ramp with varying road conditions (in this case multiple horizontal curves 

and varying barrier to the right and left). Once the predicted number of 

crashes is known, the expected number of crashes for a specific site can be 

calculated by applying the Empirical Bayes Method summarized in the 2014 

HSM Supplement, pp. B-14 to B-30. 

Detailed Analysis: 

Calculated Expected 

Average Crash 

Frequency 

(FI and PDO crashes) 

Nexpected 

Calculate Average 

Crash Frequency 

(FI and PDO crashes) 

N spf 

Calculated Predicted 

Average Crash 

Frequency 

(FI and PDO crashes) 

Npredicted 

As previously indicated, the analyst has selected the advanced method for 

an SPF with CMF Weighted with Observed Crashes. This assessment is for a 

specific location with a known crash history and so the procedure calculates the expected number of 

crashes. Because the ramp’s cross-sectional characteristics are consistent along its entire length, the 

ramp can be modeled as one segment. 

To calculate the expected average crash frequency for the existing ramp location, the analyst 

has decided to use ISATe, a free software self-calculating spreadsheet tool. This product can be 

downloaded free of charge at http://www.highwaysafetymanual.org/Pages/tools_sub.aspx. The 

following steps summarize the process for performing the calculations to evaluate the expected 

crashes. 

STEP 1: Enter known data. 

The analyst enters the geometry, traffic, and crash data into the ISATe program. The analysis is 

conducted by automating the calculations using the program and reviewing the results. ISATe 

provides the analyst with expected crash frequencies for the ramp and also outputs CMF values to 

help the analyst identify roadway characteristics that could be changed to reduce crash frequency. 

STEP 2: Divide the ramp into homogeneous segments. 

The beginning and ending of the ramp are defined as the gore nose where the ramp joins each 

freeway mainline. This particular ramp does not need to be divided into segments because its crosssectional characteristics are consistent throughout its length, and ISATe can model a ramp with up to 

five horizontal curves along its length. 

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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Enhanced Interchange Safety Analysis Tool 

General Information 

Project Description Sample Data 

Analyst SD Date 5/9/2016 Area Type Urban 

First Year of Analysis 2013 

Last Year of Analysis 2015 

Crash Data Description 

Freeway segments 

Ramp Segments First year of crash data 2013 Last year of crash data 2015 

Ramp Terminals 

Program Control 

1. Enter data in the Main, Input Freeway Segments, Input Ramp Segments, Input Ramp Terminals worksheets. 

2. Click Perform Calculations button to start calculation process. 

3. Review results in the Output Summary worksheet. Optionally, click the Print buttons to print the summary 

worksheets. 

4. Optionally, detailed results can be reviewed in the Output Freeway Segments, Output Ramp Segments, Output 

Ramp Terminals worksheets. 

Perform Calculations Print Results (optional) Print Site Summary (optional) 

STEP 3: Specify analysis period, analysis type, and area type in the ISATe “Main” worksheet. 

For this analysis, the time period of interest is defined as the years 2013-2015. Crash data are available, 

so the empirical Bayes analysis procedure that is built into ISATe will be used. The area type is 

specified as urban. 

STEP 4: Enter input data into the ISATe “Input Ramp Segments” worksheet. 

This worksheet contains two columns to describe each ramp segment (for the crash period and the 

study period) and rows for each input data element. In this example problem, the crash period and 

the study period are the same time period, and no potential modifications to this ramp are being 

considered, so the input data for the crash period and the study period are the same. For some 

analyses (e.g., if a projection of future crash counts is desired), a study period in the future can be 

specified, while the crash period always must be in the past. 

As shown in the aerial photo, the loop ramp is a single-lane ramp with two horizontal curves. The first 

curve is gradual (radius = 11,460 ft) and is located under the freeway mainline bridge. The second 

curve is sharp (radius = 150 ft) and is located in the southeast quadrant of the interchange. 

77
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Input Worksheet for Ramp Segments

 (View results in Column CJ) (View results in Advisory Messages) 

Clear Echo Input Values Check Input Values Segment 1 

Crash Period Study Period 

Basic Roadway Data 

Number of through lanes (n): 1 1 

Ramp segment description: 

Segment length (L), mi: 0.5 0.5 

Average traffic speed on the freeway (Vfrwy ), mi/h: 65 65 

Segment type (ramp or collector-distributor road): Connector Connector 

Type of control at crossroad ramp terminal: 

Alignment Data 

Horizontal Curve Data See notes 

1 

Horizontal curve?: In Seg. In Seg. 

Curve radius (R1

), ft: 11460 11460 

Length of curve (Lc1), mi: 0.04 0.04 

Length of curve in segment (Lc1,seg), mi: 0.04 0.04 

Ramp-mile of beginning of curve in direction of travel 

(X1

), mi: 0.26 0.26 

2 

Horizontal curve?: In Seg. In Seg. 

Curve radius (R2), ft: 150 150 

Length of curve (Lc2), mi: 0.12 0.12 

Length of curve in segment (Lc2,seg), mi: 0.12 0.12 

Ramp-mile of beginning of curve in direction of travel 

(X2), mi: 0.34 0.34 

3 Horizontal curve?: No No 

Cross section data and roadside data are entered as shown. The analyst should input specific 

information for a total of five barrier sections – three located on the left side of the ramp (in the 

direction of travel) and two located on the right side of the ramp. 

The analyst should next enter ramp access and traffic data. These data include the presence and 

lengths of ramp entrances or exits on each segment, and presence of weaving sections. The analyst 

provides traffic volumes for the years 2013, and ISATe uses the same traffic volume for the years 2014 

and 2015. 

Crash data are entered and broken down by crash type, severity, and year. 

78
 

SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

Cross Section Data 

Lane width (Wl

), ft: 13 13 

Right shoulder width (Wrs), ft: 10 10 

Left shoulder width (Wls), ft: 7 7 

Presence of lane add or lane drop by taper: No No 

Length of taper in segment (Ladd,seg or Ldrop,seg), mi: 

Roadside Data 

Presence of barrier on right side of roadway: Yes Yes 

1 Length of barrier (Lrb,1), mi: 0.06 0.06 

Distance from edge of traveled way to barrier face (Woff,r,1), ft: 11 11 

2 Length of barrier (Lrb,2), mi: 0.05 0.05 

Distance from edge of traveled way to barrier face (Woff,r,2), ft: 11 11 

3 Length of barrier (Lrb,3), mi: 

Distance from edge of traveled way to barrier face (Woff,r,3), ft: 

4 Length of barrier (Lrb,4), mi: 

Distance from edge of traveled way to barrier face (Woff,r,4), ft: 

5 Length of barrier (Lrb,5), mi: 

Distance from edge of traveled way to barrier face (Woff,r,5), ft: 

Presence of barrier on left side of roadway: Yes Yes 

1 Length of barrier (Llb,1), mi: 0.02 0.02 

Distance from edge of traveled way to barrier face (Woff,l,1), ft: 8 8 

2 Length of barrier (Llb,2), mi: 0.05 0.05 

Distance from edge of traveled way to barrier face (Woff,l,2), ft: 8 8 

3 Length of barrier (Llb,3), mi: 0.1 0.1 

Distance from edge of traveled way to barrier face (Woff,l,3), ft: 8 8 

4 Length of barrier (Llb,4), mi: 

Distance from edge of traveled way to barrier face (Woff,l,4), ft: 

5 Length of barrier (Llb,5), mi: 

Distance from edge of traveled way to barrier face (Woff,l,5), ft: 

Ramp Access Data See Note 

Ramp 

Entrance 

Ramp entrance in segment? (If yes, indicate type.): No No 

Length of entrance s-c lane in segment (Len,seg), mi: 

Ramp 

Exit 

Ramp exit in segment? (If yes, indicate type.): No No 

Length of exit s-c lane in segment (Lex,seg), mi: 

Weaving 

Section 

Weave section in collector-distributor road segment?: 

Length of weaving section (Lwev), mi: 

Length of weaving section in segment (Lwev seg), mi: 

Traffic Data Year 

Average daily traffic (AADTr

 or AADT c) by year, veh/d: 

(enter data only for those years for which it is available, leave 

other years blank) 

2013 9800 

2014 

2015 

79
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

- -

- -

Crash Data Year Segment Crashes 

Count of Fatal and Injury (FI) Crashes by Year 

Multiple-vehicle crashes (No,w,n,mv,fi) 

2013 0 

2014 0 

2015 0 

2016 

2017 

Single-vehicle crashes (No,w,n,sv,fi) 

2013 1 

2014 0 

2015 0 

2016 

2017 

Count of Property Damage Only (PDO) Crashes by Year 

Multiple-vehicle crashes (No,w,n,mv,pdo) 

2013 0 

2014 1 

2015 0 

2016 

2017 

Single-vehicle crashes (No,w,n,sv,pdo) 

2013 1 

2014 0 

2015 1 

2016 

2017 

Once all input data are entered into the “Input Ramp Segments” worksheet, the analyst should click 

the “Check Input Values” button to verify that no illogical or erroneous data values were entered. 

STEP 5: Perform the analysis calculations. 

The analyst clicks the “Perform Calculations” button on the “Main” worksheet of ISATe to initiate 

calculations. 

STEP 6: Review the calculation results. 

ISATe provides a summary of the calculation results on the “Output Summary” worksheet. In this 

example problem, the freeway facility of interest consists of one ramp segment. ISATe can also be 

used to analyze a facility that consists of a combination of mainline segments, ramp segments, and 

crossroad ramp terminals. 

For the entire analysis period (2013-2015), the ramp is expected to experience about nine crashes, 

of which about six will be PDO. These values are equivalent to expected crash frequencies of about 

three crashes per year, of which about two will be PDO. 

80
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Output Summary 

General Information 

Project description: Sample Data 

Analyst: JAB Date: 5/9/2016 Area Type: Urban 

First year of analysis: 2013 

Last year of analysis: 2105 

Crash Data Description 

Freeway segments 

Segment crash data available? No First year of crash data: 

Project-level crash data 

available? No Last year of crash data: 

Ramp segments 

Segment crash data available? Yes First year of crash data: 2013 

Project-level crash data 

available? No Last year of crash data: 2015 

Ramp terminals 

Segment crash data available? No First year of crash data: 

Project-level crash data 

available? No Last year of crash data: 

Estimated Crash Statistics 

Crashes for Entire Facility Total K A B C PDO 

Estimated number of crashes 

during Study Period, crashes: 8.5 0.1 0.2 1.3 1.7 5.3 

Estimated average crash freq. 

during Study Period, crashes/yr: 2.8 0.0 0.1 0.4 0.6 1.8 

Crashes by Facility 

Component 

Nbr. 

Sites Total K A B C PDO 

Freeway segments, 

crashes: 0 0.0 0.0 0.0 0.0 0.0 0.0 

Ramp segments, 

crashes: 1 8.5 0.1 0.2 1.3 1.7 5.3 

Crossroad ramp 

terminals, crashes: 0 0.0 0.0 0.0 0.0 0.0 0.0 

Crashes for Entire 

Facility by Year Year Total K A B C PDO 

Estimated number of 

crashes during the 

Study Period, crashes: 

2013 2.8 0.0 0.1 0.4 0.6 1.8 

2014 2.8 0.0 0.1 0.4 0.6 1.8 

2015 2.8 0.0 0.1 0.4 0.6 1.8 

81
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

FINDINGS 

Interpreting the Results: 

By using the ISATe spreadsheet program to evaluate the loop ramp, the analyst estimates that the 

expected crash frequency is about three crashes per year. Over the entire analysis period, this 

equates to about nine crashes. These computed numbers are consistent with the observed crash 

counts. The analyst can conduct similar analyses on other loop ramps and compare the values for 

this location to assess whether they are notably higher than those for similar locations. 

Possible Errors to Avoid: 

It is possible for the analyst to enter erroneous input data or to enter values that are illogical when 

compared across several input elements. For example, a roadside barrier lateral offset cannot be 

located closer to the travel lane than the width of the shoulder. The ISATe program contains many 

data validation mechanisms to minimize this possibility, and a checking feature is also available by 

clicking the “Check Input Values” button on the input data worksheets. 

Alternative Analysis Approaches: 

This analysis can also be performed by hand-calculating the ramp safety prediction methodology 

described in Chapter 19 of the HSM (see Appendix item A-3.4). Due to the extensive nature of the 

calculations, however, the hand-calculated method is time consuming and can be cumbersome 

due to the large number of equations included in the methodology. 

The FHWA software tool IHSDM, however, could also be used to conduct this analysis. The crash 

module for the IHSDM includes a “site-based analysis tool” that can be used without having to 

include the detailed horizontal and vertical alignment and cross-section data noted in Example 

Problem 3.3. The selection of ISATe versus IHSDM is a matter of personal preference since both of these 

free tools perform the same analysis. The IHSDM software is available for download at 

http://www.ihsdm.org. 

82
CHAPTER 4. Preliminary and Final Design Applications 

The preliminary and final design 

phases are clearly defined for most 

jurisdictions, yet key elements of 

these two phases can differ for each 

transportation agency. This Guide, 

therefore, combines the preliminary 

and final design into a single chapter. 

During the design phase, design 

decisions must be refined and 

finalized prior to construction. In 

general, safety assessments in the 

design phase focus on documenting 

design decisions, including those 

that require exceptions to the design 

standards, and calculating the 

estimated number of crashes that 

can be anticipated for the final facility 

design. 

This chapter provides information to help select safety assessment methods suitable for addressing 

questions about safety performance that arise during these preliminary and final design activities 

based upon the related task and project type. This Guide describes the design tasks in four general 

categories: 

• Selecting specific design elements and their dimensions, 

• Design exceptions, 

• Value engineering, and 

• Establishing the work zone transportation management plan. 

Selecting specific design elements and their dimensions is a critical design activity for all elements 

of the project. The objective of the safety assessment in this task is to compare the estimated safety 

performance for the alternative dimensions or elements. This safety assessment information can then 

be used to help inform the final facility design process. 

Design exceptions are needed on National Highway System roadways when controlling criteria are 

not met. States may also require documentation of deviations from their own design criteria on or 

off the National Highway System. The objective of safety assessment in this task is to estimate how 

the design exception impacts safety performance and to identify and evaluate potential strategies 

for mitigating the impact. The goal of this task is to quantify the design exception and potential 

mitigation strategies so that corridor safety performance is not adversely affected. 

NOTE: Elements of ongoing life cycle activities are incorporated into 

Chapters 2 and 4 as appropriate. 

Project

Development 

Planning and

Scoping 

Environmental Analysis 

Alternatives 

Identification/Analysis 

Preliminary Final Design Design 

Chapter 3 

Chapter 2 Chapter 4 

Figure 6.The Project Development Cycle and 

Corresponding Design Chapters 

SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

83
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

On certain projects, value engineering provides recommendations for providing needed functions, 

optimizing value and quality, and reducing the time to develop and deliver the project (23 CFR 627). 

The objective of safety assessment in this task is to quantify safety performance so that it can be 

weighed with other project considerations. 

Establishing the work zone transportation management plan enables efficient construction of a 

project without compromising safety or operations. The objective of safety assessment in this task, as it 

relates to safety performance, is to compare the safety impacts of the various traffic control strategies 

considered during development of the transportation management plan. 

Table 8 identifies the seven assessment methods applicable to the preliminary and final design 

tasks and their safety performance related analysis objective. The check marks in Table 8 suggest 

suitable safety assessment methods for each related task and objective and, in some cases, are 

distinguished by project type. In this context, the term “suitable” means that the method generally has 

the capability to address the safety performance related analysis objective with the data typically 

available for the related task and project type. 

The following example questions demonstrate the type of questions the analyst may develop at the 

beginning of the safety assessment. These questions are based on the example problems included in 

this chapter. 

1.	 How can the analyst estimate the annual number of crashes for this new facility? 

2.	 How does the analyst calculate the estimated number of crashes for each roundabout? 

3.	 How can the analyst estimate how the change in roadway geometry may affect the 

number of total and fatal and injury (FI) crashes? 

Table 9 highlights that the level of predictive reliability generally increases moving from basic to 

advanced methods. At the same time, the required resources for the analysis also will increase. In 

some cases, it may not be feasible to fully implement the preferred safety assessment method due 

to limitations in site information, crash data, traffic volume, or similar information. As an example, 

the Basic safety assessment method for a CMF Applied to Observed Crashes cannot be executed 

if historic crash data is not available. Consequently, the approach for selecting a safety assessment 

method for alternatives evaluation and identification is graphically depicted as follows. 

Determine 

Preliminary or 

Final Design Safety 

Assessment Related 

Task 

Identify Project 

Type 

Select Safety Assessment Method 

(Confirm Data Requirements) 

84
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS - Table 9. Preliminary and Final Design Safety Assessment Objective Related Task Objective Project Type Basic Intermediate Advanced Site Evaluation or Audit Historical Crash Data Evaluation CMF Applied to Observed Crashes CMF Relative Comparison AADT Only SPF SPF with CMF Adjustment SPF with CMF Weighted with Observed Crashes Observed Crashes Predicted Crashes Expected Crashes Safety Assessments: Increasing Level of Predictive Relaibility Selecting specific design elements and their dimensions To compare safety impacts of alternative dimensions 2R     3R, 4R     8    NL    5 Design Exception To estimate design exception impacts on safety performance and to identify and evaluate strategies for mitigation 3R, 4R     7  NL   Value Engineering To quantify safety performance so that it can be weighed with other project considerations 4R    6   NL   Establishing the Work Zone Transportation Management Plan To compare safety impacts of traffi c control strategies 2R   3R, 4R NL Data Requirements: Increasing Level of Resources Needed (Staff,Analysis,Time, etc.) Road Type1       Road Characteristics 2      Traffi c Volume (vpd) 3    Observed Crash Data 4    1Road Type refers to rural two-lane highway, rural multi-lane highway, urban freeway, etc. 2 Road Characteristics includes physical features such as lane widths, access density, etc. 3 Traffic Volume is the ADT or AADT in vehicles per day. 4 Observed Crash Data represents the historic crash data at the study site for a period of more than 1 year (preferably 3 to 5 years). 5 See Example Problem 4.1. 6 See Example Problem 4.2. 7 See Example Problem 4.3. 8 See Example Problem 5.3. Note: Project type definitions are as follows: R2 = resurfacing existing facilities. R3 = major rehabilitation of an existing facility. R4 = major retrofit construction efforts. NL = highway construction at a new location. See Table 3 for a full definition of each Project Type. = suitable safety assessment method.ADT = average daily traffic.AADT = annual average daily traffic. CMF = crash modification factor. SPF = safety performance function. vpd = vehicles per day.

85
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

This chapter provides examples that demonstrate the selection process for the preliminary and final 

design safety assessment methods. These examples are simplified, hypothetical problems intended to 

illustrate the thorough process for selecting a method and demonstrate how to apply the method to 

answer the associated safety-based question. 

4.1 Predicting Crashes for a New Urban Multilane Arterial 

PROBLEM OVERVIEW 

Safety Assessment Method: SPF with CMF Adjustment 

Project Phase: Preliminary & Final Design Project Type: NL 

Related Task: Selecting Specific Design 

Elements and Their Dimensions 

Calculation Method: 

 Hand Calculated  Tool Based 

Comments: 

This example problem demonstrates how to calculate the predicted number of crashes for a four-lane 

urban arterial following the construction of a median and the addition of lighting and auto speed 

enforcement. 

LEVEL OF ANALYSIS

 Basic Intermediate Advanced 

Note: See table 3 for a full definition of Project Type designations. 

PROBLEM DESCRIPTION 

As part of an expansion effort, a city will be constructing a new four-lane urban arterial. The analyst 

has been asked to estimate the number of crashes that could occur annually on this facility. The new 

corridor will be 1-mile long and will only have four minor residential driveways. Other characteristics 

of the road include a 20-ft. wide median, street lighting, and automated speed enforcement. The 

Project Type is NL and the Related Task is Selecting Specific Design Elements and Their Dimensions. 

How can the analyst estimate the annual number of crashes for this new facility? 

Summary of Available Data: 

The predicted AADT (for the major approach) is 28,000 vpd, and there are four minor residential 

driveways. Additional road information is as follows: 

•	 No on-street parking. 

•	 No roadside fixed objects closer than 30 ft. 

•	 Posted speed limit of 40 mph. 

•	 Regional Highway Safety Manual (HSM) urban arterial segment safety performance function 

(SPF) calibration factor of 1.05. 

•	 Because it is new construction, there is no available crash history. 

86
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

87

ANALYSIS 

Selecting Appropriate Safety Assessment Method(s): 

The analyst can review the applicable safety assessment methods summarized in Table 9. The 

Selecting Specific Design Elements and Their Dimensions project task for a NL project type is 

associated with three safety assessment methods (one basic and two intermediate). The CMF-based 

methods are needed to evaluate the influence of unique geometric characteristics, so the analyst 

eliminates the AADT-Only SPF option. The CMF Relative Comparison method can help the analyst 

better understand the influence of each individual design feature (for which a CMF is available), 

but does not provide a way to estimate the overall predicted corridor number of crashes and so this 

method is also eliminated from consideration. The selected safety assessment method, therefore, is 

the SPF with CMF Adjustment. 

Linkage to the HSM: 

The HSM can be used to estimate the number of predicted crashes for an urban or suburban arterial 

by applying the procedures introduced in HSM Chapter 12 (pp. 12-1 to 12-122). By determining 

predicted crashes, the analyst can estimate how many crashes are anticipated for a specific road 

type with varying road conditions. 

Selecting Specific 

Design Elements and 

Their Dimensions 

NL 

CMF Relative Comparison 

AADT Only SPF 

SPF with CMF Adjustment 

Detailed Analysis: 

For the SPF with CMF Adjustment safety assessment method, the analyst can use the SPF from the 

HSM for a divided segment. This evaluation can be performed using hand calculations (see Appendix 

A-4.1) or by using the free self-calculating spreadsheets or FHWA IHSDM tool. 

For a single location, the spreadsheets can be easily applied. For more 

complex corridors, IHSDM is the recommended method. 

The following steps provide the calculations based on using the HSM “Smart 

Spreadsheets” available for download at: http://www.highwaysafetymanual. 

org/Pages/tools_sub.aspx#4. For this example problem, the analyst should 

use the “HSM prediction urban and suburban arterial” spreadsheet tool. 

As noted in the “Summary of Available Data” section, the SPFs from the HSM 

have been previously calibrated to the region and have a calibration value of 

1.05. 

STEP 1: Input the known data for the proposed design. 

The analyst should input the data for the study segment. 

Predict Segment 

Average Crash 

Frequency for Base 

Conditions 

Nspf 

Predict Segment 

Specific Average 

Crash Frequency for 

Alternative Options 

Npredicted 
Segment Input Data: 

For segment analysis, input data in the top section (Worksheet 1A) of the “Segment 1” tab. The 

proposed design input data is shown as follows: 

Worksheet 1A - General Information and Input Data for Urban and Suburban Roadway Segments 

General Information Location Information 

Analyst 

Agency or Company 

Date Performed ABC City 05/10/16 

Roadway 

Roadway Section 

Jurisdication 

Analysis Year 2015 

Input Data Base Conditions Site Conditions 

Roadway type (2U, 3T, 4U, 4D, ST) -­ 4D 

Length of segment, L (mi) -­ 1 

AADT (veh/day) AADTMAX =66,000 (veh/day) -­ 28,000 

Type of on-street parking (none/parallel/angle) None None 

Proportion of curb length with on-street parking -­ 0 

Median width (ft) - for divided only 15 20 

Lighting (present / not present) Not Present Present 

Auto speed enforcement (present / not present) Not Present Present 

Major commercial driveways (number) -­ 0 

Minor commercial driveways (number) -­ 0 

Major industrial / institutional driveways (number) -­ 0 

Minor industrial / institutional driveways (number) -­ 0 

Major residential driveways (number) -­ 0 

Minor residential driveways (number) -­ 4 

Other driveways (number) -­ 0 

Speed Category -­ Posted Speed 

Greater than 30 mph 

Roadside fixed object density (fixed objects / mi) 0 0 

Offset to roadside fixed objects (ft) [If greater than 30 or 

Not Present, input 30] 

30 30 

Calibration Factor, Cr 1.00 1.05 

STEP 2: Following input in Step 1, review the predicted number of crashes automatically calculated by 

the spreadsheet tool see Worksheet 1L below). 

Worksheet 1L - Summary Results for Urban and Suburban Roadway Segments 

(1) (2) (3) (4) 

Crash Rate Predicted average crash frequency, (crashes/mil/ N (crashes/year) Roadway predicted rs year) Segment 

Crash Severity Level (Total) from Worksheet 1K Length, L (mi) (2)/(3) 

Total 5.3 1.00 5.3 

Fatal and injury (FI) 1.5 1.00 1.5 

Property damage only (PDO) 3.8 1.00 3.8 

SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Additional information related to crash type by severity is also calculated and can be reviewed in the 

spreadsheet tool. 

88
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

STEP 3: Summarize Findings. 

The analyst’s ultimate goal is to calculate the estimated number of crashes predicted for the road 

constructed on the new location. In addition to the total number of crashes, crash severity information 

is important to note. The following table summarizes these results. 

Predicted Number 

Roadway Improvement Predicted Number Fatal Predicted Number of of Property Damage 

Scenario and Injury Crashes Total Crashes Only Crashes 

Urban Arterial with 

median, lighting, and 

automated enforcement 

1.5 

(say 2) 

5.3 

(say 6) 

5.3 – 1.5 = 3.8 

(say 4) 

FINDINGS 

Interpreting the Results: 

Based on an evaluation of the predicted number of crashes, an urban four-lane arterial with a 

30-ft. median, four minor driveways, street lighting, and automated enforcement is estimated to have 

approximately six total crashes per year, of which 2 crashes are fatal and injury crashes, for a 1-mile 

segment of road. 

The city may want to consider alternative configurations (e.g., wider versus narrower medians, the 

effects of on-street parking, etc.) to ultimately identify the optimal design that meets the needs of 

the corridor, fulfills access requirements for adjacent properties, and minimizes crashes while also 

considering cost effective roadway elements. 

Possible Errors to Avoid: 

The HSM procedures require the analyst to understand how all SPFs and CMFs equate to the base 

conditions associated with the procedure. Incorrect use of these values can introduce erroneous 

results. 

Alternative Analysis Approaches: 

While the SPF with CMF Adjustment safety assessment method provides predicted crash information 

for the total length of road segment, selection of individual road features could be incrementally 

considered using the CMF Relative Comparison approach. For example, a preliminary analysis 

to evaluate varying the median width by a simple comparison of the CMF values can enable the 

analyst to narrow down the design options prior to applying the more complex (i.e., more data 

intensive) SPF method. 

89
 

 

 

SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

4.2 Calculating Predicted Crash Frequency for Proposed Two-Lane 

Roundabout 

PROBLEM OVERVIEW 

Safety Assessment Method: AADT-Only SPF 

Project Phase: Preliminary & Final Design Project Type: 4R 

Related Task: Safety Input for Value Engineering 

Assessment 

Calculation Method: 

 Hand Calculated  Tool Based 

Comments: 

This example problem demonstrates how to consider the number of crashes as one input into the value 

engineering process. For this case, the problem evaluates three proposed roundabouts (four-leg, twocirculating lanes) that are located within the larger project boundaries. 

LEVEL OF ANALYSIS

 Basic Intermediate Advanced 

Note: See table 3 for a full definition of Project Type designations. 

PROBLEM DESCRIPTION 

A city is reconstructing a corridor that includes several at-grade intersections. As part of the larger 

project, the city is documenting value engineering decisions related to the improvements. Within 

the corridor are three at-grade intersections that the city is designing to be converted to multilane 

roundabouts. Their analyst has been asked to calculate the estimated number of crashes for each 

roundabout so that this information can be used as input into the overall value engineering decisions. 

The Project Type is 4R and the Related Task is Value Engineering. How does the analyst calculate the 

estimated number of crashes for each roundabout? 

Summary of Available Data: 

The following table shows the predicted AADT values for the design year. For design consistency 

purposes, the geometric design will be similar for each proposed roundabout. 

Roundabout Average Annual Daily Traffic (vehicles per day) 

1 12,700 

2 33,150 

3 26,000 

Other information includes: 

•	 Two circulating lanes. 

•	 Four intersection legs. 

•	 Regional calibration factor of 0.89 for total crashes and 0.93 for fatal and injury crashes (based on 

SPFs included in NCHRP Report 672). 

90
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

91

• Due to the substantially modified corridor design, available corridor crash history will not be 

representative of future design conditions. 

ANALYSIS 

Selecting Appropriate Safety Assessment Method(s): 

The analyst can review Table 9 to identify potential safety assessment methods for this effort. For a 

4R project and the Safety Assessment for Value Engineering Analysis related task, five candidate 

safety assessment methods are available: two basic methods, two intermediate, and one advanced. 

The analyst notes that the crash conditions prior to corridor reconstruction represent a substantially 

different configuration than those shown in the design plans and so concludes that safety 

assessment methods based on historical (observed) crashes can be eliminated. The evaluation 

of a CMF Relative Comparison option would potentially provide valuable information if there were 

geometric differences between the three proposed roundabouts, but all three have consistent 

designs, and so the analyst eliminates this assessment method from consideration. 

CMF Applied to Observed Crashes 

CMF Relative Comparison 

AADT Only SPF 

SPF with CMF Adjustment 

SPF with CMF Weighted with 

Observed Crashes 

Safety Assessment 

for Value Engineering 

Analysis 

4R 

The final two candidate safety assessment methods are the AADT-Only SPF and the SPF with CMF 

Adjustment options. The analyst reviews candidate SPFs for these two prospective procedures and 

discovers that the 2010 HSM did not yet include an SPF for roundabouts; however, NCHRP Report 672 

does have SPFs for roundabouts and, as previously indicated, the city has already calibrated these to 

local conditions. As a result, the analyst selects the AADT-Only SPF option. 

Linkage to the HSM: 

The application of an SPF is similar to predictive methods presented in Volume 2 (Part C) of the HSM; 

however, the roundabout SPFs provided in NCHRP Report 672 (p. 5-23) are not included in the HSM. 

Detailed Analysis: 

The crash prediction models in the NCHRP Report 672 only require the use of entering AADT and 

geometric configuration (number of legs and circulating lanes) to calculate the estimated total 

number of crashes at the roundabout. As previously indicated, the city uses the SPFs from NCHRP 

Report 672 – Roundabouts: An Informational Guide, which have been calibrated for local conditions 

(CTotal = 0.89, CFI

 = 0.93). The following steps summarize the calculations for estimating the number of 

crashes for each roundabout. 
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

STEP 1: Calculate the predicted average total crash frequency and fatal and injury (FI) frequency for 

the roundabout (NSPF ). 

The NCHRP Report 672 equations for a roundabout with four legs and two circulating lanes are used 

for this calculation. The base conditions associated with the SPF require entering AADT values from 

2000 to 35,000 vpd for the “Total Crash” equation and AADT values from 2000 to 37,000 for the “FI 

Crash” equation. All three of the study roundabouts meet these criteria. Note: KAB and FI crashes are 

assumed to be equivalent terms for the purpose of this analysis 

Calculations Notes 

Unadjusted Total Crashes per year: 

Nspf int (Total)=0.0038×AADT 0.7490 NCHRP Report 672, Exhibit 5-19, p. 5-23 

(4-leg and 2 circulating lanes) 

Nspf int (Total)=0.0038×(12,7000.7490 )=4.50 SPF Total Crashes -- #1 (AADT = 12,700 vpd) 

Nspf int (Total)=0.0038×(33,1500.7490 )=9.24 SPF Total Crashes -- #2 (AADT = 33,150 vpd) 

Nspf int (Total)=0.0038×(26,0000.7490 )=7.70 SPF Total Crashes -- #3 (AADT = 26,000 vpd) 

Unadjusted Fatal + Injury (FI) Crashes per year: 

Nspf int (FI)=0.0013×AADT 0.5923 NCHRP Report 672, Exhibit 5-20, p. 5-23 (4-leg and 

2 circulating lanes) 

Nspf int (FI)=0.0013×(12,7000.5923)=0.35 FI Crashes: Roundabout #1 (AADT = 12,700 vpd) 

Nspf int (FI)=0.0013×(33,1500.5923)=0.62 FI Crashes: Roundabout #2 (AADT = 33,150 vpd) 

Nspf int (FI)=0.0013×(26,0000.5923)=0.54 FI Crashes: Roundabout #3 (AADT = 26,000 vpd) 

Definitions 

Nspf int (Total)= average total crash frequency for intersection-related crashes 

Nspf int (FI)= average fatal and injury crash frequency for intersection-related crashes 

AADT entering= entering (largest) AADT (vehicles per day) 

AADT = average annual daily traffic, FI = fatal and injury, NCHRP = National Cooperative Highway Research Program, SPF = safety performance function, vpd = vehicles per day 

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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

Calculate Predicted 

Roundabout Average 

Crash Frequency* 

Nspf int. 

Calculate Predicted 

Roundabout Alternative 

Specific Average Crash 

Frequency** 

Npredicted int 

* Roundabout SPF does not include 

CMFs (see NCHRP Report 672, Ch. 5) 

**Local Calibration for the NCHRP 

Report 672 SPFs resulted in calibration 

factors of 0.89 for total crashes and 

0.93 for fatal and injury crashes 

STEP 2: Calculate the predicted average crash frequency for 

each roundabout (Npredicted). 

The predicted average crash frequency for each option can 

be calculated using HSM Equation C-1, p. C-4 of HSM Volume 

2: 

Npredicted =Nspf x (CMF1x x CMF2x x... xCMF yx ) × Cx 

Where: CMF = crash modification factor/function and C = 

calibration factor for SPF 

For the NCHRP Report 672 equations, companion CMFs are 

not available, so this equation is reduced to the following 

(effectively assigning a value of 1.0 to each CMF value): 

Npredicted =Nspf x Cx 

As previously indicated, the SPF calibration factor for the local 

region has a total crash value of 

C = 0.89 and a fatal and injury value of C = 0.93. Total FI

Calculations Notes 

Npredicted int = Nspf int x C Reduced HSM equation 

Adjusted Total Crashes per Year 

Npredicted int (Roundabout #1-Total)= 4.50×0.89=4.01 Predicted Total Crashes -- #1 

Npredicted int (Roundabout #2-Total)= 9.24×0.89=8.22 Predicted Total Crashes -- #2 

Npredicted int (Roundabout #3-Total)= 7.70×0.89=6.85 Predicted Total Crashes -- #3 

Adjusted FI Crashes per Year 

Npredicted int (Roundabout #1-FI)= 0.35×0.93=0.33 Predicted FI Crashes -- #1 

Npredicted int (Roundabout #2-FI)= 0.62×0.93=0.58 Predicted FI Crashes -- #2 

Npredicted int (Roundabout #3-FI)= 0.54×0.93=0.50 Predicted FI Crashes -- #3 

Definitions: 

FI = fatal and injury. HSM = Highway Safety Manual. 

STEP 3: Summarize the findings. 

Location Predicted FI Crashes per Year Predicted Total Crashes per Year 

Roundabout #1 0.3 (say 1) 4.0 (say 5) 

Roundabout #2 0.6 (say 1) 8.2 (say 9) 

Roundabout #3 0.5 (say 1) 6.9 (say 7) 

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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

FINDINGS 

Interpreting the Results: 

Based on the results calculated using the model found in NCHRP Report 672 – Roundabouts: An 

Informational Guide, the analyst noted that the predicted number of crashes is substantially based 

on the entering traffic volume at the roundabout location. For all three roundabouts, however, the 

predicted number of FI crash per year is less than one. These estimates are an important input into 

the overall value engineering assessment. The city has elected to maintain consistent roundabout 

geometry for all three intersection locations and so the higher volume roundabout (in this case #2) is 

predicted to have more total crashes each year than the other two locations. An important aspect of 

value engineering is the incremental improvement of features. 

Possible Errors to Avoid: 

It is important to confirm that, when selecting a model to predict crashes, the SPF and its associated 

base conditions are appropriate for the studied transportation facility. For this example, the AADT 

values should be consistent with those used to develop the NCHRP Report 672 (Section 5-4). 

Another potential error is the expectation that using the more advanced method will provide better 

results. For this analysis, the corridor prior to redesign is not similar to the modified corridor and 

intersection configuration. Consequently, the historical (observed) crashes should not be used in the 

evaluation for a completely different “after” intersection configuration. 

Alternative Analysis Approaches: 

The HSM also has CMFs that can be used to predict crashes for converting a two-way, stop-controlled 

intersection or signalized intersection to a roundabout. As noted previously, Part C (Volume 2) of the 

HSM does not contain an SPF for roundabouts. If the installation of a roundabout is being considered 

as one of the alternatives, however, the predicted number of crashes can be calculated by using the 

SPF for the existing conditions and then applying the appropriate CMF from HSM Part D (Volume 3). 

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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

4.3 Documenting a Design Decision for a Sharp Horizontal Curve on a 

Rural Two-Lane Highway 

PROBLEM OVERVIEW 

Safety Assessment Method: SPF with CMF Adjustment 

Project Phase: Preliminary & Final Design Project Type: 4R 

Related Task: Design Exception 

Calculation Method: 

 Hand Calculated  Tool Based 

Hand calculated example included in Guide appendix. 

Comments: 

This example problem demonstrates how to evaluate the effect a design decision can have on the 

estimated number of crashes for a rural two-lane, two-way highway horizontal curve re-alignment 

project. 

LEVEL OF ANALYSIS

 Basic Intermediate Advanced 

Note: See table 3 for a full definition of Project Type designations. 

PROBLEM DESCRIPTION 

A two-lane highway that runs parallel to a river is experiencing failure of the embankment on the 

south side of the road. To avoid a complete failure due to a landslide, designers are planning to shift 

the roadway to the north. This will cause a sharper curvature along the section just prior to an atgrade rail crossing. The proposed new design results in a horizontal curve, with a 250 ft. radius, which 

is shorter than the minimum recommended curve radius (for e=6 percent, curve radius= 340 ft.) for 

its existing design speed of 35 mph, as set forth by A Policy on Geometric Design of Highways and 

Streets by the American Association of State Highway and Transportation Officials (AASHTO). Over the 

most recent 5-year period, there have been approximately 30 crashes in the vicinity of this 0.5-mile 

segment. 

The Project Type is 4R and the Related Task is to evaluate a Design Exception. How can the analyst 

estimate how the change in roadway geometry may affect the number of crashes (Total and FI)? 

Summary of Available Data: 

To perform this evaluation, the analyst must know the existing and proposed roadway geometry. The 

roadway curve geometry is summarized as follows: 

Geometric Element (ft.) Existing Proposed 

Curve Length 200 230 

Curve Radius 380 250 

In addition, the corridor has the following characteristics: 

• Existing and future curve will have constant curve radius without any transitional spiral curves. 

• AADT=11,000 vpd. 

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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

• Segment length= 0.5 miles. 

• Detailed crash data is not currently available. 

• Using the SPF from the HSM, the regional calibration factor for rural two-lane highways is C = 0.97. 

The existing configuration with the proposed curve superimposed is shown in the following graphic. 

Source: ©2015 Google Earth Aerial View of the Horizontal Curve 

Source: ©Google Streetview 

Google StreetView of the Existing Conditions along the Horizontal Curve 

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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

ANALYSIS 

Selecting Appropriate Safety Assessment Method(s): 

The analyst can review Table 9 to identify suitable safety assessment methods for evaluating a 4R 

project type that requires a design exception. There are five candidate safety assessment methods: 

two basic, two intermediate, and one advanced. The specific goal of the analyst is to assess whether 

the alternative curve design will result in a larger number of total or fatal injury crashes based on 

a change in the horizontal curve geometry. This design-based evaluation requires the use of a 

CMF-based procedure, so the AADT-Only SPF method can be eliminated. In addition, the historical 

(observed) crash detailed information is not available and so the two methods that rely on observed 

crashes can also be eliminated (i.e. CMF Applied to Observed Crashes and SPF with CMF Weighted 

with Observed Crashes). 

CMF Applied to Observed Crashes 

CMF Relative Comparison 

AADT Only SPF 

SPF with CMF Adjustment 

SPF with CMF Weighted with 

Observed Crashes 

Design Exception 4R 

The two remaining safety assessment methods are the CMF Relative Comparison method and the 

SPF with CMF Adjustment method. Either of these two methods will provide useful information, but 

the analyst would like to estimate the number of additional crashes, and the relative comparison 

approach provides the percentage of additional crashes. For this reason, the analyst ultimately 

selects the SPF with CMF Adjustment for the subsequent evaluation. 

Linkage to the HSM: 

The HSM can be used to estimate the number of predicted crashes for a rural, two-lane highway by 

applying the procedures introduced in HSM Chapter 10 (pp. 10-1 to 10-74). By determining predicted 

crashes, the analyst can estimate how many crashes may be expected for a specific road type with 

varying road conditions (in this case the curve radii and curve length). The HSM provides manual 

calculations, but a spreadsheet tool is available and can be used to simplify this analysis. 

Detailed Analysis: 

This example demonstrates how to use the HSM to estimate the likely effect of designing curves with 

radii less than the recommended minimum values in AASHTO’s A Policy on Geometric Design of 

Highways and Streets. 

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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

The rural two-lane predictive method is located in Chapter 10 of the HSM. The following steps provide 

the calculations based on using the HSM “Smart Spreadsheets” available for download at: http:// 

www.highwaysafetymanual.org/Pages/tools_sub.aspx#4. For this example problem, the analyst 

should use the “HSM prediction rural two lane” spreadsheet tool. Example hand calculations are 

included in the Guide Appendix (see A-4.3). 

STEP 1: Input the known data for the roadway segment. 

Segment Input Data: 

For segment analysis, input data in the top section (Worksheet 1A) of the “Segment 1” tab. The Existing 

Condition input data is shown as follows: 

Worksheet 1A - General Information and Input D

General Information 

ata for Rural Two-Lane Two-Way Roadway Segments 

Location Information 

Analyst 

Agency or Company 

Date Performed Analyst 

Roadway 

Roadway Section 

Jurisdication 

Analysis Year 

SH 111 

MP 0.0 to MP 0.5 

Study Site 

2015 

Input Data Base Conditions Site Conditions 

Length of segment, L (mi) -­ 0.5 

AADT (veh/day) 

AADTMAX = 17,800 

(veh/day) -­ 11,000 

Lane width (ft) 12 12 

Shoulder width (ft) 6 Right 

Shld: 6 Left 

Shld: 6 

Shoulder type Paved Right 

Shld: Paved Left 

Shld: Paved 

Length of horizontal curve (mi) 0 0.038 

Radius of curvature (ft) 0 380 

Spiral transition curve (present/not present) Not Present Not Present 

Superelevation variance (ft/ft) < 0.01 0 

Grade (%) 0 0 

Driveway density (driveways/mile) 5 5 

Centerline rumble strips (present/not present) Not Present Not Present 

Passing lanes [present (1 lane)/present 

(2 lane)/not present)] 

Not Present Not Present 

Two-way left-turn lane (present/not present) Not Present Not Present 

Roadside hazard rating (1-7 scale) 3 3 

Segment lighting (present/not present) Not Present Not Present 

Auto speed enforcement (present/not present) Not Present Not Present 

Calibration Factor, Cr 1 0.97 

The applicable input data for the segment of the proposed alternative curve section is demonstrated 

in the following graphic. 

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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Worksheet 1A - General Information and Input D

General Information 

ata for Rural Two-Lane Two-Way Roadway Segments 

Location Information 

Analyst 

Agency or Company 

Date Performed Analyst 

Roadway 

Roadway Section 

Jurisdication 

Analysis Year 

SH 111 

MP 0.0 to MP 0.5 

Study Site 

2015 

Input Data Base Conditions Site Conditions 

Length of segment, L (mi) -­ 0.5 

AADT (veh/day) 

AADTMAX =17,800 

(veh/day) -­ 11,000 

Lane width (ft) 12 12 

Shoulder width (ft) 6 Right 

Shld: 6 Left 

Shld: 6 

Shoulder type Paved Right 

Shld: Paved Left 

Shld: Paved 

Length of horizontal curve (mi) 0 0.044 

Radius of curvature (ft) 0 250 

Spiral transition curve (present/not present) Not Present Not Present 

Superelevation variance (ft/ft) < 0.01 0 

Grade (%) 0 0 

Driveway density (driveways/mile) 5 5 

Centerline rumble strips (present/not present) Not Present Not Present 

Passing lanes [present (1 lane)/present 

(2 lane)/not present)] 

Not Present Not Present 

Two-way left-turn lane (present/not present) Not Present Not Present 

Roadside hazard rating (1-7 scale) 3 3 

Segment lighting (present/not present) Not Present Not Present 

Auto speed enforcement (present/not present) Not Present Not Present 

Calibration Factor, Cr 1 0.97 

STEP 2: Following input in Step 1, the spreadsheet tools automatically calculate the predicted number 

of crashes. The following tables show the results for the exiting condition and the proposed alternative. 

Existing:

 Worksheet 1B - Crash Modification Factors for Rural Two-Lane Two-Way Roadway Segments 

(1) (2) (3) (4) (5) (6) (7) 

CMF for Lane 

Width 

CMF for 

Shoulder Width 

and Type 

CMF for 

Horizontal 

Curves 

CMF for Superelevation CMF for 

Grades 

CMF for 

Driveway 

Density 

CMF for 

Centerline 

Rumble Strips 

CMF 1r CMF 2r CMF 3r CMF 4r CMR 5r CMF 6r CMF 7r 

from Equation 

10-11 

from 

Equation 10-12 

from 

Equation 

10-13 

from Equations 

10-14, 10-15, or 

10-16 

from Table 

10-11 

from 

Equation 

10-17 

from Section 

10.7.1 

1.00 1.00 4.58 1.00 1.00 1.00 1.00 

99
Worksheet 1E - Summary Results for Rural Two-Lane Two-Way Roadway Segments 

(1) (2) (3) (4) (5) 

Crash Severity 

Distribution 

(proportion) 

Predicted Average 

Crash Frequency 

(crashes/year) Roadway 

Segment Length 

Crash Rate 

(crashes/mi/ 

year) 

Crash Severity Level (4) from Worksheet 1C (8) from Worksheet 1C (mi) (3)/(4) 

Total 1.000 6.53 0.5 13.1 

Fatal and Injury (FI) 0.321 2.10 0.5 4.2 

Property Damage 

Only (PDO) 0.679 4.44 0.5 8.9 

Worksheet 1E - Summary Results for Rural Two-Lane Two-Way Roadway Segments 

(1) (2) (3) (4) (5) 

Crash Severity 

Distribution 

(proportion) 

Predicted Average 

Crash Frequency 

(crashes/year) Roadway 

Segment Length 

Crash Rate 

(crashes/mi/ 

year) 

Crash Severity Level (4) from Worksheet 1C (8) from Worksheet 1C (mi) (3)/(4) 

Total 1.000 8.13 0.5 16.3 

Fatal and Injury (FI) 0.321 2.61 0.5 5.2 

Property Damage 

Only (PDO) 0.679 5.52 0.5 11.0 

SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Proposed:

 Worksheet 1B - Crash Modification Factors for Rural Two-Lane Two-Way Roadway Segments 

(1) (2) (3) (4) (5) (6) (7) 

CMF for Lane 

Width 

CMF for 

Shoulder Width 

and Type 

CMF for 

Horizontal 

Curves 

CMF for Superelevation CMF for 

Grades 

CMF for 

Driveway 

Density 

CMF for 

Centerline 

Rumble Strips 

CMF 1r CMF 2r CMF 3r CMF 4r CMR 5r CMF 6r CMF 7r 

from Equation 

10-11 

from 

Equation 10-12 

from 

Equation 

10-13 

from Equations 

10-14, 10-15, or 

10-16 

from Table 

10-11 

from 

Equation 

10-17 

from Section 

10.7.1 

1.00 1.00 5.70 1.00 1.00 1.00 1.00 

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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

101

STEP 3: Summarize the results for predicted crashes. 

Configurations Total Predicted Crashes 

per Year 

Fatal and Injury Crashes per Year 

Existing Horizontal Curve 

Section (R = 380 ft) 

6.53 

(say 7) 

2.10 

(say 3) 

Alternative Horizontal 

Curve Section (R = 250 ft) 

8.13 

(say 9) 

2.61 

(say 3) 

Increase in Predicted 

Number of Crashes due to 

Design Exception 

8.13 - 6.53 = 1.60 

(say 2) 

2.61 – 2.10 = 0.51 

(say 1 FI crash every 2 years) 

Percent Increase in Predicted Number of Crashes 

due to Design Exception 

1.60/6.53×100%=24.5% 

(say 25%) 

0.51/2.10×100%=24.3% 

(say 25%) 

FINDINGS 

Interpreting the Results: 

The construction of a sharper horizontal curve at this location will result in approximately two more 

crashes per year with an additional fatal or injury crash predicted to occur approximately once every 

2 years. These changes correspond to an increase in crashes of approximately 25 percent at the 0.5­

mile section. The large percentage relative to the very small frequency can lead to an overstatement of 

the site conditions. The actual total number of crashes is expected to increase from 7 to 9 crashes per 

year. If deemed necessary, it may be feasible to “offset” this increase in crashes. The DOT may elect to 

acquire detailed crash data and evaluate whether additional safety treatments may help mitigate this 

increase in crashes. These treatments could include, for example, additional signing and marking. 

Possible Errors to Avoid: 

The HSM recommends using engineering judgment to assess whether combined CMFs reasonably 

represent an estimated crash frequency. Analysts should be cautious to only multiply CMFs that 

correspond to the correct baseline conditions and appropriate crash types and severity. The CMF 

Clearinghouse FAQ titled “How can I apply multiple CMFs” provides additional information and 

clarification (see http://www.cmfclearinghouse.org/faqs.cfm#q4). 

Alternative Analysis Approaches: 

If data are available, a detailed evaluation of observed crash severity could help to offer additional 

insights regarding corridor operations. In addition, the use of the CMF Relative Comparison 

approach, as noted during the safety assessment method selection, may be a convenient tool 

for quickly considering how varying site features may ultimately influence the percent increase or 

decrease in crashes for a candidate safety treatment. 

SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

103

CHAPTER 5. Urban Street Continuous Case Study 

Safety assessment methods can be incorporated into all phases of the project development process. 

To demonstrate how an agency could continue to assess safety throughout the various project 

stages, the following urban street case study, referred to as a continuous case study, provides 

example problems that answer the following questions: 

• How can the analyst estimate which, if any, of the road segments or intersections have more 

crashes than expected for a facility of this type? (Planning and Scoping) 

• How can the analyst compare the estimated crash frequency for the existing configuration to 

Option #1, Option #2, and Option #3? (Alternative Analysis) 

• How can the analyst estimate the annual percent reduction in crashes for installing a left-turn 

lane contrasted to installing a right-turn lane at Intersection #1? (Preliminary and Final Design) 

The detailed calculations for these questions are summarized in Sections 5.1, 5.2, and 5.3. 

5.1 Urban Street Continuous Case Study – Planning and Scoping 

PROBLEM OVERVIEW 

Safety Assessment Method: SPF with CMF Weighted with Observed Crashes 

Project Phase: Planning & Scoping Project Type: 3R 

Related Task: Establish Project Purpose and Calculation Method: 

Need  Hand Calculated  Tool Based 

Comments: 

This example problem demonstrates how to estimate the number of expected crashes for an urban 

arterial and calculate excess crash conditions. 

LEVEL OF ANALYSIS

 Basic Intermediate Advanced 

Note: See table 3 for a full definition of Project Type designations. 

PROBLEM DESCRIPTION 

As part of planning and scoping activities, a transportation agency has identified an urban street 

targeted for renovation that experiences multiple-vehicle crashes involving vehicles turning left. The 

specific section of the urban street includes two segments and two signalized intersections. The street 

has a narrow divided median but does not have any left-turn lanes. The section is 1.85 miles in length 

and passes through a community that consists of commercial development near multiple-family 

residential dwellings, as shown on the aerial photo. The associated Project Type is a 3R project and the 

Related Task is to Establish Project Purpose and Need. How can the analyst estimate which, if any, of 

the road segments or intersections have more crashes than expected for a facility of this type? 
Intersection #1 Intersection #2 

Segment #1 Segment #2 

Source: ©Google Earth 

SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Summary of Available Data: 

Segment Data 

The existing urban corridor has the following segment characteristics: 

• Roadway is a four-lane divided urban arterial with parallel parking along the entire corridor. 

• Adjacent land use is commercial / industrial. 

• Median width = 10 ft. 

• Street lights present along corridor. 

• No automated speed enforcement. 

• Posted speed limit = 40 mph. 

• Estimated number of roadside fixed objects = 100 per mile. 

• Typical offset to roadside fixed objects = 10 ft.
 

Characteristics unique to each segment are summarized in the following table:
 

Roadway Segment Characteristic Segment 1 Segment 2 

Segment Length (miles) 1.1 0.75 

Annual average daily traffic (vehicles per day) 

[3-year average value] 

13,300 11,500 

Commercial driveway count 1 major, 17 minor 3 major, 7 minor 

Industrial/institutional driveway count 2 major, 0 minor 2 major, 0 minor 

Residential driveway count 1 major, 0 minor 0 

Other driveway count 0 0 

Note: For the purposes of this example, a major driveway is assumed to have a minimum of 10 vehicles per hour during 

the peak periods. 

Segment crashes for the 3-year study period are shown as follows: 

Crash Type/ 

Location 

Observed Crash Frequency (crashes/yr) 3-Year Average for Observed 

Year 1 Year 2 Year 3 Crash Frequency (crashes/yr) 

Multiple-vehicle non-driveway 

Segment 1 5 7 6 6 

Segment 2 2 1 3 2 

Single-vehicle 

Segment 1 3 3 3 3 

Segment 2 2 4 0 2 

Multiple-vehicle driveway-related 

Segment 1 1 3 2 2 

Segment 2 2 1 0 1 

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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

105

Intersection Data 

The two public intersections located along the corridor have the following common characteristics: 

• Each intersection is a four-leg signalized intersection with permissive phasing on all approaches 

and no turn lanes. 

• Street lights present. 

• Right-turn-on-red permitted for all approaches. 

• Intersection red-light cameras are not present. 

• Sum of pedestrian crossings per hour = 10. 

• Maximum number of lanes crossed by pedestrian = 4. 

• Number of bus stops within 1000 ft. = 2. 

Characteristics unique to each intersection are summarized in the following table: 

Signalized Intersection Characteristic Intersection 1 Intersection 2 

Major-road annual average daily traffic (vehicles per day) 

[3-year average value] 

13,300 11,500 

Minor-road annual average daily traffic (vehicles per day) 

[3-year average value] 

8,800 9,600 

Number of Bus Stops within 1,000 ft. of the Intersection 11 7 

Schools within 1000 ft. of the Intersection Present Not present 

Number of Alcohol Sales Establishments within 1,000 ft. of 

the Intersection 

0 1 

Intersection crashes for the 3-year study period are shown as follows: 

Crash Type/ 

Location 

Observed Crash Frequency (crashes/yr) 3-Year Average for Observed Crash 

Year 1 Year 2 Year 3 Frequency (crashes/yr) 

Multiple-vehicle 

Intersection 1 3 2 4 3 

Intersection 2 2 2 2 2 

Single-vehicle 

Intersection 1 0 1 2 1 

Intersection 2 0 0 0 0 

ANALYSIS 

Selecting Appropriate Safety Assessment Method(s): 

The analyst can first inspect Table 5 to identify potential safety assessment methods for a 3R project 

type and the Establish Project Purpose and Need task. Five potential methods may be considered for 

this analysis: two basic, two intermediate, and one advanced. Because the goal of this analysis is to 

assess whether the corridor experiences more crashes than would be expected for a facility of this 

type, a Site Evaluation or Audit would not provide this type of crash-specific information. The analyst 

plans to use the safety performance functions (SPF) from the Highway Safety Manual (HSM) for this 

assessment, so the AADT-Only SPF method can be removed from consideration. The Historical Crash 
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Data Evaluation can be used to identify the type and location of crashes at the site, but does not 

provide information related to the number of crashes that could be expected at a similar facility 

and so this method is also removed from consideration. The two remaining methods can be used 

for the analysis. 

The transportation agency noted the possibility that left-turn maneuvers may be an issue, and so 

the analyst selects the SPF with CMF Weighted with Observed Crashes option for this evaluation. 

This method enables subsequent evaluation of roadway characteristics, if needed, as the project 

development process progresses while also considering the crash history for the study corridor. The 

SPF with CMF Adjustment method, though not selected, is also a viable safety assessment method for 

this evaluation because it does allow the calculation of predicted crashes for similar facility types that 

could then be compared to the observed crashes. 

Site Evaluation or Audit 

Historical Crash Data Evaluation 

AADT Only SPF 

SPF with CMF Adjustment 

SPF with CMF Weighted with 

Observed Crashes 

Establish Project 

Purpose and Need 3R 

The SPF with CMF Weighted with Observed Crashes method results in expected crash information 

and can be used for estimating the future performance of an existing facility or the future impact of 

minor geometric changes to an existing road (see Table 1). To most effectively use this approach, 

an agency should calibrate the SPF for its local jurisdiction. A calibration factor of 1.0 can be used if 

this information is not available, but the results will not be refined to local conditions. The results can, 

however, be used for comparative purposes. 

After the expected number of crashes is calculated, a variety of analysis 

approaches can be used to then evaluate whether the corridor is overrepresented by crashes. For this assessment, the analyst will calculate the 

excess expected average crash frequency by comparing the number of 

expected crashes (unique to the study corridor) to the predicted number of 

crashes (representing roads with similar characteristics to the study corridor). 

Linkage to the HSM: 

The HSM can be used to estimate the number of predicted crashes for an 

urban and suburban arterial by applying the procedures introduced in 

HSM Chapter 12 (pp. 12-1 to 12-122). By determining predicted crashes, the 

analyst can estimate how many crashes may be estimated for a specific 

road type with varying road conditions. Once the predicted number of 

crashes is known, the expected number of crashes for a specific site can be 

calculated by applying the Empirical Bayes Method summarized in HSM, 

Part C (Volume 2), pp. A-15 to A-23. 

Calculated Expected 

Average Crash 

Frequency 

(FI and PDO crashes) 

Nexpected 

Calculate Average 

Crash Frequency 

(FI and PDO crashes) 

N spf 

Calculated Predicted 

Average Crash 

Frequency 

(FI and PDO crashes) 

Npredicted 

106
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

To further evaluate the calculated number of expected crashes, the analyst can then assess the 

various safety assessment performance measures summarized in Table 6 (based on HSM Table 4-1, 

p. 4-8). Because the selected Advanced safety assessment method (SPF with CMF Weighted with 

Observed Crashes) will result in the number of expected crashes, the analyst selects the excess 

expected crash frequency method to assess whether the crashes for the corridor exceed what can 

be typically estimated for a similar corridor. Additional information about this procedure is located in 

HSM Chapter 4 (p. 4-75 to 4-78). 

Detailed Analysis: 

The expected average crash frequency for the corridor segments and intersections can 

be calculated using the HSM “Smart Spreadsheets” available for download at: http://www. 

highwaysafetymanual.org/Pages/tools_sub.aspx#4. For this example problem, the analyst can use 

the “HSM prediction urban and rural arterials” spreadsheet tool. 

STEP 1: Input the data for each segment and intersection into the spreadsheet tool. 

The following graphic shows a representation of Worksheet 1A for Segment #1. This roadway segment 

worksheet includes input information similar to that shown in the HSM worksheet (see HSM p. 12-108). 

Segment #2 data is similarly input into a worksheet (not shown). 

Worksheet 1A - General Information and Input Data for Urban and Suburban Roadway Segments 

General Information Location Information 

Analyst 

Agency or Company 

Date Performed 

ABC 

DOT 

06/15/16 

Roadway 

Roadway Section 

Jurisdication 

Analysis Year 

Urban Corridor - Segment #1 

MP 1.0 to MP 2.1 

Small Town, USA 

2015 

Input Data Base Conditions Site Conditions 

Roadway type (2U, 3T, 4U, 4D, ST) -­ 4D 

Length of segment, L (mi) -­ 1.1 

AADT (veh/day) AADTMAX = 66,000 (veh/day) -­ -­ 13,300 

Type of on-street parking (none/parallel/angle) None Parallel (Comm/Ind) 

Proportion of curb length with on-street parking -­ 1 

Median width (ft) - for divided only 15 10 

Lighting (present / not present) Not Present Present 

Auto speed enforcement (present / not present) Not Present Not Present 

Major commercial driveways (number) -­ 1 

Minor commercial driveways (number) -­ 17 

Major industrial / institutional driveways (number) -­ 2 

Minor industrial / institutional driveways (number) -­ 0 

Major residential driveways (number) -­ 1 

Minor residential driveways (number) -­ 0 

Other driveways (number) -­ 0 

Speed Category -­ Posted Speed Greater than 

30 mph 

Roadside fixed object density (fixed objects / mi) 0 100 

Offset to roadside fixed objects (ft) [If greater than 

30 or Not Present, input 30] 

30 10 

Calibration Factor, Cr 1.00 1.00 

107
 

SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Data for each of the study intersections can then be input into Worksheet 2A (see HSM p. 12-113). The 

following graphic depicts a representation of the Intersection #1 worksheet. Intersection #2 data is 

similarly included into a worksheet (not shown). 

Workseet 1L - General Information and Input Data for Urban and Suburban Arterial Intersections 

General Information Location Information 

Analyst 

Agency or Company 

Date Performed 

ABC 

DOT 

06/15/16 

Roadway 

Intersection 

Jurisdication 

Analysis Year 

Urban Corridor

 Intersection #1 

Small Town, USA 

2015 

Input Data Base Conditions Site Conditions 

Intersection type (3ST, 3SG, 4ST, 4SG) – 4SG 

AADTMAJOR (veh/day) AADTMAX = 67,700 (veh/day) – 13,300 

AADTMINOR (veh/day) AADTMAX = 33,400 (veh/day) – 8,800 

Intersection lighting (present/not present) Not Present Present 

Calibration factor, Ci 1.00 1.00 

Data for unsignalized intersections only: – – 

Number of major-road approaches with left-turn 

lanes (0,1,2) 0 0 

Number of major-road approaches with right-turn 

lanes (0,1,2) 0 0 

Data for signalized intersections only: – – 

Number of approaches with left-turn lanes (0,1,2,3,4) 

[for 3SG, use maximum value of 3] 0 0 

Number of approaches with right-turn lanes 

(0,1,2,3,4) [for 3SG, use maximum value of 3] 0 0 

Number of approaches with left-turn signal phasing 

[for 3SG, use maximum value of 3] – 0 

Type of left-turn signal phasing for Leg #1 Permissive Permissive 

Type of left-turn signal phasing for Leg #2 – Permissive 

Type of left-turn signal phasing for Leg #3 – Permissive 

Type of left-turn signal phasing for Leg #4 

(if applicable) – Permissive 

Number of approaches with right-turn-on-red 

prohibited [for 3SG, use maximum value of 3] 0 0 

Intersection red light cameras (present/not present) Not Present Not Present 

Sum of all pedestrian crossing volumes (PedVol) – 

Signalized intersections only 10 

Maximum number of lanes crossed by a pedestrian 

(nlanesx) – 4 

Number of bus stops within 300 m (1,000 ft) of the 

intersection 0 2 

Schools within 300 m (1,000 ft) of the intersection 

(present/not present) Not Present Present 

Number of alcohol sales establishments within 300 

m (1,000 ft) of the intersection 0 0 

108
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

109

STEP 2: Tabulate the predicted crash frequency for each segment and intersection. 

The following graphics show representations of Worksheet 1L (see HSM p. 12-113) for Segment #1 and 

Worksheet 2L (see HSM p. 12-117) for Intersection #1. The analyst developed similar summaries (not 

shown) for Segment #2 and Intersection #2. 

Segment #1 Summary Results: 

General Information and Input Data for Urban and Suburban Arterial Intersections 

Crash Severity Level 

Predicted Average Crash 

Frequency Npredicted rs

(crashes/year) Roadway Segment 

Length, L(mi) 

Crash Rate 

(crashes/mi/year) 

(Total) from Worksheet 1K (2)/(3) 

Total 5.8 1.10 5.3 

Fatal and injury (FI) 1.6 1.10 1.5 

Property damage only (PDO) 4.2 1.10 3.8 

Intersection #1 Summary Results:

 Summary Results for Urban and Suburban Arterial Intersections 

1 2 

Crash Severity Level 

Predicted Average Crash Frequency Npredicted int 

(crashes/year) 

(Total) from Worksheet 2K 

Total 3.6 

Fatal and injury (FI) 1.2 

Property damage only (PDO) 2.4 

STEP 3: Calculate the expected number of crashes for each segment and intersection. 

The “Urban Site Total” worksheet tab in the spreadsheet can be used to summarize the predicted and 

observed crashes, apply the weighted adjustment factor, and calculate the expected average crash 

frequency. The summary results are depicted in the following representations for Worksheet 3A (see 

HSM p. 12-118). The highlighted values represent the historical crash data. 
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

 

 

- -

Worksheet 3A - Predicted Crashes by Severity and Site Type and Observed Crashes Using the SiteSpecific EB Method for Urban and Suburban Arterials 

1 2 3 4 5 6 7 8 

Collision Type/Site 

Type 

Predicted Average Crash Frequency 

Observed 

Crashes 

Npredicted 

(crashes/year) 

Overdispersion 

Parameter, k 

Weighted 

Adjustment, w 

Expected Average 

Crash Frequency 

Npredicted 

(TOTAL) 

Npredicted 

(FI) 

Npredicted 

(PDO) 

Equation A 5 

from Part C 

Appendix 

Equation A 4 from 

Part C 

Appendix 

ROADWAY SEGMENTS 

Multiple-vehicle non-driveway 

Segment 1 3.931 1.137 2.795 6 1.320 0.162 5.666 

Segment 2 2.199 0.643 1.557 2 1.320 0.256 2.051 

Single-vehicle 

Segment 1 1.231 0.194 1.037 3 0.860 0.486 2.141 

Segment 2 0.784 0.120 0.664 2 0.860 0.597 1.274 

Multiple-vehicle driveway-related 

Segment 1 0.546 0.155 0.391 2 1.390 0.568 1.174 

Segment 2 0.372 0.106 0.266 1 1.390 0.659 0.586 

INTERSECTIONS 

Multiple-vehicle 

Intersection 1 3.208 1.007 2.201 3 0.390 0.444 3.092 

Intersection 2 2.801 0.864 1.937 2 0.390 0.478 2.383 

Single-vehicle 

Intersection 1 0.248 0.073 0.175 1 0.360 0.918 0.310 

Intersection 2 0.230 0.071 0.159 0 0.360 0.924 0.212 

COMBINED 

(sum of column) 15.551 4.369 11.182 22 -­ -­ 18.889 

STEP 4: Calculate the excess crashes by segment or intersection. 

Based on the weighting of the observed and predicted crashes in Step 3, the analyst can calculate 

the excess expected average crash frequency to identify corridor segments or intersections with 

more than the expected number of crashes. Computing this measure requires the tabulation of the 

following quantities: 

• Predicted average crash frequency (crashes/year). 

• Expected average crash frequency (crashes/year). 

• Excess value. 

The excess value is calculated by subtracting the predicted average crash frequency from the 

expected average crash frequency. The following table summarizes the total crash statistics using this 

approach. 

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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Crash Type / Site Type 

Predicted Average 

Crash Frequency 

(crashes/yr) 

Expected Average 

Crash Frequency 

(crashes/yr) 

Excess Calculated as 

Expected minus Predicted 

(crashes/yr) 

Multiple-vehicle non-driveway 

Segment 1 3.9 5.7 1.8 

Segment 2 2.2 2.1 -0.1 

Single-vehicle 

Segment 1 1.2 2.1 0.9 

Segment 2 0.8 1.3 0.5 

Multiple-vehicle driveway-related 

Segment 1 0.6 1.2 0.6 

Segment 2 0.4 0.6 0.2 

Multiple-vehicle 

Intersection 1 3.2 3.1 -0.1 

Intersection 2 2.8 2.4 -0.4 

Single-vehicle 

Intersection 1 0.2 0.3 0.1 

Intersection 2 0.2 0.2 0.0 

Corridor Total 15.6 18.9 3.3 

FINDINGS 

Interpreting the Results: 

Based on the excess expected average crash frequencies calculated in Step 4, the largest excess of 

crashes is found to occur on Segment 1. The overall street section is found to experience 3 to 4 more 

crashes per year than would be predicted for a similar facility. 

Possible Errors to Avoid: 

The study location does not have left-turn lanes present but does include a 10-ft. median. This 

physical constraint requires the analyst to assume a segment roadway type that is a four-lane divided 

arterial (4D). Similarly, the parallel parking along the entire corridor length requires a value of 1 for the 

proportion of curb length with on-street parking. Because the curb length does not extend into the 

intersections, the total corridor length should not be used for determining this proportion value. 

Alternative Analysis Approaches: 

During the selection of the appropriate safety assessment method, the analyst also identified the 

SPF with CMF Adjustment as a candidate assessment method to consider. Because this method is 

classified as an Intermediate safety assessment method, the procedure results in predicted crashes 

for a facility type. The use of this alternative analysis method would require a safety assessment 

performance measure (see Table 6) other than the excess expected average crash value. The 

companion performance measure procedure to use with predicted crashes that will address the 

analyst’s question of over-represented locations is the Excess Predicted Average Crash Frequency 

using SPFs noted in the performance measures table. 

111
 

SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

5.2 Urban Street Continuous Case Study – Alternatives Analysis 

PROBLEM OVERVIEW 

Safety Assessment Method: SPF with CMF Adjustment 

Project Phase: Alternatives Analysis Project Type: 3R 

Related Task: Alternative Selection 

Calculation Method: 

 Hand Calculated  Tool Based 

Comments: 

This example problem demonstrates how to estimate the number of predicted crashes for alternative 

design options 

LEVEL OF ANALYSIS

 Basic Intermediate Advanced 

Note: See table 3 for a full definition of Project Type designations. 

PROBLEM DESCRIPTION 

The analyst next conducted the alternative evaluations task for the improvement project for the 

four-lane divided urban arterial corridor identified in Section 5.1. After determining that the corridor 

does experience a higher-than-expected crash frequency, the analyst examined the crash 

predictions more closely to evaluate low-cost redesign options that could be implemented within 

the current right-of-way limits. The initial study identified Segment #1 as the section of road with the 

greatest number of expected crashes compared to predicted crashes, so the analyst is focused on 

alternatives that can be applied to that 1-mile segment. 

The alternatives currently under consideration include: 

• Option 1: Reduce on-street parking to 50 percent of the section’s curb length. 

• Option 2: Reduce the number of roadside objects to no more than 50 objects per mile. 

• Option 3: Combine Option 1 and Option 2 (reduce on-street parking and remove roadside objects). 

The Project Type is 3R and the Related Task is Alternative Selection. How can the analyst compare 

the estimated crash frequency for the existing configuration to Option #1, Option #2, and Option #3? 

Summary of Available Data: 

The site data is the same as that presented in the Section 5.1 data summary. The modifications are 

expected to occur in 2 years, and at that time the AADT value is projected to increase moderately 

from the current 13,300 vpd value to 13,725 vpd. 

ANALYSIS 

The analyst can first review the potential safety assessment methods shown in Table 8. The Alternative 

Selection task and the 3R project type are associated with five safety assessment methods: two basic, 

two intermediate, and one advanced. For this evaluation, the analyst intends to compare the number 

112
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

113

of estimated crashes for three low-cost options and contrast these values to the number of crashes 

for the current segment. Since a CMF-based method enables the specific consideration of a change 

in road characteristics, the safety assessment methods that use CMFs are applicable for this analysis. 

Traffic volume information is also available, so a safety assessment method that is volume-based can 

be used. Based on these considerations, the AADT-Only SPF method, which does not directly capture 

changes in road characteristics, can be eliminated from further consideration. The analyst would like 

to incorporate both the calculations conducted for the initial assessment as well as the moderate 

increase in traffic volume. Consequently, the analyst eliminates the methods that do not use an SPF 

(i.e., CMF Applied to Observed Crashes and CMF Relative Comparison). 

The two remaining safety assessment methods include SPF with CMF Adjustment and SPF with CMF 

Weighted with Observed Crashes. The analyst may elect to use one or both of these methods. 

Because the modifications can be expected to change the future number of crashes at the site, 

the analyst selects the SPF with CMF Adjustment method so that weighting with observed crashes 

for a modified roadway does not introduce unexpected biases. The SPF with CMF Weighted with 

Observed Crashes method can be used to evaluate the future impact of minor geometric changes 

to an existing road (per Table 1), but since the threshold of “minor geometric changes” can vary, the 

analyst elects not to use this particular method. 

Ultimately, the analyst selects the SPF with CMF Adjustment method for the assessment. To use 

this analysis method most effectively, an agency should calibrate the SPF for its local jurisdiction. 

A calibration factor of 1.0 can be used if this information is not available, but the results will not be 

refined to location conditions. For the purposes of this assessment, the SPF method can be used for 

comparison. 

Linkage to the HSM: 

The HSM can be used to estimate the number of predicted crashes for 

an urban and suburban arterial by applying the procedures introduced 

in HSM Chapter 12 (pp. 12-1 to 12-122). By determining predicted crashes, 

the analyst can estimate how many crashes may be associated with 

a specific road type with varying road conditions. Once the predicted 

number of crashes is known, the analyst can compare the estimated safety 

performance of the varying options to identify optimal designs. 

CMF Applied to Observed Crashes 

CMF Relative Comparison 

AADT Only SPF 

SPF with CMF Adjustment 

SPF with CMF Weighted with 

Observed Crashes 

Alternative Selection 3R 

Predict Segment 

Average Crash 

Frequency for Base 

Conditions 

N spf 

Predict Segment 

Specific Average 

Crash Frequency for 

Alternative Options 

Npredicted 
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

Detailed Analysis: 

The predicted average crash frequency for the corridor segments and intersections can be calculated 

using the HSM “Smart Spreadsheets” available for download at: http://www.highwaysafetymanual. 

org/Pages/tools_sub.aspx#4. For this example problem, the analyst can use the “HSM prediction urban 

and rural arterials” spreadsheet tool. 

STEP 1: Input the data for the study segment (referred to as Segment #1 in Section 5) into the 

spreadsheet tool. 

This evaluation should use the future AADT value of 13,725 vpd. The following graphic shows a 

representation of Worksheet 1A for Segment #1, Option #3. This roadway segment worksheet includes 

input information similar to that shown in the HSM worksheet (see HSM p. 12-108). Information for 

existing Segment #1 conditions, Option 1, and Option 2 are similarly input into a worksheet (not shown).

 General Information and Input Data for Urban and Suburban Roadway Segments 

General Information Location Information 

Analyst 

Agency or Company 

Date Performed 

ABC 

DOT 

06/15/16 

Roadway 

Intersection 

Jurisdication 

Analysis Year 

Urban Corridor-Segment #1 

MP 1.0 to MP 2.1 

Small Town, USA 

2015 

Input Data Base Conditions Site Conditions 

Intersection type (2U, 3T, 4U, 4D, ST) – 4D 

Length of segment, L (mi) – 1.1 

AADT (veh/day) AADTMAX = 66,000 (veh/day) – 13,725 

Type of on-street parking (none/parallel/angle) None Parallel (Comm/Ind) 

Proportion of curb length with on-street parking – 0.5 

Median width (ft) - for divided only 15 10 

Lighting (present / not present) Not Present Present 

Auto speed enforcement (present / not present) Not Present Not Present 

Major commercial driveways (number) – 1 

Minor commercial driveways (number) – 17 

Major industrial / institutional driveways (number) – 2 

Minor industrial / institutional driveways (number) – 0 

Major residential driveways (number) – 1 

Minor residential driveways (number) – 0 

Other driveways (number) – 0 

Speed Category – Posted Speed Greater than 

30 mph 

Roadside fixed object density (fixed objects / mi) 0 50 

Offset to roadside fixed objects (ft) [If greater than 30 

or Not Present, input 30] 30 10 

Calibration Factor, Cr 1.00 1.00 

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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

STEP 2: Calculate the number of predicted crashes for the study year. 

At the completion of Step 1, the spreadsheet tools automatically calculated the predicted number 

of crashes for the existing conditions and for the three candidate options. To review example results 

for the intersection calculations, see the summary results of the predicted crashes for Option 3 

(shown in Worksheet 1L). The analyst calculated similar summary results (not shown) for the Existing 

configuration, Option 1, and Option 2 (using the AADT value of 13,725 vpd as previously noted). 

Worksheet 1L - General Information and Input Data for Urban and Suburban Roadway Segments 

Crash Severity Level 

Predicted Average Crash 

Frequency Npredicted rs

(crashes/year) Roadway Segment 

Length, L(mi) 

Crash Rate 

(crashes/mi/year) 

(Total) from Worksheet 1K (2)/(3) 

Total 4.2 1.10 3.8 

Fatal and injury (FI) 1.2 1.10 1.1 

Property damage only (PDO) 3.0 1.10 2.8 

STEP 3: Summarize Findings. 

The analyst’s ultimate goal is to assess how much the three options have the potential to reduce the 

number of crashes predicted for the study segment 2 years into the future (when the AADT is 13,725 

vpd). In addition to the total number of predicted crashes, the crash severity information is important 

to note. The following table summarizes these results. 

Roadway 

Improvement 

Scenario 

Predicted 

Number of FI 

Crashes 

(crashes/yr) 

Potential 

Reduction in FI 

Crashes 

(crashes/yr) 

Percent 

Reduction in 

FI Crashes 

Predicted 

Number of 

Total Crashes 

(crashes/yr) 

Potential Reduction 

in Total Crashes 

(crashes/yr) 

Percent 

Reduction in 

Total Crashes 

Existing 

Configuration 1.7 N/A N/A 6.1 N/A N/A 

Option 1 – Reduce 

on-street parking 

by 50% 

1.3 1.7 – 1.3 = 0.4 23.5% 4.8 6.1 – 4.8 = 1.3 21.3% 

Option 2 – Reduce 

the number of 

roadside objects 

to 50 per mile 

1.5 1.7 – 1.5 = 0.2 11.8% 5.3 6.1 – 5.3 = 0.8 13.1% 

Option 3 – 

Reduce on-street 

parking by 50% 

and reduce 

the number of 

roadside objects 

to 50 per mile 

1.2 1.7 – 1.2 – 0.5 29.4% 4.2 6.1 – 4.2 = 1.9 31.1% 

Note: N/A = not applicable. 

115
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

FINDINGS 

Interpreting the Results: 

Based solely on an evaluation of the predicted number of crashes, reducing the on-street parking to 

approximately 50 percent of the curb length (Option #1) results in a 21.3 percent total crash reduction. 

If the number of roadside objects is reduced to 50 per mile (from the current 100 per mile) and the 

road is not otherwise modified (Option #2), the reduction in total number of crashes can be estimated 

to be approximately 13.1 percent. For the alternative that reduces the on-street parking and the 

roadside object density (Option #3), the reduction in the total number of crashes can be similarly 

estimated as a 31.1 percent. The number of fatal and injury crashes for the existing roadway segment 

is equivalent to less than two per year. This value is based on SPFs that have not been calibrated to 

the region. For all three options, the reduction in the number of fatal and injury crashes is similar to the 

trend observed for total crashes with reductions of 23.5 percent , 11.8 percent, and 29.4 percent for 

Option 1, 2, and 3 respectively. 

For the conditions outlined in this problem, the minor modifications to the corridor appear to result in 

modest crash reductions. The number of predicted crashes can be used as input into a cost benefit 

study to assess whether the investment is economically justified. The analyst should use caution when 

assessing the results of this analysis due to the small number of crashes (less than 10). 

These calculations are based only on predicted crash performance, but do not consider potential 

operational issues. For example, limiting the on-street parking can potentially provide additional 

operational benefits to the adjacent travel lane. 

Possible Errors to Avoid: 

The HSM procedures require the analyst to understand how SPFs and CMFs equate to the base 

conditions associated with the procedure. Incorrect use of these values can introduce erroneous 

results. This example compared the crash predictions, so a calibration factor with a value of 1.0 can 

be used for comparison purposes. If the analyst would like to record findings as crash frequency 

instead of percent reduction in the number of crashes, calibrated SPFs with known calibration factors 

should be used for the analysis. A more detailed analysis during the design phase would provide 

additional useful information about predicted crashes. 

Alternative Analysis Approaches: 

A common method for evaluating design alternatives is to use CMF comparisons that do not directly 

consider traffic volume. This approach is less reliable than the predictive methods, but can be used 

to evaluate alternatives when an SPF is not available for the condition or when a simple comparison 

between two alternatives is all that is needed. 

The analyst can also use the advanced safety assessment method to further evaluate existing 

conditions and future designs with minor changes. Future configurations that include major changes 

are not suitable for the SPF with CMF Weighted with Observed Crashes method as the observed 

crashes would no longer be representative of the new configuration. 

116
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

117

5.3 Urban Street Continuous Case Study – Preliminary & Final Design 

PROBLEM OVERVIEW 

Safety Assessment Method: CMF Relative Compariso

Project Phase: Preliminary & Final Design Project Type: 3R 

Related Task: Selecting Specific Design Calculation Method: 

Elements and Their Dimension  Hand Calculated 

Comments: 

n 

 Tool Based 

This example problem demonstrates how to calculate the predicted number of crashes for alternative 

design options at an urban arterial. 

LEVEL OF ANALYSIS

 Basic Intermediate Advanced 

Note: See table 3 for a full definition of Project Type designations. 

PROBLEM DESCRIPTION 

This problem is based on the scenario for Segment #1 as shown in Section 5.2. 

During the project development design phase, the transportation agency notified the analyst that 

on-street parking will be reduced by 50 percent for the entire corridor. This design change will result 

in complete removal of on-street parking at the intersection approaches for the corridor described 

in Section 5.1. The transportation agency intends to use the “recovered” space to make room for 

turn lanes at Intersection #1. The turn lanes will only be added on the primary corridor approaches 

(and not on the intersecting streets). The Project Type is 3R and the Related Task is Selecting Specific 

Design Elements and Their Dimensions. How can the analyst estimate the annual percentage 

reduction in crashes for installing a left-turn lane compared with the estimated reduction from 

installing a right-turn lane at Intersection #1? 

Summary of Available Data: 

The site data is the same as that presented in the Section 5.1 data summary. Right-turn-on-red will 

continue to be permitted on all approaches. 

ANALYSIS 

Selecting Appropriate Safety Assessment Method(s) 

The analyst can first review the potential safety assessment methods shown in Table 9. The Selecting 

Specific Design Elements and Their Dimensions task and the 3R project type are associated with all 

seven candidate safety assessment methods. For this evaluation, the analyst intends to compare 

the estimated percent reduction in crashes for the two turn-lane options, so a CMF-based method 

that considers varying geometric characteristics is needed. As a result, the analyst may eliminate 

the Site Evaluation or Audit, the Historical Crash Data Evaluation, and the AADT-Only SPF safety 

assessment methods from further consideration. The addition of a turn lane at Intersection #1 is 

a minor geometric change, so any of the remaining four methods should be suitable for the turn 
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

lane analysis. Because all four of the CMF-based methods should provide similar results for this 

comparison, the analyst selects the basic method that does not require extensive data – CMF 

Relative Comparison. The analyst could also have used one of the remaining SPF-based methods 

with minimal additional effort as a continuation of the previous calculations conducted, as illustrated 

in Sections 5.1 and 5.2. 

Site Evaluation or Audit 

Historical Crash Data Evaluation 

CMF Applied to Observed Crashes 

CMF Relative Comparison 

AADT Only SPF 

SPF with CMF Adjustment 

SPF with CMF Weighted with 

Observed Crashes 

Alternative Selection 3R 

Linkage to the HSM: 

Chapter 12 (p. 12-24 to 12-26) and Chapter 14 (p. 14-23 to 14-26) of the HSM, as well as the FHWAsponsored CMF Clearinghouse (www.cmfclearinghouse.org) collectively include a wide variety of 

CMFs for varying turn lane configurations. These CMF resources identify the base conditions and 

applicable site applications for the individual countermeasure of interest. 

Detailed Analysis: 

The CMF Relative Comparison safety assessment method can be used to compare potential 

countermeasures or treatments to identify the treatment most likely to have the greatest impact on 

reducing crashes. The only data requirement for this basic safety assessment method is the value for 

each CMF representing the candidate treatments for the same “before” characteristics and crash 

types. 

To perform this assessment, the analyst reviews the CMF values for the two turn-lane options and 

compares their relative values. The information for the two CMFs is summarized in the following 

table. Recall that a CMF value less than 1.0 is associated with a larger reduction in future crashes 

when compared to a CMF with a value equal to 1.0 (assumed to have no real effect on reducing 

crashes). Based on this simple comparison, the analyst concludes that the recommended 

treatments, in order of priority, should be: 

1.	 Install a left-turn lane at the signalized intersection (associated with a CMF value of 0.81 or 

an estimated 19 percent reduction in crashes). 

2.	 Install a right-turn lane at the signalized intersection (associated with a CMF of 0.92 or an 

estimated 8 percent reduction in crashes for this treatment). 

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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

Proposed Treatment 

CMF 

(S.E.) 

Crash Type 

(Base Condition) 

Crash 

Severity Source 

Install left-turn lane 

on two signalized 

intersection 

approaches 

0.81 

(0.13) 



All crash types 

and roadway 

types 

All 

HSM Table 12-24, p. 12-43, 

HSM Table 14-12, p. 14-23, and 

CMF Clearinghouse at 

http://www.cmfclearinghouse.org/detail. 

cfm?facid=270#commentanchor 

Install right-turn lane 

on two signalized 

intersection 

approaches 

0.92 

(0.03) 



All crash types 

and rural roadway types 

All 

HSM Table 12-26, p. 12-44, 

HSM Table 14-15, p. 14-26, and 

CMF Clearinghouse at 

http://www.cmfclearinghouse.org/detail. 

cfm?facid=290#commentanchor 

Note:  = CMF Clearinghouse Star Rating. CMF = crash modification factor. HSM = Highway Safety Manual. S.E. = 

standard error. 

FINDINGS 

Interpreting the Results: 

Based on the relative comparison of CMFs method, the analyst concluded that the addition of a 

left-turn lane on the two major approaches for Intersection #1 is a more effective option for reducing 

crashes than adding a right-turn lane. This assessment is based on the historic crash performance 

of left-turn and right-turn lanes and their associated CMFs and does not account for design features 

such as length of turn lane or operational features including consideration of turning volumes. A 

comprehensive assessment that addresses these issues should be performed as well during the 

design phase of the project development process. 

Possible Errors to Avoid: 

A wide variety of CMF values are available for turn lanes. The selection of appropriate CMFs should 

include verification of appropriate base conditions, confirmation of consistent crash types between 

compared CMFs, and selection of higher quality CMFs based on small standard error values or higher 

star ratings (if using the CMF Clearinghouse). A common error associated with selection of CMFs is 

the selection of values that do not have applicable “before” conditions. 

Alternative Analysis Approaches: 

For this analysis, four candidate safety assessment methods emerged as viable options for the 

analysis. For the three methods that were not selected, the CMF value is multiplied by the observed 

or the predicted number of crashes. Though these techniques will result in numeric answers that 

generally represent the estimated reduction in crashes, they will all provide a similar answer to 

the analyst’s question providing that the same CMF values are used for all of the evaluations. For 

this reason, any of the four CMF-based safety assessment methods is suitable for this analysis. The 

analyst selected the CMF Relative Comparison method based on the comparative nature of the 

question and recognized that, for this condition, this simple approach would provide similar findings 

as one of the more complex analysis methods and could be performed quickly with minimal data 

requirements. 

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Intersection Calculations (NSPF int ): 

Appendix.
 

Alternative Calculations for Select Example Problems
 

A-3.2 Hand Calculated Example — Predicting Crash Frequency for 

Alternative Intersection Turn-lane Options at Four-Lane, Rural, Undivided 

Highways 

PROBLEM OVERVIEW 

Safety Assessment Method: SPF with CMF Adjustment 

Project Phase: Alternative Analysis Project Type: 2R 

Related Task: Alternative Selection 

Calculation Method: 

 Hand Calculated  Tool Based 

Tool based example included in Problem 3.2. 

Comments: 

This example problem demonstrates how to conduct a safety assessment for an alternative design 

for a rural, multilane highway and compare the estimated safety improvement to the existing 

configuration. 

LEVEL OF ANALYSIS

 Basic Intermediate Advanced 

Note: See table 3 for a full definition of Project Type designations. 

Detailed Analysis: 

This example demonstrates hand calculations for Problem 3.2 and Alternative #1. The full problem 

in Chapter 3 includes two alternative options. For this assessment, the analyst can use the SPF with 

CMF Adjustment method. This will provide the number of predicted crashes (estimated for a type of 

facility). The SPFs can be locally derived or the analyst can use the Highway Safety Manual (HSM) 

equations. If the analyst uses the safety performance functions (SPF) from the HSM, they should be 

calibrated for local conditions where possible. To perform a relative comparison between two options 

for the same location, this procedure can be used and a calibration value of 1.0 assumed. 

This hand-calculated example uses the rural undivided multilane predictive procedures located in 

Chapter 11 of the HSM. 

STEP 1: Calculate the predicted average total crash frequency and fatal and injury frequency for the 

intersection and major road segment base conditions (NSPF). 

From the HSM, use the SPF equation for intersection-related crashes at undivided rural multi-lane 

highways to calculate the average intersection total crash frequency. This base condition SPF applies 

to the existing configuration as well as the alternative. 

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Calculations Notes 

Total Crashes: 

Nspf int (Total)=exp[-10.008 

+(0.848 ×ln(AADTmaj )) 

+(0.448×ln(AADTmin ))] 

HSM Equation 11-11, p. 11-21 with 4ST 

Total (4-lane stop on minor) constants 

from HSM Table 11-7, p. 11-22 

Nspf int (Total)=exp[-10.008+(0.848 ×ln(30,000)) 

+(0.448 ×ln(5,000))]=12.80 Answer (Total Crashes) 

Fatal + Injury (FI) Crashes: 

Nspf int (Total)=exp[-11.554+(0.888×ln(AADTmaj)) 

+(0.525×ln(AADTmin ))] 

HSM Equation 11-11 with 4ST Fatal and 

injury constants from HSM Table 11-7, p. 

11-22 

Nspf int (FI)=exp[-11.554+(0.888 ×ln(30,000)) 

+(0.525 ×ln(5,000))]=7.94 

Answer (FI Crashes) 

Note: 

Nspf int (Total)= average total crash frequency for intersection-related crashes 

Nspf int (FI)= average fatal and injury crash frequency for intersection-related crashes 

AADTmaj= major road AADT (vehicles per day) 

AADTmin = minor road AADT (vehicles per day) 

FI = Fatality and injury. HSM = Highway Safety Manual. 

Segment Calculations (NSPF ru): 

The HSM equation for undivided roadway segments can be used to calculate the average segment 

crash frequency. This SPF applies to the existing design as well as the alternative intersection 

configuration. Recall that each approach segment is 0.19 miles for a total study length of 0.38 miles. 

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Calculations Notes 

Total Crashes: 

HSM Equation 11-7 with 4-lane total 

constants from HSM Table 11-3, p. 11-15 

Calculation for one major road 

approach segment 

Answer including Both Approaches 

(0.38 mile segment) 

FI Crashes: 

HSM Equation 11-7 with 4-lane Fatal and 

injury constants from HSM Table 11-3, p. 

11-15 

Calculation for one major road 

approach segment 

Answer including Both Approaches 

(0.38 mile segment) 

STEP 2: Calculate the predicted average crash frequency for the segment and intersection unique 

design conditions (Npredicted). 

The predicted average crash frequency (Npredicted) for each option can be calculated using the 

following equation (HSM Equation 11-1, p. 11-2): 

N = N predicted spf × (CMF1x× CMF2x ×...×CMF yx ) × Cx 

Where: CMF = crash modification factor/function and C = calibration factor for SPF 

[Notice that any countermeasure that matches base conditions will have a CMF value of 1.0 and 

does not change the value of N .] predicted

Predicted Average Crash Frequency (Nint) for Intersections 

The intersection-related CMFs that do not match base conditions are next included in the 

calculations. For this example, a calibration factor of 1.0 is assumed (assuming the HSM predictive 

equations are representative of local conditions). The required CMFs are: 

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Total Crashes Fatal and Injury 

Crashes 

Notes 

Left-turn Lane CMF: 

CMF 2i=0.52 CMF 2i=0.42 Use HSM Table 11-22, p. 11-34 for left-turn lanes 

(Alternative) 

Note: CMF = crash modification factor, CMF2i = left-turn lane CMF 

The predicted number of intersection-related crashes is then calculated by multiplying the CMFs and 

the calibration factor times the appropriate N value. spf int

Calculations Notes 

N(predicted int = N(spf int) × CMF 2i × C x 

Equations with turn lane CMF 

and the local calibration factor 

Total Crashes: 

Npredicted int(Existing-Total = 12.80 × 1.0 × 1.0 =12.80 Existing 

Npredicted int(Alt-Total= 12.80 × 0.52 × 1.0 = 6.66 Alternative 

FI Crashes: 

Npredicted int(Existing-Fi= 7.94 × 1.0 × 1.0 = 7.94 Existing 

Npredicted int(Alt-Fi= 7.94 × 0.42 × 1.0 = 3.33 Alternative 

Definitions: 

Note: 

C x = Local calibration factor (can assume Cx =1.0 for this calculation because evaluation is based on 

relative comparisons) 

CMF = crash modification factor 

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Segment Predicted Average Crash Frequency 

The segment-related CMFs that do not match base conditions are next included in the calculations. 

These are the required CMFs: 

Calculations Notes 

Lane Width CMFs: 

CMF 1ru= (CMF RA-1.0) × PRA +1.0 

HSM Equation 11-13, p. 11-26 with Table 

11-11 values and default value of 0.27 for 

PRA 

CMF 1ru= (1.00-1.0) × 0.27 + 1.0 = 1.00 ≥ 12’ Lanes (Existing) 

CMF 1ru= (1.04-1.0) × 0.27 + 1.0 = 1.01 11’ Lanes (Alternative) 

Shoulder Width CMFs: 

CMF 2ru= (CMFWRA × CMFTRA - 1.0) × PRA+1.0 

HSM Equation 11-14, p. 11-27 with Table 

11-12 and Table 11-13 values and default 

value of 0.27 for PRA 

CMF 2ru= (0.87 × 1.00 - 1.0) × 0.27 +1.0 = 0.96 8’ paved shoulder (Existing) 

CMF 2ru= (1.00 × 1.00 -1.0) × 0.27 + 1.0 = 1.00 6’ paved shoulder (Alternative) 

Note: 

CMF = crash modification factor 

CMF 1ru = Lane width CMF 

CMF 2ru = Shoulder width and type CMF 

CMF RA = for segment-related crashes (run-off-the-road, head-on, sideswipe) 

CMF WRA = for segment-related crashes based on shoulder width 

CMFTRA = for segment-related crashes based on shoulder type 

PRA = Proportion that segment-related crashes make up of all crashes (default is 0.27) 

The predicted number of segment-related crashes is then calculated by multiplying the CMFs and the 

calibration factor times the appropriate N value. spf ru

Calculations Notes 

Npredicted ru= Nspf ru × (CMF 1ru × CMF 2ru )× C 

Equations with lane and shoulder 

CMFs and the local calibration 

factor 

Total Crashes: 

Npredicted ru(Existing-Total)=4.49 × (1.0 × 0.96) × 1.0 = 4.31 Existing 

Npredicted ru(Alt-Total) = 4.49 × (1.01 × 1.0) × 1.0 = 4.53 Alternative 

Fatal and Injury Crashes: 

Npredicted ru(Existing-Fi)= 2.46 × (1.0 × 0.96) × 1.0 = 2.36 Existing 

Npredicted ru(Alt-Fi)= 2.46 × (1.01×1.0) × 1.0 = 2.48 Alternative 

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STEP 3: Add the intersection and segment crashes per year to calculate the total and FI predicted 

crashes (N ) . (predicted (Total) ) and (N(predicted (FI) 

Total Crashes per Year Fatal and Injury Crashes per Year 

Alternative Intersection 

Crashes 

Segment 

Crashes 

Predicted 

Crashes 

Intersection 

Crashes 

Segment 

Crashes 

Predicted 

Crashes 

Existing 12.80 4.31 17.11 

(say 18) 7.94 2.36 10.30 

(say 11) 

Alternative #1 6.66 4.53 11.19 

(say 12) 3.33 2.48 5.81 

(say 6) 

Difference 6.14 -0.22 5.92 

(say 6) 4.61 -0.12 4.49 

(say 5) 

Percent 

Reduction 48.0% -5.1% 34.6% 

(say 34%) 58.1% -5.1% 43.6% 

(say 43%) 

Manual calculations are shown only for total crashes. Calculations for fatal and injury (FI) and property 

damage only (PDO) crashes are calculated in a similar manner. The hand calculations include rounded 

values. The self-calculating spreadsheets do not truncate the numbers and so rounding errors are not 

included. Consequently, the hand calculated results may have minor differences than the spreadsheet 

calculated values. 

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A-3.4 Hand Calculated Example — Calculating Expected Crashes for 

Urban Freeway Ramps 

PROBLEM OVERVIEW 

Safety Assessment Method: SPF with CMF Weighted with Observed Crashes 

Project Phase: Alternative Analysis Project Type: 4R 

Related Task: Interchange Justification Request 

Calculation Method: 

 Hand Calculated  Tool Based 

Tool based example included in Problem 3.4. 

Comments: 

This example problem demonstrates how to estimate the expected number of crashes for an urban 

freeway loop ramp. 

LEVEL OF ANALYSIS

 Basic Intermediate Advanced 

Note: See table 3 for a full definition of Project Type designations. 

Detailed Analysis: 

This example demonstrates hand calculations for Problem 3.4. The full problem in Chapter 3 includes 

calculations that demonstrate the use of the free ISATe self-calculating “Smart Spreadsheet.” For this 

assessment, the analyst can use the SPF with CMF Weighted with Observed Crashes method. This 

assessment is for a specific location with a known crash history, so the expected number of crashes 

can be calculated. Because the ramp’s cross-sectional characteristics are consistent along its entire 

length, the ramp can be modeled as one segment. 

The following steps summarize the process for performing hand calculations to evaluate the expected 

crashes. 

STEP 1: Calculate the predicted average crash frequency, Nspf: 

Multiple-vehicle crashes: 

The HSM equations for multiple-vehicle FI and PDO crashes at freeway ramps are summarized as 

follows: 

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Calculations Notes 

Predicted Average Crash Frequency for Multiple-Vehicle Crash Types: 

HSM Equation 19-22, p. 19-31 with urban 

one-lane C-D road segment FI crash 

severity constants from HSM Table 19-7, 

p. 19-32. 

Approximately 0.164 FI multiple-vehicle 

crashes 

HSM Equation 19-22, p. 19-31 with urban 

one-lane C-D road segment PDO crash 

severity constants from HSM Table 19-7, 

p. 19-32. 

Approximately 0.321 PDO multiplevehicle crashes 

Note: HSM = Highway Safety Manual. PDO = property damage only. 

Single-vehicle crashes: 

The HSM equations for single-vehicle FI and PDO crashes at freeway ramps are summarized as follows: 

Calculations Notes 

Predicted Average Crash Frequency for Single-Vehicle Crash Types: 

HSM Equation 19-26, p. 19-34 with urban 

one-lane C-D road segment FI crash severity 

constants from HSM Table 19-10, p. 19-35. 

Approximately 0.149 FI single-vehicle crashes 

HSM Equation 19-26, p. 19-34 with urban onelane C-D road segment PDO crash severity 

constants from HSM Table 19-10, p. 19-35. 

Approximately 0.169 PDO single-vehicle 

crashes 

Note: FI = fatal and injury. HSM = Highway Safety Manual. PDO = property damage only. 

STEP 2: Apply the appropriate CMFs and calculate the predicted average crash frequency (Npredicted) 

Calculate CMF values for any conditions that differ from the base conditions. 

CMF for Horizontal Curve: 

The horizontal curve CMF requires an average entry speed (v ), the radius for the entry curve (Ri ), ent,i 

the proportion of the segment length with the curve (Pc, i ), and the number of horizontal curves in the 

segment (m). To calculate this CMF value, the limited curve speed (curve with the sharpest radius) must 

first be identified. 

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Calculations Notes 

Limited Curve speed 

Limiting curve speed for 

horizontal curve [HSM equation 

19-59, p. 19-67] 

Limiting curve speed for curve 1 

and curve 2 are 151.35 ft/s and 

41.27 ft/s, respectively 

Average entry speed at curve 1 

HSM equation 19-68, p.19-70 

Average exit speed at curve 1 

HSM equation 19-70, p.19-70 

Average entry speed at curve 2 

HSM equation 19-72, p.19-70 

From HSM Table 19-42, p. 19-67, 

Vcdroad=40 

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Calculations Notes 

CMF for horizontal curve 

HSM equation 19-33, p.19-47 

Note: HSM = Highway Safety Manual. 

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CMF for Lane Width: 

Calculations Notes 

CMF for Lane Width 

CMF 2, w, x, y, fi = exp (a × [W1-14]) HSM equation 19-34, p.19-48 

CMF 2, cds, 1, mv, fi = exp (-0.0458 × [13-14]) 

CMF 2, cds, 1, mv, fi = 1.047 

CMF 2, cds, 1, sv, fi = exp (-.0458 × [13-14]) 

CMF 2, cds, 1, sv, fi = 1.047 

CMF 2, cds, 1, mv, pdo =1.000 

CMF 2, cds, 1, sv, pdo =1.000 

HSM Table 19-25, p.19-48 

Note: CMF = crash modification factor. HSM = Highway Safety Manual. 

CMF for Shoulder Width: 

CMF for Right Shoulder Width 

CMF 3, w, x, y, z= exp (a × [Wrs-8]) HSM equation 19-35, p.19-48 

CMF 3, cds, 1, mv, fi = exp (-0.0539 × [10-8]) 

CMF 3, cds, 1, mv, fi = 0.897 

CMF 3, cds, 1, mv, pdo = exp (-0.0259 × [10-8]) 

CMF 3, cds, 1, mv, pdo= 0.949 

CMF 3, cds, 1, sv, fi= exp (-0.0539 × [10-8]) 

CMF 3, cds, 1, sv, fi = 0.897 

CMF 3, cds, 1, sv, pdo = exp (-0.0259 × [10-8]) 

CMF 3, cds, 1, sv, pdo = 0.949 

HSM Table 19-26, p.19-49 

Note: CMF = crash modification factor. HSM = Highway Safety Manual. 

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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

 

 

 

 

 

 

 

 

CMF for Left Shoulder Width 

CMF4, w, x, y, fi = exp (a × [Wls-4]) HSM equation 19-36, p.19-49 

CMF4, cds, 1, mv, fi = exp (-0.0539 × [7-4]) 

CMF4, cds, 1, mv, fi = 0.851 

CMF4, cds, 1, mv, pdo = exp (-0.0259 × [7-4]) 

CMF4, cds, 1, mv, pdo = 0.925 

CMF4, cds, 1, sv, fi = exp (-0.0539 × [7-4]) 

CMF4, cds, 1, sv, fi = 0.851 

CMF4, cds, 1, sv, pdo = exp (-0.0259 × [7-4]) 

CMF4, cds, 1, sv, pdo = 0.925 

HSM Table 19-27, p.19-49 

Note: CMF = crash modification factor. HSM = Highway Safety Manual. 

CMF for Roadside Barrier: 

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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

CMF for Right Side Barrier 

HSM equation 19-37, p.19-50 

HSM equation 19-74, p.19-71 

HSM Table 19-28, p.19-50 

HSM Table 19-28, p.19-50 

HSM Table 19-28, p.19-50 

HSM Table 19-28, p.19-50 

Note: CMF = crash modification factor. HSM = Highway Safety Manual. 

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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

 

CMF for Left Side Barrier 

HSM equation 19-38, 

p.19-51 

HSM equation 19-76, 

p.19-72 

HSM Table 19-29, p.19-51 

HSM Table 19-29, p.19-51 

HSM Table 19-29, p.19-51 

HSM Table 19-29, p.19-51 

Note: CMF = crash modification factor. HSM = Highway Safety Manual. 

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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

STEP 3: Calculate the predicted average crash frequency for the segment under unique design 

conditions (Npredicted ) 

Note: FI = fatal and injury. HSM = Highway Safety Manual. PDO = property damage only. 

135
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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

STEP 4: Calculate the expected crashes.
 

The next step is to evaluate the expected number of total crashes.
 

Note: HSM = Highway Safety Manual. 

Summary table: 

Note: FI = fatal and injury. PDO = property damage only. 
 

SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

A-4.1 Hand Calculated Example — Predicting Crashes for a New Urban 

Multilane Arterial 

PROBLEM OVERVIEW 

Safety Assessment Method: SPF with CMF Adjustment 

Project Phase: Preliminary & Final Design Project Type: NL 

Related Task: Selecting Specific Design 

Elements and Their Dimensions 

Calculation Method: 

 Hand Calculated  Tool Based 

Tool based example included in Problem 4.1. 

Comments: 

This example problem demonstrates how to calculate the predicted number of crashes for a four-lane 

urban arterial following the construction of a median and the addition of lighting and auto speed 

enforcement. 

LEVEL OF ANALYSIS

 Basic Intermediate Advanced 

Note: See table 3 for a full definition of Project Type designations. 

Detailed Analysis: 

The urban multilane predictive method is located in Chapter 12 of the HSM. The following steps 

summarize the manual calculation for the proposed configurations as described in Problem 4.1. 

STEP 1: Calculate the predicted average crash frequency and fatal and injury frequency for the 

proposed road segment. 

Segment Calculations (N ): SPF RS

Use the SPF equation in the HSM for segment-related crashes at divided urban multi-lane arterials to 

calculate crash frequency: 

137
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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS
SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

STEP 2: Calculate the Crash Modification Factors: 

STEP 3: Calculate the predicted average crash frequency for the design conditions (Npredicted).
 

The predicted average crash frequency (N ) can be calculated using the following equations:
 predicted 

Nbr = Nspf × (CMF1r × CMF2r × CMF3r × CMF4r × CMF5r )

Npredicted = (Nbr + Npedr + Nbiker) × Cr 

Manual calculations are shown only for total crashes. Calculations for FI and PDO crashes are 

calculated in a similar manner. The hand calculations include rounded values. The self-calculating 

spreadsheets do not truncate the numbers and so rounding errors are not included. Consequently, 

the hand calculated results may have minor differences than the spreadsheet calculated values. 

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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

Detailed Analysis: 

A-4.3 Hand Calculated Example — Documenting a Design Decision for 

a Sharp Horizontal Curve on a Rural Two-Lane Highway 

PROBLEM OVERVIEW 

Safety Assessment Method: SPF with CMF Adjustment 

Project Phase: Preliminary & Final Design Project Type: 4R 

Related Task: Design Exception 

Calculation Method: 

 Hand Calculated  Tool Based 

Tool based example included in Problem 4.3. 

Comments: 

This example problem demonstrates how to evaluate the effect a design decision can have on the 

estimated number of crashes for a rural two-lane, two-way highway horizontal curve re-alignment 

project. 

LEVEL OF ANALYSIS

 Basic Intermediate Advanced 

Note: See table 3 for a full definition of Project Type designations. 

The predictive methods for rural two-lane highways and curved roadway segments are located in 

Chapter 10 of the HSM. The following steps summarize the process for performing hand calculations 

to evaluate safety treatments that can be included in the preliminary or final design phases. 

STEP 1: Calculate the average crash frequency for the rural two-lane road segments considering 

base conditions (NSPF ). This value represents the number of crashes for any rural two-lane highway 

with a similar traffic volume and only base conditions. 

Segment Calculations (NSPF RS ): 

Use the SPF equation from the HSM for rural two-lane undivided roadway segments to calculate the 

average segment crash frequency: 

Calculations Notes 

Total Crashes: 

Nspf rs = AADT × L × 365 × 10-6 × e (-0.312) HSM Equation 10-6, p. 10-15 

Nspf rs = 11,000 × 0.5 × 365 × 10-6 × e (-0.312)=1.469 crashes Answer (Total Crashes) 

Note: HSM = Highway Safety Manual 

STEP 2: Calculate predicted average crash frequency for the segment under unique design 

conditions (N ). This value represents the number of crashes for any rural two-lane highway with predicted 

a similar traffic volume and similar geometric characteristics. 

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SCALE AND SCOPE OF SAFETY ASSESSMENT METHODS IN THE PROJECT DEVELOPMENT PROCESS

 

To calculate the estimated CMF due to decreasing the curve radii, the designer must first estimate the 

CMF of installing a curve compared to a base condition of a straight section of road. 

Next calculate the CMF for the new curve using the same method. 

The predicted average crash frequency (Npredicted ) can then be calculated using the following 

equation: 

Npredicted = Nspf rs × (CMF1x × CMF2x x ...........× CMF yx ) × Cx 

 

Calculations Notes 

Npredicted = Nspf × CMF1x × C x 

Base Equations Using C x = 0.97 as the local 

calibration factor 

Total Crashes 

Npredicted = 1.469×4.58×0.97=6.52 Answer (Total Crashes, Existing) 

Npredicted = 1.469×5.74×0.97=8.14 Answer (Total Crashes, Proposed) 

Crash reduction=(6.52-8.14)/6.52 = -25% Crash reduction 

141


For more information: 

Scale and Scope of Safety Assessment Methods in the Project 

Development Process, visit: http://safety.fhwa.dot.gov/ 

November 2016 http://safety.fhwa.dot.gov 

FHWA-SA-16-016