U.S. Department of Transportation
Federal Highway Administration
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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.
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.
Several existing documents provide concepts for integrating safety into the project development process. For example:
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.
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.
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 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.
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.
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.
|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 |
|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.
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:
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.
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.
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.
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.
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.
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.
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.
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.Table 2. Safety Assessment Methods for Project Phase, Task, and Type
|Project Phase||Related Task||Project Type1||Safety Assessment Method to Consider|
|Planning and Scoping (Chapter 2)||Preliminary Planning and Needs Assessment||1R, 2R, 3R, 4R, NL||✔|
|Establish Project Purpose and Need||2R||✔|
|Establish Project Scope||2R||✔|
|Alternatives Identification and Evaluation (Chapter 3)||Alternative Selection||2R||✔|
|Interchange Access Justification and Documentation||3R, 4R||✔||✔||✔|
|Preliminary and Final Design (Chapter 4)||Selecting specific design elements and their dimensions||2R||✔|
|Design Exception||3R, 4R||✔||✔||✔|
|Establishing the Work Zone TransportationManagement Plan||2R||✔|
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.
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.|
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||Date Needs|
|Road Type1||Road Characteristics2||Traffic Volume3||Observed Crash Data4|
|Site Evaluation or Audit||✔||✔||X|
|Historical Crash Data Evaluation||X||X||✔|
|CMF Applied to Observed Crashes||✔||✔||✔|
|CMF Relative Comparison||✔||✔|
|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. X = recommended data
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 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.
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:
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.