5.3 AASHTO’s Highway Safety Manual (HSM)

5.3.1 Overview

The HSM was developed by AASHTO. The first edition of the HSM was published in 2010 and is the premier guidance document for incorporating quantitative safety analysis in the project planning and development processes. With an emphasis on using analytical methods to quantify safety, the HSM helps practitioners understand the safety effects of decisions in planning, design, operations, and maintenance efforts. This science-based approach is intended to take the guesswork out of safety analysis. A supplement to the HSM was published in 2014 to provide additional guidance on performing analysis on freeways. The HSM is for practitioners at the State, county, MPO, local agency, tribal, and consultant level.

This section is intended to be an introduction into the HSM, not a comprehensive overview. Reference the HSM or the HSM website for additional detailed information.

The most recent HSM, 1st Edition, 2010, consists of three volumes and a supplement. It is divided into four parts, which are further described in the following sections:

Part A – Introduction, Human Factors, and Fundamentals (Volume 1);

Chapter 1 – Introduction and Overview

Chapter 2 – Human Factors

Chapter 3 – Fundamentals

Part B – Roadway Safety Management Process (Volume 1);

Chapter 4 – Network Screening

Chapter 5 – Diagnosis

Chapter 6 – Select Countermeasures

Chapter 7 – Economic Appraisal

Chapter 8 – Prioritization

Chapter 9 – Safety Effectiveness Evaluation

Part C – Predictive Method (Volume 2 and Supplement); and

Chapter 10 – Predictive Method for Rural Two-Lane, Two-Way Roads (Volume 2)

Chapter 11 – Predictive Method for Rural Multilane Highways (Volume 2)

Chapter 12 – Predictive Method for Urban and Suburban Arterials (Volume 2)

Chapter 18 – Predictive Method for Freeways (2014 Supplement)

Chapter 19 – Predictive Method for Ramps (2014 Supplement)

Part D – CMF (Volume 3)

Chapter 13 – Roadway Segments

Chapter 14 – Intersections

Chapter 15 – Interchanges

Chapter 16 – Special Facilities and Geometric Situations

Chapter 17 – Road Networks

Figure 5-1

shows these HSM parts and chapters in flowchart format.

Figure 5-1: HSM Parts

The information contained within the HSM is not a legal standard of care and is intended to aid agencies in their efforts to integrate safety into their decision-making processes. The HSM was not created to replace or substitute the exercise of sound engineering judgement. The HSM does not supersede publications such as the TMUTCD, the AASHTO A Policy on Geometric Design of Highways and Streets
(Green Book), or other AASHTO and agency guidelines, manuals, and policies.

5.3.2 HSM Part A – Introduction, Human Factors, and Fundamentals

Part A describes the purpose, scope, and intended audience of the HSM. The HSM exists to provide analytical tools, techniques, and processes for quantifying the potential effects that decisions made in planning, design, operations, and maintenance have on overall crashes. The information in the HSM intends to help agencies integrate safety into their decision-making processes. When used properly, the HSM can be integrated into all facets of the project development process. It also provides a brief introduction into the core elements of human factors that affect the interaction between drivers and the roadway. Although there is no specific design guidance provided in the HSM related to human factors, the information related to human factors is intended to support the information provided in Parts B, C, and D. A more detailed discussion on human factors is available in the NCHRP

Report 600: Human Factors Guidelines for Road Systems. The last chapter in Part A introduces the fundamental concepts needed to understand the roadway safety management techniques and crash estimating methods presented in the subsequent parts of the HSM. This chapter includes an overview of the following fundamental concepts:

Differences between objective safety and subjective safety;

Key definitions for crash-related terms;

Overview of crashes as random and rare events;

Contributing factors of crashes;

Reducing crashes by changing the roadway and environment;

Data needed for crash analysis; and

Evolution of crash estimation methods

For greater understanding of Part A, please refer to the HSM.

5.3.3 HSM Part B – Roadway Safety Management Process

Part B of the HSM presents procedures and information that is useful in monitoring and reducing crash frequency and severity on existing roadway networks. This includes the following six steps of the Roadway Safety Management Process:

Network Screening – Reviewing the existing transportation network to identify and rank individual sites based on the potential for reduction in average crash frequency.

Diagnosis – Evaluating individual sites based on crash data, historic site data, and field conditions to identify crash patterns.

Select Countermeasures – Selecting possible countermeasures to reduce the average crash frequency at the site based on the identified factors that may contribute to the crashes.

Economic Appraisal – Identifying individual projects that are cost effective or economically justified by evaluating the benefits and costs of the possible countermeasures.

Prioritize Projects – Identifying and prioritizing the set of improvement projects that meet the agency’s objectives, including safety, by evaluating the list of economically justified improvements at a specific site or sites.

Safety Effectiveness Evaluation – Evaluating the effectiveness of a countermeasure or improvement project at one site or multiple sites in reducing average crash frequency or severity.

The Roadway Safety Management Process is an iterative process to improve the overall safety of the existing roadway network by implementing economically justified projects through data-driven processes. The basic outline of the Roadway Safety Management Process is demonstrated in Figure 5-2

.

Figure 5-2: Road Safety Management Process

5.3.4 HSM Part C – Predictive Method

Part C of the HSM provides a predictive method to estimate average crash frequencies for both existing and proposed future conditions of a roadway network, facility, or individual site. This predictive method not only estimates total crashes over a given time but can also be used to estimate average crash frequencies by crash severity and collision type. It is recommended that these estimates be determined for existing conditions, alternatives to the existing conditions, or for proposed new roadways. A roadway network can be divided into individual sites of either homogenous roadway segments or intersections to run the predictive method.

The estimated average crash frequency of an individual facility type is based on the traffic volumes, geometric design features, and traffic control type for that site. Predictive models were developed to estimate average crash frequencies for a variety of facility types using regression models. These regression models were developed from data of similar sites across the country and are known as safety performance functions (SPFs) in the HSM. SPFs have been developed for specific facility types under what are referred to as “base conditions,” the most common conditions for similar sites across the country. For example
, Table 5-1

shows a sample of the base conditions for rural multilane highways.

Table 5-1: Rural Multilane Highway Base Conditions
Undivided Roadways	Divided Roadways	Intersections
Lane width: 12 feet	Lane width: 12 feet	Intersection skew angle: 0 degrees
Shoulder width: 6 feet	Right shoulder width: 8 feet	No intersection left-turn lanes except on stop-controlled approaches
Shoulder type: Paved	Median width: 30 feet	No intersection right-turn lanes except on stop-controlled approaches
No lighting	No lighting	No lighting

The HSM has developed the SPFs for the following facility types:

Rural two-lane, two-way roads;

Rural multilane highways;

Urban and suburban arterials; and

Freeways and ramps (2014 supplement)

Table 5-2

and Table 5-3

demonstrate the various facility types that SPFs were developed for as part of the HSM.

Table 5-2: SPFs by Roadway and Arterial Facility Types
Facility Type	Undivided Roadway Segment	Divided Roadway Segment	Intersections
			Stop Control on Minor Leg(s)		Signalized
			3-Leg	4-Leg	3-Leg	4-Leg
Rural Two-Lane, Two-Way Road	✓		✓	✓		✓
Rural Multilane Highways	✓	✓	✓	✓		✓
Urban and Suburban Arterials	✓	✓	✓	✓	✓	✓

Table 5-3: SPFs by Freeway and Ramp Facility Types
Facility Type	Classification	Rural	Urban
Freeway Segment	Four-Lane	✓	✓
	Six-Lane	✓	✓
	Eight-Lane	✓	✓
	Ten-Lane		✓
Ramp Segments	One-Lane Entrance Ramp	✓	✓
	One-Lane Exit Ramp	✓	✓
	Two-Lane Entrance Ramp		✓
	Two-Lane Exit Ramp		✓
Collector-Distributor Road (C-D)	One-Lane C-D Road	✓	✓
Collector-Distributor Road (C-D)	Two-Lane C-D Road		✓
Ramp Terminal	One-Way Stop-Controlled Two-, Three-, or Four-Lane Crossroad	✓	✓
	Signalized Two-Lane Crossroad	✓	✓
	Signalized Three-Lane Crossroad	✓	✓
	Signalized Four-Lane Crossroad	✓	✓
	Signalized Five-Lane Crossroad		✓
	Signalized Six-Lane Crossroad		✓

A simplified example
of how SPFs were developed for the HSM is illustrated in Figure 5-3

. This figure illustrates how crash data from across the country was used in the development of the SPFs located in the HSM.

Figure 5-3: Example SPF Development

Adjustments to the predicted crashes, determined by using an SPF, may account for the differences between the base conditions used in developing the SPF and the site-specific conditions.

Part C CMFs are used to account for these differences when site-specific conditions vary from the base conditions. These CMFs were developed specifically for individual SPFs and are intended to be applied only to the crash prediction results of that particular SPF. The SPF-related CMFs are different than the CMFs on the CMF Clearinghouse website. The Highway Safety Manual 2nd Edition (HSM2) is expected to rename SPF-related CMFs to AFs to reduce the confusion between Part C CMFs developed for a specific SPF and CMFs located on the Clearinghouse website.

A calibration factor (C) may also be used to make agency-specific adjustments to the predicted crash totals. This accounts for the differences between the agency or agencies for which the models were developed and the agency/agencies for which the predictive model is being applied. Calibration factors to the HSM predictive method calculations should be applied whenever possible. Each agency is responsible for developing their own calibration factors for specific facility types.

The HSM Part C predictive method is summarized in the following general equation:

N_Predicted = N_SPFX × (CMF_1x × CMF_2x × ... × CMF_nx) × C_x

Where:

N_Predicted
= predicted average crash frequency for a specific site type

N_SPFX
= predicted average crash frequency for the base conditions of the SPF for a specific site type

CMF_1x
= CMF specific to the SPF for a specific site type

C_x
= calibration factor to adjust the specific SPF to the local conditions for a specific site type

x
= specific site type

Because crash totals fluctuate over time, it is difficult to know whether changes in the observed crash totals are due to changes in site conditions or are due to natural fluctuations. A period with a high number of observed crashes is statistically likely to be followed by a period with a low number of observed crashes. The opposite of this tendency also applies; it is probable that a period with low crash totals will be followed by a period with high crash totals. This tendency to regress to the mean or average is known as regression-to-the-mean (RTM). Failure to account for the effects of RTM introduces the potential for RTM bias, also known as selection bias. RTM bias results in overestimating or underestimating the effectiveness of a treatment. The effects of RTM bias are illustrated in Figure 5-4

. For existing sites, facilities, or roadways, the Empirical Bayes (EB) method can be applied within the predictive method to account for both the predicted average crash frequency and the observed crash frequency. This method accounts for the reliability of a particular SPF and RTM bias.

Figure 5-4: Regression to Mean Bias

The application of the EB method within the predictive method results in an expected average crash frequency. Figure 5-5

illustrates the relationship between observed, predicted, and expected crashes when performing the predictive method for a particular SPF. See Appendix F, Section 2 – NCHRP 17-50 Project Example of HSM in Design
for more details on how expected crashes can be estimated.

Figure 5-5: Relationship of Observed, Predicted, and Expected Crashes within the Predictive Method

Table 5-4

identifies the scenarios when the EB method is applicable and not applicable, as identified in the HSM.

Table 5-4: Applicability of the EB Method
EB Method is Not Applicable (Predicted Crashes)	EB Method is Applicable (Expected Crashes)
EB method is not applicable for the following types of situations: New alignments for a substantial proportion of the project length Change in the basic number of intersection legs or traffic control type	EB method is applicable for the following situations: Geometrics and traffic control features are not changed (for example the “do-nothing alternative”) Where the roadway cross-section is modified but the basic number of through-lanes remains the same Minor changes in alignment are made while keeping most of the alignment intact A passing lane or short four-lane section is added to a rural two-lane, two-way road Any combination of the above improvements

The HSM predictive method is summarized as an 18-step process used to estimate average crash frequencies for a roadway network, facility, or site and is illustrated in Figure 5-6

. An example of applying the HSM Part C predictive method is provided in Appendix F, Section 2 – NCHRP 17-50 Project Example of HSM in Design
.

Figure 5-6: HSM Predictive Method 18-Step Process

5.3.5 HSM Part D – Crash Modification Factors

Part D of the HSM presents information regarding the effects of various safety treatments or countermeasures. This information is intended to estimate how effective a particular countermeasure or set of countermeasures will be in reducing crashes at a specific location in the form of a CMF. Most of the information in Part D of the HSM manual is now located on the CMF Clearinghouse website. It is anticipated that, with the publication of the HSM2, Part D will focus on the methodologies for selecting, applying, and developing CMFs and will no longer list individual CMFs. The CMF Clearinghouse website will become the sole repository of CMFs.

5.3.6 HSM 2nd Edition

It is anticipated that the second edition of the HSM (or HSM2) will expand upon the methodologies in the first edition of the HSM, incorporate new models and research that has been completed, and modify practices based on user experiences and needs. New or additional content is expected to be included in the areas of human factors, bicycle and pedestrian safety, systemic safety, SPF calibration, additional crash prediction models, and CMF development. It is the responsibility of the users of the manual to incorporate any relevant information when the HSM2 becomes publicly available. Additional information about the HSM2 is available on the AASHTO website related to the HSM. A link to the website can be found in Appendix F, Section 3 – External References (Reference 4)
.