Application of the AASHTO Highway Safety Manual (HSM)

Karen Dixon, Ph.D., P.E.

Email: k-dixon@tamu.edu

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HSM Overview

2

Integrate Safety

into Decision

Processes

3

Need

Safety Trade-Offs?

What Do You Do?

Safety Right-of-Way

Costs

Environment

4

Nominal Safety Substantive

Safety

Standards

Compliance

Highway Safety has Two

Dimensions

5

Expected or Actual

Crash Frequency

and Severity

Nominal Safety is

an ABSOLUTE

Substantive

Safety is a

CONTINUUM

CR

ASH

RIS

K

DESIGN DIMENSION

Gre

ate

r

Greater

Design Exception Insights

6

HSM Vision Akin to HCM

Definitive; represents quantitative ‘state-of-the-

art’ information

Widely accepted within professional practice of

transportation engineering

Science-based; updated regularly to reflect research

1

2

37

Common Impacts for

Project Level Decisions?

Safety Environmental Traffic Right-of-Way CostsImpacts Impacts Operations

Design

Criteria

(nominal

safety)

Mo

re Q

uan

tita

tive

Gre

ate

r W

eig

ht

Substantive Safety

The HSM

• Traffic Noise

Models

• Air Quality Models

• 3-D Visualization

• Environmental

Assessments

•Traffic

Operations /

Microscopic

Simulation

•Construction

Plans

• Cost Models

• Real Estate

Appraisals

• DOT

Databases

8

>> 0 >> 1 >> 2 >> 3 >> 4 >>

Design

Existing Conditions Alternative 1

Alternative 2 Alternative 3 9

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Vol. 1 (Part A)

IntroductionHuman Factors Fundamentals

Vol. 3 (Part D)

Crash Modification Factors (CMFs)

Vol. 1 (Part B)

Roadway Safety Management

Process

Vol. 2 (Part C)

Predictive Method

SPFs

• Calibrate SPFs for LOCAL conditions

• Weight with EB Method

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Expected Crashes

= CMF x

(base condition crashes)

You can remember it as

“M is for multiply”

Crash

Modification

Factor

CMF

a''conditionwithfrequencycrash averageExpected

b''conditionwithfrequencycrash averageExpectedCMF

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CMF Example

Expected crashes

= CMF x (base condition crashes)

= 0.9 x (base condition crash frequency)

CMF = 0.90

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Represents a 10%

Reduction in Crashes

Snapshot of How HSM Works

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The Value of the HSM

• Provides proven and vetted science-based approach to quantifying safety effects of decisions and actions we contemplate

• Provides common knowledge base, language and basis for reasoned safety judgments

• Allows incorporation of safety to same level of importance as other factors

• Does not increase risk of tort liability

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It does not force or require you to

do anything, it merely helps you do

a better job

Measuring Safety

SUBJECTIVE SAFETY

• Perception

• Values vary among

observers

OBJECTIVE SAFETY

• Quantifiable

• Independent of the

observer

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The HSM uses crashes

as a measure of

safety

What is

Safety?

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>> 0 >> 1 >> 2 >> 3 >> 4 >>

Crashes

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>> 0 >> 1 >> 2 >> 3 >> 4 >>

Random Events

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Rare Events

Relative Proportion of Crash Events

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A

B

C

O

K

Crash Severity

Categories in the HSM

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A

B

C

O

K K crash

One or more persons died

within 30 days of the crash

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A

B

C

O

K A crash

Incapacitating injury

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A

B

C

O

K B crash

Non-incapacitating evident injury

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A

B

C

O

K C crash

Possible injury

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A

B

C

O

K O crash

No injuries - reportable

property damage resulted from

crash

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A

B

C

O

K Crash Severity

Most severe injury controls

level of crash severity

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Available Tools and Resources to Help Apply

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29

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Diamonds represent data needs

Squares represent assessment tools

Circles represent assessment methods

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Companion Software

HSM Part Supporting Tool

Part B: Roadway Safety Management Process

SafetyAnalyst

www.safetyanalyst.org

Part C: Predictive Methods

IHSDM

www.ihsdm.org

Part D: Crash Modification Factors

FHWA CRF/CMF Clearinghouse

www.cmfclearinghouse.com

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HSM Part C Self-Calculating

Spreadsheets at

highwaysafetymanual.org

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Evaluating Options (example application)

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Evaluating Options (continued)

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Evaluating Options (continued)

Total Crashes per Year FI Crashes per Year

AlternativeIntersection

CrashesSegment

Crashes

Predicted Crashes

Intersection Crashes

Segment

Crashes

Predicted Crashes

Existing 12.80 4.3417.14

(say 17)7.94 2.37

10.31(say 10)

Alternative 6.66 4.5411.20

(say 11)3.33 2.49

5.82(say 6)

FI = fatality and injury

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Companion Resources &

Documents• AASHTO Site: highwaysafetymanual.org

• HSM Online Overview Course (FHWA-NHI

380106)

• Application of Crash Modification Factors (NHI

380093) workshop

• FHWA CMF Clearinghouse

(cmfclearinghouse.org)

• ITE Integration of Safety in the Project

Development Process and Beyond: A Context

Sensitive Approach

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HSM Is Not Just for

Safety Experts!

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What‘s Next?

• First Edition Published in 2010

• Resources and Tools Continue to be Developed

• Next Edition of the HSM Expected in 2019 or 2020

• Production Contract for HSM V2 Signed Last Week (Team of Kittelson & Associates and Texas A&M Transportation Institute)

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Questions & Discussion

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