Incorporating Safety into Design

CE 453 – Highway Design

October 2, 2006

Jerry Roche, P.E.

Transportation Safety Engineer

FHWA – Iowa DivisionFederal Highway

Administration

Outcomes

• Define Safety

• Describe the background of design criteria to achieve safe highway operations

• Define Nominal and Substantive Safety

• Apply Engineering Judgment

The State of Safety in the US

• 43,443 traffic fatalities in 2005 (the highest since 1990)

• Fatality rate increased (first increase since 1986)

• Traffic crashes and deaths cost the country $230B annually

Jack Trice Stadium – Iowa State University

Our ‘customers’ -- the traveling public, highly value safety

“Participants from focus groups agreed that safety is the most important transportation concern.”

NCHRP Report 376

NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM

Transportation Research BoardNational Research Council

Customer-Based Quality in Transportation

What does Safety mean?

Safety

“Safer”- a relative term

– an absolute

A highway engineer’s view of “safer”

Is this road ‘less safe’ or ‘more safe’?

What factors contribute to safety performance of a roadway?

• Traffic volume (“exposure to risk), including traffic mix

• Driver behavior– Speed (including speed differential)– Alcohol & drug use– Seatbelt and helmet use– Distraction

• Highway Geometry– Alignment– Cross-section– Roadside– Intersections

Role of Road Design in Crash Prevention

Design can reduce:– Incidence of human error– Chance of human error resulting in

crash– Severity of the consequences of

crashes

• How a particular highway is built (Engineering) impacts both the number and severity of crashes

AASHTO Policies and State design manuals represent “safe” design practice in the minds of many

= Safe Design Practices

The often unasked questions are--

DOT

Design

Manual

Is a road designed to meet current standards as safe as it can be?

Is a road designed to meet current standards as safe as it should be?

What factors influence Design Criteria?

• Costs

• Traffic Operations

• Maintenance

• Constructability

• Safety

Are these design decisions based on safety?

• Select minimum lane and shoulder width per functional class and traffic volume

• Select curves and tangents to fit terrain, right-of-way

• Select grades to balance earthwork and minimize right-of-way

• Select minimum vertical curves per design speed and grades

• Cross Section

• Horizontal Alignment

• Vertical alignment–Grade

–Vertical curvature

An insight -- Design criteria are generally structured and used as limiting -- why is this so?

• Minimum value of Middle Ordinate

• Minimum width of shoulder

• Minimum clear zone

Is the minimum always sufficient?

Are there ever times when the minimum might be excessive?

Link Between Standards and Safety

• How can we make highways sufficiently safe?

• Does applying standards achieve it?

• How about cost-benefit?

• What can road professionals do?

STANDARDS

DESIGN

CRASHES

AASHTO Green Book

“The intent of this policy is to provide guidance to the designer by referencing a recommended range of values of critical dimensions. It is not intended to be a detailed design manual that could supercede the need for the application of sound principles by the knowledgeable design professional. Sufficient flexibility is permitted to encourage independent designs tailored to particular situations.”

- Foreword, page xli, A policy on Geometric Design of Highways and Streets

Two Ways to Look at Safety as Engineers and Planners*

• Nominal Safety is examined in reference to compliance with standards, warrants, guidelines and sanctioned design procedures

• Substantive Safety is the performance of the roadway as measured in terms of crashes, including their frequency, type and severity.

*Ezra Hauer, ITE Traffic Safety Toolbox Introduction, 1999

Aspects of Nominal Safety

– Roadway design must enable road users to behave legally

– Roadway design should not create situations with which a minority of road users have difficulties

– Owning agency requires protection against claims of moral, professional and legal liability

Some nominally safe locations are substantively unsafe

Sharp Curve after long Tangent

• Curve Design Speed of 35 mph is acceptable and provided for in AASHTO Policy

• Speed differential value > 12 mph pose high safety risk of 10%+ higher crash rates

Design Speed on Tangent = 60 mph

Speed Differential = 60 mph – 35 mph = 25 mph

When design criteria can not be met, is it always true that substantive safety is “compromised”?

= Safe Design Practices

=

DOT

Design

Manual

Design

Exceptions

Report

Project xx?

Some locations with nominal safety problems have no history of crashes

Aspects of Substantive Safety

– A function of what resources are available (roadway design, maintenance, enforcement, emergency medical services)

– A function of the “context” of the location

Substantive Safety is “Context Sensitive”

What types, frequency and severity of crashes would

you expect here?

How much different would crashes be

for this road?

Issues to ponder when considering the relationship between nominal and substantive safety

• What is the basis for the design values referred to in criteria?

• How do we apply the values in actual design?

• What factors beyond the control of the designer influence safety?

• How much do we really know about the relationship of highway features to safety?

Philosophical Considerations in Highway Design --

“The direct application of established design criteria or standards (i.e., nominal safety) is no assurance that a certain quality of design (i.e., level of substantive safety) will be achieved--indicating that such criteria are not sufficient in themselves.”

Philosophical Considerations in Highway Design, from “Dynamic Design for Safety” -- Jack E. Leisch, 1974

Where should we focus our efforts?

Nominal SafetyS

ubst

anti

ve S

afet

y

Meets

Meets

Does Not Meet

Does Not Meet

How can we provide substantive safety?

• Determine if there are any safety problems– Using detailed crash data– Perform a safety audit (field review

by professionals)– Talk to law enforcement

• Then, design accordingly and document your decisions

Safety Analysis• “The crash rate is 41/HMVM which is lower

than the statewide rural average of 63/HMVM.”

Concept for I-80 reconstruction project

Nominal Safety

Sub

stan

tive

Saf

ety Meets

Mee

ts

Does Not Meet

Doe

s N

ot

M

eet

• However,– 5 fatal 40 personal injury crashes (20%)– One of the highest cross-median fatal and major

injury crash locations in the state

Safety Analysis• “There are three horizontal curves with a degree of

curvature greater than 6. Crash data analysis revealed no serious problems related to the curves.

These curves will not be reconstructed as part of this project. However, safety measures, such as maintaining a right of way free of obstructions and providing appropriate warning signs, will be considered.”

Concept for IA 281 reconstruction project

Nominal Safety

Sub

stan

tive

Saf

ety Meets

Mee

ts

Does Not Meet

Doe

s N

ot

M

eet

Key Safety Principles and Design

• No highway is safe, only safer or less so

• We know how to make highways safer

• Law of diminishing marginal returns applies

• Money should be spent effectively

Closing Points

• Regardless of your position, “safer” IS your responsibility

“The care of human life & happiness…is the first and only objective of good government”

Thomas Jefferson

Questions and Discussion