Incorporating Safety into Design

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Incorporating Safety into Design

• CE 453 Highway Design
• February 21, 2007

Jerry Roche, P.E. Transportation Safety
Engineer FHWA Iowa Division
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Outcomes

• Define Safety
• Describe the background of design criteria to
  achieve safe highway operations
• Define Nominal and Substantive Safety
• Apply Engineering Judgment

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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
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Our customers -- the traveling public, highly
value safety

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

NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM
NCHRP Report 376
Customer-Based Quality in Transportation
Transportation Research Board National Research
Council
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What does Safety mean?

• Safety

an absolute

• Safer- a relative term

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A highway engineers view of safer
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Is this road less safe or more safe?
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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

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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

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AASHTO Policies and State design manuals
represent safe design practice in the minds of
many
DOT Design Manual
Safe Design Practices The often
unasked questions are--
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?
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What factors influence Design Criteria?

• Costs

• Traffic Operations

• Maintenance

• Constructability

• Safety

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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

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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?
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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?

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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

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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
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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

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Some nominally safe locations are substantively
unsafe
Design Speed on Tangent 60 mph

• Sharp Curve after long Tangent
• Curve Design Speed of 35 mph is acceptable and
  provided for in AASHTO Policy
• Speed differential value gt 12 mph pose high
  safety risk of 10 higher crash rates

Speed Differential 60 mph 35 mph 25 mph
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When design criteria can not be met, is it always
true that substantive safety is compromised?
DOT Design Manual
Safe Design Practices
Design Exceptions Report Project xx
?
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Some locations with nominal safety problems have
no history of crashes
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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

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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?

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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?

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• 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

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Where should we focus our efforts?
Nominal Safety
Does Not Meet
Meets
Meets
Substantive Safety
Does Not Meet
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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

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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

• However,
• 5 fatal 40 personal injury crashes (20)
• One of the highest cross-median fatal and major
  injury crash locations in the state

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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

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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

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Closing Points

• Regardless of your position, safer IS your
  responsibility

The care of human life happinessis the first
and only objective of good government Thomas
Jefferson
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Questions and Discussion