Transcript Flexibilities in VDOT’s Design Policies and Standards

Flexibilities in VDOT’s Design Policies
and Standards
2012
Outline
• VDOT’s Flexible Design Philosophy
• Why Use Flexible Design
• Understanding what “flexibility in design”
means, and why it is important
• Reveal how design flexibility is
incorporated in Green Book contents
• The Value of Design Exceptions
• Risk Management and Safety
• Tort Liability
• Real Life Scenario
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Why Flexible Design?
• Facilitates program/project delivery
and achieving performance goals
• Allows consideration of a wider range
of design options and alternatives to fit
conditions
• Enables more cost-effective designs
• Promotes “common sense”
engineering
• Recognize and design for context
sensitive issues
• Minimize project impacts
• Focus on project purpose and need
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Flexible Design Philosophy
• Recognizes that flexibility is a necessary and
desired aspect of the design process
• Uses a risk assessment and risk management
approach for all aspects of the design
• Apply performance criteria to evaluate flexible
design decisions, as well as condition criteria
• Understand the risks and consequences for design
decisions – this often requires more information and
higher level analysis than simply applying criteria
“by the book”
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AASHTO/VDOT Flexibilities in
Design Policies and Standards
• VDOT incorporates a great deal of the flexibilities
offered by AASHTO into our current project
development process.
• Our current policies allow an engineer a number of
choices and alternative design “tools” to allow for
maximum flexibility.
• Designers have choices in design controls that effect
the outcome of their project.
• Such choices allow for creative thinking, innovation
in possible solutions, the ability to engage
stakeholders early and fully study all design options.
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VDOT IIM 235 (CSS)
• Emphasize VDOT’s commitment to providing
flexibility and innovation to our transportation
challenges.
• Pave-in-Place - allows non-hard roads with traffic
between 50 to 750 vpd upon approval by BOS, to
be paved in place.
• Rural Rustic – Low density developments with an
ADT no more than 1500 vpd
• 3R
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Guidelines for Geometric Design
of Very Low-Volume Local Roads (ADT ≤ 400)
• Addresses the unique design
issues of determining appropriate
cost-effective geometric design
policies for very low-volume local
roads
• Covers both new and existing
construction projects.
• May be used in lieu of the Green
Book for those facilities.
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Green Book Encourages
3R Designation Where Appropriate
“Specific site investigations and
crash history analysis often
indicate that the existing design
features are performing in a
satisfactory manner. The cost of
full reconstruction for these
facilities, particularly where major
realignment is needed, will often
not be justified.”
Green Book Forward
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3R Design Guidance
• TRB Special Report 214 is a commonly used
reference for 3R criteria
• A “safety conscious” approach for improvement of
safety performance
• Evaluation of existing
• Geometric design
• Roadside conditions
• Traffic operations
• Pavement and drainage structures
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Major Issues Found in the
AASHTO Guide of Flexibility in Highway Design
•Published in 1997 & 2004
•Importance of establishing
“purpose and need” early in plan
development
•Key decisions made early in design
process
•Basic design controls are choices
made by engineers
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“Risk Management Best Practices”
•2007 publication of FHWA
•Adopted by VDOT in 2008
•Offers engineer a better
understanding of the operational
and safety effects of potential
design exceptions
• Discusses proven, effective
mitigation strategies
• Importance of documenting the
design exception and mitigation
approach
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“Bridging” Flexibility in Highway Design
AASHTO/FHWA “A
Guide for Achieving
Flexibility in Highway
Design”
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Engineering is a Balancing Act
Balancing Competing Objectives
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Safety
Operations
Standards
Costs
Environmental
Impacts
Stakeholders
Quality
Risk
Laws/Regulations
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AASHTO Flexible Design Policy
“As highway designers, highway engineers strive to
provide for the needs of highway users while
maintaining the integrity of the environment. Unique
combinations of design requirements that are often
conflicting result in unique solutions to the design
problems”
“...Sufficient flexibility is permitted to encourage
independent designs tailored to particular situations.”
Source: Foreword, p. xliii, 2004 Edition
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Know how to use the Green Book
“The intent of this policy is to provide
guidance to the designer by referencing a
recommended range of values for critical
dimensions. It is not intended to be a detailed
design manual that could supersede the need
for the application of sound principles by the
knowledgeable design professional. Minimum
values are either given or implied by the lower
value in a given range of values. The larger
values within the ranges will normally be used
where the social, economic, and environmental
(S.E.E.) impacts are not critical.‟
Green Book Foreword, pg xliii
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What Flexibility is in the Green Book?
• Many dimensions and values are shown as ranges
• Many criteria described as “guidelines” or “typical”
• Many concepts are not dimensioned and discussed
only in functional terms
• In many cases, choices are offered for how to
complete a design
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Solutions or concepts not specifically
included are not precluded
Specific solutions are not mandated
Designer judgment is implied or
explicitly suggested
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Application of Design Exceptions
• Designers should understand that design
exceptions are an acceptable and useful tool
when all standard design solutions are
exhausted.
• Acceptance of design exceptions should not
be viewed as an admission of failure
• Designers should avoid labeling a number
outside of the “norms” as unsafe
• A properly documented design exception is
essential for achieving flexibility in design
• Lack of supporting information, inadequate
guidance, limited resources, and requests
made too late in project development can all
pose potential problems in the approval of a
design exception
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Risk Management of Design Exceptions
• Understand the operational
and safety effects of any
potential DE
• What is the potential risk of
the DE
• Employ proven, effective and
applicable mitigation
strategies as outlined in the
Mitigation Strategies Manual
• Fully document and mitigate
the DE
• Serves to justify the
decision and manage
agency risk
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Analyzing Roadway Safety
•Provides tools for
considering safety in the
project development process
•Assists practitioners in
selecting countermeasures
and prioritizing projects
•Compare project
alternatives
•Quantify and predict the
safety performance of
roadway elements
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Safety Has Two Dimensions
• Nominal Safety
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Examined in reference to compliance with standards,
warrants, guidelines and design procedures
Comparing geometric design features (lane/shoulder, SSD,
grades) to prevailing design standards (Green Book)
If a roadway meets minimum design criteria it is said to be
nominally safe
• Substantive Safety
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Is the expected or long term safety performance for a
roadway
Define the nature/extent of safety problem by frequency,
type, severity of accidents
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Defining the Safety Problem
• Not every location is a “high
accident” site.
• Understand types and patterns
of accidents; focus on more
serious accident types
• Not all “nominally unsafe” roads
have substantive safety
problems
• Substantive safety may not be a
problem on any given project
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Tort Risk
• Adherence to a standard does not
automatically demonstrate reasonable
care
• Deviation from a standard does not
automatically demonstrate negligence
• A structured process for evaluating,
approving and documenting the rationale
for key decisions is necessary to address
professional responsibility and tort
liability
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Example Situation
• Route 633 over Garnett Creek located in
Fredericksburg District
• The bridge and approach work was
constructed in the last few years.
• How could VDOT have altered the design of
the project to maximize the flexibility within
our standards?
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Project Background
• Bridge Replacement over Garnett's Creek
•Bridge sufficiency rating 29.9
• Rural
• Un-posted speed limit
• Accident report shows 1 crash in last 6 years
• Speed study shows that 85th percentile is 52 mph.
• Existing two way road – 16’ width
• Existing Bridge – 21’ width
• Existing 2’ shoulders, 2’ front width ditches
• Road can be closed during construction
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Proposed Typical Section
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Rural Collector
Existing Year ADT
Design Year ADT
Truck Percentage
Design Speed
Lane Width
Shoulder Width
Front Ditch Width
Front Ditch Slope
Bridge Width
550 vpd
800 vpd
2.5%
55 mph
11’
5’
6’
4:1
28’
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Existing Bridge and Approaches
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3R Standards
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So, What Really Was Built?
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Bridge Typical Section
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As-Built
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What Would You Have Done Differently?
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Route 633 over Garnett’s Creek
Could we have applied more context sensitivity or
practical engineering concepts?
Utilize 3R vs. GS-3
Shoulder width – Applied 2’ shoulders vs. 5’
Ditch width – 3’ at 3:1 vs. 6’ at 4:1
Lane width – 10’ vs. 11’
3R site prep for GR-9 vs. full site prep
Bridge width 26’ vs. 28’
Reduced footprint, better context sensitivity
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Conclusion
• VDOT and AASHTO provide ample flexibility
regarding the use of geometrics
• Are you applying that flexibility?
• Understand how and when the Green Book (or other
AASHTO/VDOT references) applies
• How often do you and/or your staff read/utilize
these references?
• Understand that as a designer you have a multitude of
engineering choices available
• There is never only “one solution to a problem”
• Stay committed to your decisions but flexible in your
approach
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Questions?
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