Module 6: The Art & Science of Geometric Design Flashcards
(43 cards)
What is Design?
-Design is the process of selecting the elements that, when combined, will make up an end product,
-In engineering, these elements are primarily features, dimensions, & materials.
-The geometric design guide of roads involves selecting a road’s visible features & dimensions.
-A balance between physical demands, the constraints, & the setting.
TAC Geometric Design Guide.
-‘Go-To’ guide for road design.
-GDG offers design guidance & validation when properly applied.
-Not a ‘planning’ document - GDG is a design guide
Design.
-Design is a process in which sound engineering judgement & experience play significant roles.
-Designers use judgement , technical references, & calculations to assist in selecting the appropriate primary design elements.
-However, selecting elements in isolation from each other is not design. The final design is the sum of all the decisions made during the design process.
Correct or Incorrect Geometric Design?
-In geometric design of roads, the process cannot generally be called correct or incorrect.
-But rather more or less:
-efficient
-attractive
-safe
-costly
The Goal of Design.
-The goal is a well-designed road
-An acceptable balance between level of service, cost, environmental impact, & level of safety to all users.
-This balance will vary from location to location, & even from time to time.
The Design Challenge.
-The evaluation of competing alternatives to arrive at the most appropriate product.
Design Philosophy.
-Safety
-Design domain
-Design flexibility
Safety.
-Not in the context of ‘safe’ or ‘unsafe’
-Only ‘more safe’ or ‘less safe’
-Nominal safety
-Substantive safety
Design Domain.
-Provides a range of values or options to select from for design.
Design Domain - Benefits.
-Shortcomings of standards & guidelines
-Allows more realistic designs
-Requires greater judgement of designer
-Direct relationship with safety
-Table picking won’t work
Design Flexibility.
New Challenges demand design flexibility or Context Sensitive Design:
-constrained urban environments
-community pressures
-environmental constraints
-budget constraints
Flexible Design.
- A flexible design is a concept that allows the use of design values from any design domain as long as the combination produces a good design & an acceptable level of expected performance.
-Using a flexible design approach can produce acceptable performance while minimizing the impact on agreed constraints, & meeting the requirements & expectations of stakeholders & road users.
-Flexible design discourages the practice of blindly following a standard or guideline without understanding of performance resulting from the design outcome.
Design Exception.
-A design exception us a documented decision to design a highway element or a segment of highway to design criteria that do not meet minimum values or ranges established for that highway or project.
Design Exception - Applications.
-avoid adversely impacting the natural environment
-improve the natural environment
–avoid adverse social effects
-avoid land acquisitions
-preserve right of way
-preserve historical or cultural resources
-accommodate the context of the site
-limit construction costs
-Design Exception.
-Outside the norm of Design Domain values
-Clear understanding of its potential effects
-Must be accompanied with mitigations measures
-Documentation of justification is key
-fear of the legal aspects has deter many agencies to not adopt DE
-design decisions must be defendable in litigation
-the documentation would contain, as a minimum, the process & reasoning that led to the decision, including the circumstances of each project, the choices available, & the considerations reviewed, as well as a complete explanation for the decision itself
DE and Safety.
-The consideration of safety is arguably the centra; issue involved in a decision to accept or approve a design exception
-understanding the relationship of safety to the criteria, the design process, & a desired or expected outcome of the design is important.
-The concepts of nominal & substantive safety are fundamental to the topic of design exceptions & their mitigation
‘Nominally’ Unsafe but ‘Substantively’ Safe.
-By definition, locations with design exceptions are nominally unsafe, in that one or more design elements do not meet minimum criteria
-That does not mean, however, that the highway cannot function at an acceptable level of substantive safety
-The objective should be to understand the quantifiable (substantive) safety effects expected with a nominally unsafe design
Design Elements.
- Design speed
- Horizontal alignment
- Superelevation
- Vertical alignment
- Grade
6, Stopping sight distance - Vertical clearance
- Cross slope
- Lane width
- Shoulder width
- Bridge Width
Design Speed.
-A selected speed used to determine the various geometric features of the roadway.
-It is different from the other controlling criteria in that it is a design control, rather than a specific design element
-Design speed > posted speed
-Crash risk increases with increasing differentials in speed
Horizontal Alignment.
-Refers to the horizontal curvature of the roadway
-Design criteria specify a minimum radius for the selected design speed
-Directly related to superelevation & side friction
Superelevation.
-Superelevation is the rotation of the pavement on the approach to and through a horizontal curve
-Superelevation is intended to assist the driver by counteracting the lateral acceleration produced by tracking the curve
Vertical Alignment.
-Vertical alignment includes grade as well as vertical curvature.
-Design basis for minimum length of vertical curvature is to provide the minimum stopping sight distance
Grade.
-Grade is the rate of change of the vertical alignment
-Grade affects vehicle speed & vehicle control, particularly for large trucks
Stopping Sight Distance.
-Defined as the distance needed for drivers to see an object on the roadway ahead & bring their vehicles to safe stop before colliding with the object
-Influenced by both vertical & horizontal alignment
