Module 6: The Art & Science of Geometric Design Flashcards
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
Alignment.
-Alignment is a 3D problem broken down into two 2D problems:
-horizontal alignment (plan view)
-vertical alignment (profile view)
-Horizontal & vertical alignment should be designed to complement each other for safe & more attractive highways
-Alignment design depends primarily on the design speed
Alignment - Consistent Design.
-It is important that the alignment has consistent designs to avoid sudden changes in the vertical & horizontal layout of the highway.
-Road design that uses consistent alignment & lane configuration reduces the information processing workload for the driver.
-Crashes, roadway-based geometric inconsistency, & driver workload have been shown to be strongly related to each other.
Horizontal Alignment.
-Design based on design speed & curvature
-Driver workload increases when approaching & on curves
-Drivers underestimate curvature
-Safely accommodate transitions in cross section
-Curves can present special safety problems for trucks & other large vehicles
-Curves influence speed behaviour - curvilinear roads will have lower speeds
Spiral Curves.
-A spiral curve is a curve with a constantly varying radius
-It is to provide smooth transition & a natural driving path between a tangent & a circular curve
-A spiral curve provides a convenient & desirable arrangement for developing superelevation runoff
-Spiral curves are also used in the design of interchange ramps
Superelevation - Design Values.
-Superelevation = pavement slope height / pavement width
-Typically in Canada - 0.04 m/m, 0.06 m/m, 0.08 m/m
-Inadequate superelevation can cause vehicles to skid as they travel through a curve
-Can you have too much superelevation?
-vehicles sliding transversely when traveling at very low speeds
Vertical Alignment.
-Vertical alignment consists of straight line grades (tangents or gradients) & the vertical curves used to connect them
-Two types of vertical curves:
-crest curves occur on hills
-sag curves occur in valleys
Vertical Curves.
-Main design criteria:
-Provision of minimum stopping sight distance
-Headlight sight distance
-Adequate drainage
-Comfort in operation
-Aesthetic
-Minimum length of a crest vertical curve needs to satisfy the safety, comfort, & appearance criteria
-Design of a sag vertical curve needs to satisfy at least headlight sight distance. passenger comfort, drainage & appearance criteria.
Grade.
-Grade affects vehicle speed & vehicle control, particularly for large trucks.
-The adopted criteria express values for both max & min grade
-max grades to reduce the potential for drivers of heavy trucks to
lose control as they descend steep grades
-min grades to achieve proper drainage have also been
established
-Max grades on highways should be selected to limit grade effect on vehicle performance. Speed differential on highways with steep grades can contribute to safety & operational problems
Stopping & Sight Distances.
-SSD allows alert, competent drivers to come to a quick stop under ordinary circumstances. This distance is usually inadequate when drivers must make complex decisions, when information is difficult to find, when information is unusual, or when unusual maneuvers are required.
-In these instances, the proper sight distance to use is the DSD.
-Because DSD allows drivers to maneuver their vehicles or vary their operating speed rather than just simply stop. DSD is much greater than SSD
Vertical Clearance.
-Minimum values for the various highway functional classifications
-Consequence of insufficient vertical clearance are obvious
-As a minimum, need signage for insufficient clearance
Cross Slope.
-To drain water to the side of the road
-reduces ponding on the side of the road
-minimizes icing on poorly drained pavement
-reduces maintenance problems
-Both maximum & minimum criteria exist for cross slope
-Typically 2%; most recently 3%
Lane Width.
-Different design values for through travel lanes, auxiliary lanes, ramps, & turning roadways
-Lane width has an effect on traffic operations & highway capacity
-On high speed roadways, narrow lanes increase risk of cross centerline head on or run-off road crashes due to lane departure potential. However, in urban environments, narrower lane widths may be chosen to reduce speed
-Curvilinear horizontal alignments increase the risk of lane departure crashes in general, & when combined with narrow lanes widths, will increase risk.
Shoulder Width.
-Shoulder width has a measurable effect on traffic safe operations & highway capacity, particularly for high speed roadways
Bridge Width.
-Bridge width is the total width of all lanes & shoulders on the bridge
-The widths are typically narrower due to the cost of the bridge
Road Classification - Rural.
-Local (L)
-Collector (C)
-Arterial (A)
-Freeway (F)
Road Classification - Urban.
-Lane/Alley
-Local (L)
-Collector (C)
-Arterial (A)
-Expressway (E)
-Freeway (F)
Mobility vs Access.
-All roads provide service to traffic, access to land or both
-Freeways, expressways & arterials primarily move traffic
-Local roads & lanes are used almost exclusively for access
-Collectors provide for both
Hierarchy of Cross-Section Elements.
-Reduced width or eliminate border
-Reduced width or eliminate boulevard
-Eliminate parking lane(s)
-Reduce or eliminate median width
-Use minimum lane width
-Use minimum sidewalk width
-Reduce number of travel lanes
Designs that Lead to Higher Speeds.
Cross Section:
-number of lanes
-road width
-width of obstacle free zone
-presence of emergency lane
-presence of cycle track or service road
-presence of road marking
Alignment:
-fewer bends
-level road surface
Road Environment:
-fewer buildings along road
-less vegetation along road
