Geometric Design

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Geometric Design

• Wright, Chapter 7

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Geometric Design

• Goal provide geometric elements that
• are economically feasible
• provide safe, continuous operation
• Elements
• location
• alignment
• cross-section
• access
• Challenge find optimum combination of features

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Design Standards

• State-to-state variation
• State standards follow guidance issued by AASHTO
• Regional differences due to finance, climate,
  need
• Uniformity facilitates understanding by
  construction contractors

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Design Controls and Criteria

• Design influenced by a number of factors
• Safety
• Functional class and level of service
• Traffic volume and composition (ADT, AADT, DHV,
  Directional distribution, Trucks (), Design
  speed)
• Topography
• Users
• Design Vehicle characteristics
• Environmental
• Right-of-way
• Design features influence all visible features
  and affect capacity, safety performance and
  social acceptance

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Relationship of Traffic to Design

• Traffic Elements
• ADT, AADT, DHV
• Directional distribution (D) one-way volume in
  predominant direction (rural 2-lane road, D0.60)
• Truck percentage
• Design Speed (V)
• K-factor ratio of traffic occurring during peak
  hour for an average weekday (K ranges 0.09 to 0.1)

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Design Speed

• A speed used to determine the various geometric
  design features of a roadway
• Factors
• Terrain
• Traffic volume and composition
• Cost
• Aesthetics

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Design Vehicle

• Dimensions of design vehicle
• Passenger car
• Trucks (5-19 )
• Buses
• Recreational Vehicles

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WB-50 Design Vehicle
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Critical Length of Grade
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Road and Highway Alignment

• Physical location
• Conditions are consistent driver expectancy
• Balance between grades and curvature (graphic)
• Horizontal alignment tangents connected by
  curves
• Curves arc definition or radius
• superelevation
• Stopping sight distance
• Vertical alignment

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Alignment Guidelines

• Topography be consistent
• Long tangents avoid sharp curves at end (see
  graphic)
• Avoid broken back curves
• Conform to vehicle path
• Minimize number of curves

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Horizontal Circular Curve

• Arc Definition degree of curve is the angle
  subtended by a 100 foot long arc (Fig 7-6)
• arc length travel lane

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Horizontal Curve

• degree of curvature
• rural highway (D ranges from 5-7 degrees)

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Impact of Curves on Travel Time

• Loss of time due to
• deceleration
• curve time
• acceleration
• Safe speed on curves
• speed at which side slip occurs
• sight distance

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Effect of Curvature on Truck Speed (USFS)
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Superelevation Theory

• Vehicle on curve is forced radially outward by
  centrifugal force
• Force countered by vehicle weight component of
  superelevation and side friction between tires
  and road surface

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Superelevation Theory

• From laws of mechanics
• v velocity (ft/sec)
• V velocity (mph or km/hr)
• e rate of superelevation ()
• f side friction factor
• g gravitational constant 32.2 ft/sec2
• (ef) is usually small
• so (1- 0.1ef) 1
• Use to find minimum Radius for design

US Units
SI Units
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Factors Affecting Superelevation

• Coefficient of side friction (f)
• Drivers sense of discomfort
• f 0.10 for 70 mph, f 0.16 up to 60 mph
• new tires/ wet pavement
• 0.5 (25 mph) to 0.35 (45 mph)
• Climate, terrain, location, slow vehicles
• Snow and ice e 0.08 maximum
• Highway, gravel or low volume max e 0.12
• congested traffic e 0.06 maximum

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Superelevation
US Units
SI Units
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Sight Distance

• Provide ample stopping sight distance for safe
  operation
• Vertical curves
• Relationship of drivers eye, object height and
  length of curve
• Horizontal curves
• degree of curve and obstruction inside curve

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SSD

• SSD perception reaction distance braking
  distance
• PRD 2.5 sec x v (ft/sec)
• Braking distance f(velocity, deceleration rate
  11.2 ft/sec/sec)
• Effect of Grade (G, )

US Units
SI Units
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Vertical Curve Stopping Sight Distance

• Sight Distance is the length of roadway visible
  to the driver. Generally, passing sight distance
  and stopping sight distance govern geometric
  design.
• For S
• For SL

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