CE562 Lecture 5 Horizontal Alignment 1

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CE562 Lecture 5Horizontal Alignment (1)
Text A Policy on Geometric Design, pp. 131-231.
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General Consideration
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Theo-reticalConsi-deration
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Maximum Superelevation

• Superelevation cannot be too large since an
  excessive mass component may push slowly moving
  vehicles down the cross slope.
• Limiting values emax
• 12 for regions with no snow and ice conditions
  (higher values not allowed),
• 10 recommended value for regions without snow
  and ice conditions,
• 8 for rural roads and high speed urban roads,
• 4, 6 for urban and suburban areas.

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Limiting Superelevation Rates(One-Meter Lateral
Shift)
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Maximum Friction

• Maximum side friction factor on wet concrete
  pavements ranges from 0.45 at 100 km/h to 0.5 at
  30 km/h (vehicle skids)
• Drivers feeling of discomfort
• Values much lower than the maximum side friction
  factors are used in design

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Maximum Friction
Exhibit 3-11
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Minimum Radius
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Distribution of e and f (RgtRmin)

• Extreme Policies
• use superelevation to the maximum extent
• use friction factor to the maximum extent  
• An intermediate policy is recommended

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Design for Rural Highways and High-Speed Urban
Streets
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Maximum Side Friction
Exhibit 3-11
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Distribution of e and f

• Policy number 5 is used.

Exhibit 3-15
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Superelevation Rates Determined by Policy 5
Exhibit 3-18
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Superelevation Rates Determined by Policy 5
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Minimum Radii for Curves without Superelevation
Very large radii enable using the normal cross
slopes since the centrifugal force is so weak
that can be balanced by the side friction even
where the cross section is reverse (the mass
component adds to the centrifugal force).
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Tangent-to-Curve Transition

• Provides natural path for drivers
• Improves appearance of the highways and streets
• Accommodates distance needed to attain
  superelevation
• Accommodates gradual roadway widening

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Superelevation Runoff and Tangent Runout

• Normal cross section

Tangent runout the length of highway needed to
change the normal cross section to the cross
section with the adverse crown removed.
Superelevation runoff the length of highway
needed to change the cross section with the
adverse crown removed to the cross section fully
superelevated.
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Location of Runout and Runoff
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Design Requirements for RunoffsMaximum Relative
Gradient
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Design Requirements for RunoffsMaximum Relative
Gradient
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Design Requirements for RunoffsMaximum Relative
Gradient
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Design Requirements for RunoffsMaximum Relative
Gradient
Lr minimum length of superelevation runoff
(m), ? maximum relative gradient (), n1
number of lanes, bw adjustment of number of
rotated lanes, w traffic lane width (m), ed
supperelevation rate ().
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Design Requirements for Runoffsbw
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Minimum Length of Tangent Runout
Lt minimum length of tangent runout (m), eNC
normal cross slope rate (), ed superelevation
rate (). Lr minimum length of superelevation
runoff (m).
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Minimum Lengths of Runouts and Runoffs
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Transition Curves - Spirals
The Euler spiral (clothoid) is used. The radius
at any point of the spiral varies inversely with
the distance.
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Spiral Use
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Design of Spirals

• Two-second driving on a spiral curve is desirable
  (Exh. 3-34)

p tangent-circular curve offset, pmin 0.2 m,
pmax 1.0 m, R radius (m), V design speed
(km/h), C maximum rate of change in lateral
acceleration, C 1.2 m/s3.
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Location of Runouts and Runoffs

• Tangent runout proceeds a spiral
• Superelevation runoff Spiral curve

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Attaining Superelevation (1)
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Attaining Superelevation (2)
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Attaining Superelevation (3)
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Runoffs with Medians

• Case I - The whole of the traveled way, including
  the median, is superelevated as a plane section.
  Used for narrow medians and moderate
  superelevation.
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• Case II - The median is held in the horizontal
  plane and the two traveled ways are rotated
  separately around the median edges. Used for
  medians of intermediate width to about 10 m.
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• Case III - The two traveled ways are separately
  treated for runoff with a resultant difference in
  elevation at the median edges. Used for medians
  of about 12 m or more in width.