Cross Section Elements 1

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Cross Section Elements (1)

• CE562
• Text A Policy on Geometric Design, pp.305 - 375

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Pavement Surface Types
High-type pavement has smooth riding qualities,
good skid-resistance properties in all weather,
long life span without fatigue failure. Low-type
surface ranges from surface-treated earth roads
to loose surface such as earth, crashed stone,
gravel. It requires greater steering
effort. Intermediate-type surface ranges from
surface treatments to pavements only slightly
lower quality than high-type pavements. High
traffic volume and high design speed warrant the
use of the high-type surface type. Low traffic
volumes and low design speed warrant the use of
the low-type surface.
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Pavement Cross Slopes
Undivided traveled ways (two- and multilane) on
tangents and flat curves have a crown in the
middle and slope downward toward both edges. The
downward cross slope may be a plane or rounded
section (parabolic), or a combination of the
two. One-way traveled ways on divided highways
may be crowned separately or may have a
unidirectional cross slope (almost always
downward to the outer edge).
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Exhibit 4-2
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Exhibit 4-3
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Crowns vs. Unidirectional Slopes Divided Roadways
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Cross Slopes on Tangents
Contradictory design controls A steep lateral
slope reduces water ponding and the width of the
water flow along the curb. A flat lateral slope
reduces vehicles' drift towards the low
edge. Recommended design controls Lateral drift
of vehicles at high speed is barely perceptible
on cross slopes up to 2. The slope of 1.5-2.0
is acceptable on high-speed highways. In the
areas of intense rainfall a maximum cross slope
is 2.5. Crown section Change in the cross slope
of 3-4 causes swaying of high body vehicles.
Rounded crowns reduce discomfort. Constructing
transition cross-sections from tangent to
superelevated pavement may be difficult.
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Curbed Highways

• The minimum slop values of 1.5-2 in areas with
  intense rainfalls will cause wide sheet of water
  on the curbed traveled way.
• Possible improvements
• parabolic cross section with increasing cross
  slope towards the outer edges,
• gutter along the curb with the cross slope larger
  than on the traveled way,
• on multilane traveled way, cross slope broken
  along traffic lane edges, increasing from the
  minimum value on the innermost lane up to the
  maximum value on the outermost lane. This
  solution is used on uncurbed sections as well.

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Lane Widths

• Lane width strongly influences traffic safety and
  comfort
• Lane width range is 2.7-3.6 m with the 3.6 m lane
  predominant on high-type highways
• Two-lane two-way highways with the 3.6 lane
  provide safe clearance between large commercial
  vehicles
• Circumstances that justify narrower lanes
• Urban areas with land restrictions -- 3.3 m
• Low speed facilities -- 3.0 m
• Auxiliary lanes at intersections -- 3.0 m
• Low-volume roads in rural and residential areas
  -- 2.7 m
• Continuous two-way left-turn lanes -- 3.0-4.8 m

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Anything wrong with this cross section?
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Shoulders

• Functions of shoulders
• accommodation of stopped vehicles (disabled
  vehicles, bus stops)
• emergency use
• lateral support for the pavement
• space for roadside facilities
• space for bicycles and pedestrians
• driving comfort (freedom from strain)
• improvement in sight distance
• improvement in capacity
• Graded shoulder between the edge of the
  traveled way and the intersection of the shoulder
  slope and the foreslope plane
• Usable shoulder part of the graded shoulder
  that can be used by the stopped vehicle

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Exhibit 4-3
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Shoulders
Shoulder can be surfaced by using turf, gravel,
crushed rock, mineral or chemical additives,
bituminous treatment, asphalt or concrete
pavements. Functions of shoulders on low-type
vs. high-type roads Width of shoulders Low-type
roads -- minimum 0.6 m, recommended 1.8-2.4
m, Shoulder usable by bicycles -- minimum 1.2
m, High-type roads -- minimum 3.0 m, recommended
3.6 m. Clearance to roadside elements (barriers,
walls, signs, etc.) High-type roads -- 0.6 m to
the edge of the usable shoulder, Low-type roads
-- minimum 1.2 m to the edge of the traveled way.
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Shoulders

• Other rules
• Shoulders should be continuous.
• Shoulders on bridges should have the same width
  as on the approach sections.
• Cross slopes
• Bituminous and concrete shoulders -- 2-6 ,
• Gravel and crushed rock shoulders -- 4-6 ,
• Turf shoulder -- about 8 .
• Clear zones
• Continuous and clear of exposed obstructions
  uncurbed high-speed roads governed by AASHTO
  Roadside Design Guide, - uncurbed low-speed roads
  3.0 m, curbed roads 0.5 m.

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Shoulders

• Shoulder stability
• All types of shoulders must be flush with the
  traveled way. Unstabilized shoulders need regular
  maintenance.
• Paved or stabilized shoulders reduce maintenance
  and improve lateral support for the pavement of
  the traveled way.
• In the most firm solution, the whole roadway is
  surfaced including shoulders.
• Shoulder contrast
• Different color and surface of the shoulder
  discourage drivers from using it as a traveled
  way. Bituminous, crushed stone, gravel and turf
  shoulders offer excellent contrast with concrete
  pavements. Stone aggregates and turf offer good
  contrast with bituminous pavements.
• Edge lines should be used when the whole roadway
  is paved.
• Depressed rumble strips alert drivers that cross
  over onto the shoulder but they make the shoulder
  useless for bicycles.

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Curbs

• Functions of curbs
• drainage control
• roadway edge delineation
• right-of-way reduction
• delineation of pedestrian walkways
• reduction in maintenance operation
• Types of curbs
• Vertical curbs -- discourage vehicles from
  leaving the roadway, not desirable on high-speed
  highways, desirable on urban roads, and along
  long walls, tunnels to protect safety walks.
• Sloping curbs -- vehicle can cross them readily
  when required, used at median edges, to outline
  channelizing islands, at the outer edge of the
  shoulder.

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Vertical Curb
Sloping Curbs
Exhibit 4-6
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