King Fahad University for Petroleum and Minerals Civil Engineering Department

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King Fahad University for Petroleum and
MineralsCivil Engineering Department

• CE-576-Geometric Highway Design
• Chapter III
• Instructor Dr. Nedhal T. Ratrout

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Chapter III
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Objective

• Curvature of Turning Roadways and Curvature at
  Intersections.
• Offtracking.
• Traveled Way Widening on Horizontal Curves.
• Widths for Turning Roadways at Intersections.
• Other Elements Affecting Geometric Design.

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Curvature of Turning Roadways Curvature at
Intersections
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Minimum Radius or Turning speed

• Desirable turning speed for design is the average
  running speed of traffic on the highway
  approaching the turn for movements with little or
  no conflict with pedestrians or other vehicular
  traffic
• Side friction factors used on curves at
  intersections are higher than side friction
  factors accepted used on high-speed highways.

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• The following figure shows a result of studies of
  speed on different intersections curves. 95
  percentile speed was assumed to be close to the
  design speed it is indicated on 34 locations in
  graph with their relative side friction factors
  (e is taken in to account). The average curve is
  drawn based on 95 percentile speed and f about
  0.5 (developed with low speeds).

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• The following table shows the minimum radii for
  intersections curves . Minimum radii are
  calculated from formula 3-9 (pg. 133).

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• The values in the previous table are illustrated
  on the following curves

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• The minimum radii used in design are preferably
  used on the inner edge of the traveled way rather
  than on the middle of vehicle path or centerline
  of traveled way.
• e rate of at least 8 is desirable at all
  locations rates of 8.0 to 10 if ice or snow
  are not a factor, but at signalized intersections
  lesser amount of e is appropriate.
• If there are significant number of large trucks
  for each design speed, flatter curves less e
  should be provided.
  

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Length of Spiral
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The following table shows the min. length of
spiral for intersection curves which obtained
from Shrott formula (pg.177)
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• Values of C (the rate of change of lateral
  acceleration) shown in the previous table are
  higher than the values used on open highways.
• The values of min. length of spiral can be used
  for spirals connecting two circular arcs with
  widely different radii, min. radius will be the
  difference in the radii of the two curves.

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• In case of spiral connecting compound curve at
  intersection with radius of one curve is more
  than twice the radius of the other, the length of
  spiral is less than 30m 100ft will be used.

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Compound Circular Curves

• On open highways the ratio of the flatter radius
  to the sharper radius should not exceed 1.51 .
  For intersections ratio can be 21, this ratio
  results in approximately the same difference in
  average running speeds for two curves (about 10
  km/h 6 mph). Ratio of 21 is also acceptable
  for ramps with difference in radii.

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• A desirable max. ratio is 1.751 if smaller
  difference in radii should be used.
• If the ratio is greater than 21 a spiral or
  circular arc of intermediate radius should be
  inserted between the two curves.
• Compound curves should have enough length to
  enable driver to decelerate at reasonable rate
  (3-5 km/h 2-3 mph/s at intersections)

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Based on those deceleration rates the following
table shows the min. curve length based on the
running speeds shown in Exhibit 3-44 (pg.202)
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• If the travel is not in the direction of sharper
  curvature ratio of 21 is not critical and may be
  exceeded.

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Offtracking
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General

• Offtracking happens because of slip angle by the
  tires to the direction of travel which results
  from the friction developed between the pavement
  and rolling tires.

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Derivation of Design Values for widening on
Horizontal Curves

• The amount of widening needed on horizontal
  curves increases with the size of design vehicle
  decreases with increasing in radius of
  curvature.

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• Width elements of design vehicle used in
  determining the roadway widening on curves
  include
• Track width on curve, U Is calculated using
  equation 3-35(pg.207) it can be obtained on
  curves in exhibit3-47(pg.208).
• The lateral clearance allowance, C The
  clearance between the edge of the traveled way
  nearest wheel path for body clearance between
  vehicles passing or meeting. Its assumed to be
  0.6, 0.75 0.9 2.0, 2.5, 3.0 ft, for tangent
  lane width Wn 6.0, 6.6 7.2m 20, 22 24 ft
  respectively.

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• The width of the front overhang, FA Is the
  radial distance between the outer edge of the
  tire path of the outer front wheel the path of
  the outer front edge of the vehicle body. It is
  calculated using equation 3-36(pg.209) it can
  be obtained from Exhibit 3-48(pg. 210).

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4- The width of rear overhang, FB Is
the radial distance between the outer edge of the
tire path of the inner rear wheel the inside
edge of the vehicle body.
5- The extra width allowance, Z Is an
additional radial width of pavement to allow for
the difficulty of maneuvering on a curve the
variation in driver operation. It is calculated
by using equation 3-37(pg.209) it can be
obtained from Exhibit3-49 (pg.211).
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Traveled way widening on Horizontal Curve

• For one of the following reasons traveled way
  widening on horizontal curve is required
• The design vehicle occuping greater width because
  of offtracking.
• Drivers experience difficulty in steering their
  vehicles in the center of the lane.

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The amount of widening of traveled way on
horizontal curve can be given as
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• The following figure shows the components of Wc

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The width of traveled way on curve, Wc is
calculated by the following equation
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f

• Appropriate design vehicle should be truck
  because of offtracking in truck is larger than
  passenger cars. Intermediate Semitrailer (WB-15
  WB-50) is representative design vehicle for
  two-lane highway. The values of traveled way
  widening for this design vehicle shown in
  Exhibit3-51 (pg.215)

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• Where both sharper curves (as for 50 km/h 30mph
  design speed) large tuck combinations are
  prevalent, the values in Exhibit3-51 should be
  adjusted in accordance with Exhibit3-52 (pg.217).
• The difference in track width (U) of design
  trucks are substantial for sharp curves
  associated with intersections, but for open
  highway with radii larger than 200m with design
  speed over 60 km/h 30mph the differences are
  insignificant ( see Exhibit3-47, pg. 208).

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Design Values for Traveled Way Widening

• Because drivers are not in position to judge
  clearance when passing vehicle or when meeting
  opposing vehicle because all geometric elements
  on a divided highway are well maintained the
  values of widening shown in Exhibit 3-51 can be
  used for two-lane, one-way traveled way of
  divided highway.

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• The discussed values of traveled way widening are
  applicable for open-highway but for intersection
  conditions with generally smaller radii on
  turning roadway the criteria for design widths
  are somewhat different.

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Application of Widening on Curves

• On simple (unspiral) curves, widening should be
  applied on the inside edge of the traveled way
  only, the extension of the outer-tangent avoids a
  slight reverse curve on the outer edge. On curves
  with spirals, widening may be applied on the
  inside edge or divided equally on either side of
  the centerline, the final marked centerline and
  any longitudinal joint, should be placed midway
  between the edges of the widened traveled way.

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• Curve widening should transition gradually over a
  length sufficient to make the whole of traveled
  way fully usable. Preferably, widening should
  transition over superelevation runoff length, but
  shorter lengths are sometimes used. Changes in
  width normally should be effected over a distance
  of 30 to 60m 100 to 200ft

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• The edge of the traveled way through the
  widening transition should be smooth, graceful
  curve. A tangent transition edge should be
  avoided. In any event, the transition ends should
  avoid an angular break at the pavement edge.

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• On highway alignment without spirals, widening is
  attained with 1/2 to 2/3 of the transition length
  along the tangent and the balance along the the
  curve. The inside edge of traveled way may be
  designed as a modified spiral,parabolic or cubic
  curve, or larger radius compound curve. Otherwise
  it may be aligned by eye in field. On highway
  with spiral, the increase in width is distributed
  along the length of spiral.

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• Widening areas can be fully detailed on
  construction plans. Alternatively, general
  controls can be cited on construction or standard
  plans with final details left to the field
  engineer.

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Widths for Turning Roadway at Intersections

• Three cases are commonly considered in design
• Case I- one-lane, one-way operation without
  provision for passing a stalled vehicle

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• Case II- one-lane, one-way operation with
  provision for passing stalled vehicle is used to
  allow operation at low speed with sufficient
  clearance so that other vehicles can pass a
  stalled vehicle.

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• Case III- two-lane operation, either one-way or
  two-way is applicable where operation is two way
  or operation is one-way, but two lanes are needed
  to handle the traffic volume.

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Design Values

• The derived widths for various radii for each
  design are given in exhibit 3-54 (pg.221). These
  recommended widths are not used directly because
  turning roadway usually accommodate more than one
  type of vehicle.
• Exhibit 3-55 (pg.224) shows values for three
  logical conditions of mixed traffic.

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• For case I if the design vehicles are WB-19 or
  WB-33D WB-62 or WB 109D their turning paths
  must be considered. If their width values in
  Exhibit 3-54 exceed the values of Exhibit 3-55
  the vales of Exhibit 3-54 will be considered as
  the min. width for turning.
• In traffic condition A the values in Exhibit
  3-55 are higher than those P vehicles in exhibit
  3-54.

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• In traffic condition B the values in Exhibit
  3-55 for case I III are those for SU vehicles
  in Exhibit 3-54. For case II values are reduced.
• In traffic condition Cthe values in Exhibit3-55
  for case I III are those for the WB-12 WB-40
  truck in Exhibit3-54. For case II values are
  reduced.

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The following table indicates the different
combinations used in determination values given
in Exhibit 3-55 by assuming full clearance for
the design vehicle indicated
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The larger vehicles that can be operated on
turning roadways of widths shown in Exhibit3-55
with partial clearance varies from ½C C are
shown below
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The following table is a summary of range of
usable shoulder widths or equivalent lateral
clearances outside of turning roadway, not on
structure
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• If roadside barriers are provided, the width
  should be measured to the face of the barrier,
  and graded width should be about 0.6m 2.0 ft
  greater.
• For other than low-volume conditions, it is
  desirable that right shoulders be surfaced,
  surface treated or stabilized for width of 1.2m
  4.0 ft or more.

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Other Elements Affecting Geometric Design
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Drainage

• Water surface elevation for flood or various
  return periods will influence decisions regarding
  the highway profile where an encroachment on the
  flood plain is considered.
• Stream crossing other highway encroachment on
  flood plains should be located aligned to
  preserve the natural flood flow distribution
  direction.

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• Surface channels are used in intercept remove
  surface runoff from roadways. They are lined with
  vegetation, and rock or paved channel linings.
• Curbs or dikes, inlets chutes or flumes are
  used where runoff from roadway would erode fill
  slopes. Where storm drains are needed curbs are
  usually provided.

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• Drainage inlets should be designed located to
  limit the spread of water on traveled ways to
  tolerable widths and to prevent slit debris
  carried in suspension from being deposited on the
  traveled way where the longitudinal gradient is
  decreased.

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• Drainage is more difficult costly for urban
  than for rural highways because of more inlets
  underground systems, lack of natural areas of
  water bodies to receive flood water higher
  volumes of traffic including pedestrians.
• Reduction of peak flow can be achieved by the
  storage of water that falls on the site in
  detention basins, storm drainage pipes, swale
  channels, parking lots, and rooftops.

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Erosion Control Landscape development

• Erosion maintenance are minimized by using flat
  side slopes, rounded end blended with natural
  terrain serrated cut slopes, well design for
  drainage channels inlets with keeping erosion
  in mind prevention of erosion at culvert
  outlets protective grounded covers planting
  and proper facilities for ground water
  interception, dikes, berms and other protective
  devices.

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• Landscaping of urban highways streets is good
  in mitigation the many nuisance associated with
  urban traffic and make urban highways streets
  better neighbors.

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Rest Areas, Information Centers Scenic
Overlooks

• A safety rest area is a roadside area, with
  parking facilities separated from the roadway,
  provided for the travelers to stop and rest for
  short periods.

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• An information center is a staffed or unstaffed
  facility at rest area for furnishing travel and
  other information or services to travelers.
• A scenic overlook is a roadside area provided to
  park vehicles, beyond the shoulder for viewing
  scenery or for taking photographs in safety.

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Lighting

• To minimize effect of glare for economical
  lighting installation, luminaries are mounted at
  heights of at least 9m 30ft. Mounting heights
  of 10 to 15 m 35 to 50 ft are preferable to
  improve lighting uniformity.
• High mast lighting, special luminaries on mast of
  30m 100ft, is used to light large highway
  areas such as interchanges rest areas.

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• Luminaire supports should be placed outside the
  roadside clear zones whnever practical.
• On divided highway or street, luminaire supports
  may be located either in the median or on the
  right side of the roadway.
• For median installation, dual-mast should be
  used, for which 12 to 15 m 40 to 50 ft mounting
  height are favored.

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UtilitiesUtilities in Urban

• Vents,drains, markers, manholes shutoffs that
  related to underground installations should be
  located so as not interfere with the safety or
  maintenance of the highway or street not to be
  concealed by visitation. They should be located
  near the right-of-way line.

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• In case of curbed sections, utilities should be
  located in the border areas between the curb
  sidewalk, at least 0.5m 1.5ft behind the face
  of the curb.
• In case of existing development limited
  right-of-way widths, area outside the roadway can
  be used for overhead lines with other utilities
  located under the roadway. In case of placing all
  facilities at underground roadway, it should be
  accomplished in manner with minimum adverse
  effect on traffic because of maintenance future
  utility service

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UtilitiesUtilities in Rural

• On new construction no utility should be situated
  under any part of the roadway, except where it
  crosses the highway.
• No poles should be located in the median of
  divided highway above-ground utility that may
  struck by vehicles should not be permitted with
  in the highway clear zone.

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• The only exception permitted would be where the
  utility is breakaway or installed behind traffic
  barrier erected to protect the vehicles.
• For low-speed rural collectors rural local
  roads, except for very low-volume local roads
  with ADTs less than or equal to 400 vehicles/day,
  minimum clear zone of 3m 10ft should be
  provided.

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Traffic Control Devices Signing Marking

• Highway signs are one of the three general types
• Regulatory signs used to indicate the rules of
  traffic movement.

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• Warning signs used to indicate conditions that
  may involve risk to highway users.
• Guide signs used to direct traffic along a rout
  or toward a destination.
• Supports for the highway signs should be placed
  on structures, outside the clear zone, or behind
  traffic barriers placed for other reasons. Or the
  sign supports should be breakaway or overhead
  sign supports.

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For highways streets there are three general
types of marking or markers

• Pavement marking include centerline strip, lane
  lines, and edge striping of various word
  symbol markings, approach to obstruction, stop
  cross walk lines.
• Object marking objects should be adequately
  marked by painting or by use of other high
  visibility material. If the object in direct line
  of traffic, marking should be illuminated at
  night by floodlighting or at least be effectively
  reflectorized.

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• Delineators used to guide traffic, particularly
  at night. They are installed at certain heights
  spacing to delineate the roadway where alignment
  changes may be confused not clearly defined.

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Traffic Control DevicesTraffic Signals

• Careful consideration should be given for
  horizontal vertical curvature with respect to
  signal visibility, pedestrian needs, and
  geometric schematics to ensure effective signed
  operation, potential future signal needs should
  be evaluated.

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• Lane arrangement is the key to successful
  operation of signalized intersections. Therefore,
  a complete analysis of current future traffic
  demand should be done and evaluating the need to
  provide right-and left-turn concurrently with the
  potential for obtaining any additional
  right-of-way needed.
• Parking areas entrances exits should be
  designed in manner that will simplify the
  operation of the affected traffic signals.

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Noise Barrier

• Construction of a noise barrier should be avoided
  at a given location if it limit stopping sight
  distance below minimum values in Exhibit3-1
  (pg.112) epically if the location of it is along
  the inside of a curve.
• On non-tangent alignments a separate concrete
  roadside barrier may obstruct sight distance even
  though the noise barrier does not, installation
  of metal rather than concrete will be better to
  give adequate sight distance.

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Fencing

• Any portion of highway with full control of
  access may be fenced except in areas of
  perception slopes, natural barriers, or where it
  can be established that fencing is not needed to
  preserve access control.
• Fencing is usually located at or near the
  right-of-way line or, where frontage roads are
  used, in the area between the through highway
  the frontage road (outer separation).

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Maintenance of Traffic Through Constrution Areas

• Developing traffic control plans in construction
  areas, detours temporary connections includes
  the following items that should be considered
• Diversion detour alignments to allow traffic to
  pass smoothly around the work zones with a
  reasonable speed safe movement.
• Adequate tapers for lane drops or where traffic
  is shifted laterally.

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• In urban areas, the selected diversion paths
  should include safe roadway crossing, a smooth
  surface, and adequate width to accommodate
  persons with disabilities.
• Adequate traffic control devices pavement
  marking for daytime and nighttime effectiveness.

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• Roadway illumination warning lights where
  justified.
• The location of cones, delineators, drums,
  barriers, or barricades, to channelized traffic,
  when special conditions exist or if not shown in
  the standard plans.
• Policies concerning removal of sign marking
  from the job site when they are no longer needed.

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• The removal of contractor equipment completely
  off the roadways, median shoulders should be
  done at night, on weekends, and whenever
  equipment is not in operation. Storage of the
  hazardous materials should not be on roadways,
  medians or shoulders near the flow of traffic.
• A requirement in the plans to prohibit the
  parking of employees vehicles in those areas on
  the project which may affect the safety of
  workers.

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Thank you