Chapter 6 Design of Highway At-Grade Intersections

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Chapter 6Design of Highway At-Grade Intersections

• typical intersection types and their
  characteristics
• general design principles for at-grade
  intersections
• Know how to find minimum radii for the curves at
  at-grade intersections
• Know what the channelization is, objectives for
  using channelization, and factors affecting its
  application

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Intersection types
At-grade
Grade separated
With ramps (Interchanges)
Without ramps (meaning no connection between the
intersection roads!)
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Topics to be covered

• Types of intersections
• Objectives in the design of at-grade
  intersections
• Alignment considerations
• Grade considerations
• Curve types and minimum radii
• Channelization what it is, why we need it, how
  we provide it

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At-grade intersections (pay attention to
channelization techniques) T or 3-leg
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Speed change lanes
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At-grade intersections (pay attention to
channelization techniques) 4-leg
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At-grade intersections (pay attention to
channelization techniques) Multi-leg
Mulry Square in Greenwich Village, Manhattan, New
York City
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• Rotary Intersections (Roundabouts)
• A rotary intersection is one in which all traffic
  merges into and emerges from a one- way road
  around a central island. There is little delay to
  traffic due to speed reductions and no delay at
  all due to stopping. Its advantages include
• 1. Continuos traffic movement from all legs
• 2. Accidents are likely to be less serious
• 3. Where more than 4-legs are involved , the
  design layout may be simplified
• 4. The cost of this type may be considerably less
  than that of grade separation structures.

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• Disadvantage include
• 1. Requires large area
• 2. Costs are more than other at-grade
  intersections
• 3. It is not suitable for large pedestrians
  movements
• 4. There is little delay due to speed reduction
• 5. It needs long weaving sections to ensure
  smooth flow.

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Typical markings for roundabouts with one lane
Roundabouts different from circles

• Circles may have a signal, STOP signs,
  no-control at entry Roundabouts are always
  controlled by Yield signs.
• Splitter islands
• Peds are not allowed to use the central island
• No parking in the circle
• Circulating vehicles always have the ROW.

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Roundabouts different from circles

• Advantages
• Provides non-stop movements
• Reduce crash occurrences
• Reduce crash severity
• Esthetically appealing
• Function as a traffic calming measure

In Australia
Good for low to medium traffic. Definitely NOT
for high volume intersections ? Too many weavings
In Maryland
In Norway
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At-grade intersection design objectives and
considerations

• Need to meet two conflicting objectives
• Minimize the severity of potential conflicts
  among different streams of traffic and between
  pedestrians and turning vehicles.
• Provide for the smooth flow of traffic across
  the intersection

Adequate pavement width and approach sight
distances must be provided.
Operating characteristics of both the vehicles
and pedestrians
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At-grade intersection design considerations

• Alignment and profile design

• Angle of intersecting roads

• Suitable channelization system for the traffic
  pattern

• Minimum required widths of turning roadways

• Adequate sight distance for the type of traffic
  control used (no control, Yield, Stop, Signal)

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Alignment
Suggested improvements to intersections with
acute angle problems
Longer walking distance
Angle of turn
Obtuse angle
The angle of turns should be 60 to 120 degrees.
Acute angle
Superelevation problem
Wider pavement needed for turning vehicles
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Profile

• Make it as flat as possible
• Avoid approach grades in excess of 3
• Avoid grade changes at intersections (Crest ?
  sight distance problems, Sag ? drainage problems)
• The grade line of the major highway should be
  carried through the intersection. Adjust the
  grade for the normal crown of the crossroad to an
  inclined cross section at its junction with the
  major road. ? A good example at Columbia Lane and
  Grandview Road (the signalized intersection on
  Columbia Lane just north of DI).

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Curves at at-grade intersections
Rule 1 When the turning speed at an intersection
is assumed to be 15 mph or less, the curves for
the pavement edges are designed to conform to at
least the minimum turning path of the design
vehicle. If the speed is greater than this, the
design speed is also considered to determine the
radius (Remember? R u2/(ge fs). ? This
means that you are not supposed to use the values
in Table 7-2 and 7-3 in the textbook. Rule 2 The
angle of intersection affects the curve design.

• Typical types
• Simple curve
• Simple curve with taper
• 3-centered compound curve

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Three typical curve design methods
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Minimum edge of pavement designs Simple curve
and simple curve with taper (Table 7.2)
R
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Channelization objectives

• Direct the paths of vehicles
• Control the merging, diverging, and crossing
  angle of vehicles
• Reduce the amount of paved area
• Provide a clear indication of the proper path
  for different movements
• Give priority to the predominant movements
• Provide pedestrian refuge
• Provide separate storage lanes for turning
  vehicles

• Provide space for traffic control devices for
  visibility
• Control prohibited turns
• Separate different traffic movements at
  signalized intersections with multiple-phase
  signals
• Restrict the speeds of vehicles

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Channelization considerations

• Motorists should not be required to make more
  than one decision at a time.
• Sharp reverse curves and turning paths greater
  than 90 degrees should be avoided.
• Merging and weaving areas should be as long as
  possible, but other areas of conflict between
  vehicles should be reduced to a minimum
• Crossing traffic streams that do not weave or
  merge should intersect at 90 degrees, although a
  range of 60-120 degrees is acceptable

• The intersecting angle of merging streams should
  be such that adequate sight distance is provided.
• Refuge areas for turning vehicles should not
  interfere with the movement of through vehicles
• Prohibited turns should be blocked wherever
  possible
• Decisions on the location of essential traffic
  control devices should be a component of the
  design process.

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Chapter 7Grade Separations- and interchanges
Intersections

• Although the construction cost of the grade
  separated structures is very high, but the
  following advantages justify its uses
• 1. To ensure free movement of high speed traffic
  on motorways
• 2. Increase capacity lane to its initial value
• 3. Increase safety for all vehicles

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• 4. More economic in operation and losses and
  cheaper in expensive areas
• 5. Save time and money and more comfort
• 6. It can be implemented at any site.

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• The separated at-grade intersections have the
  following disadvantages
• 1. High construction costs and maintenance
• 2. Not suitable for five or more legs
  intersections
• 3.The layout of grade separation may be confusing
  to some drivers .

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• Interchanges can be divided into the following
  main types
• a) 3- way interchanges may be a T -or Y types
  intersection as shown in Fig.34. Both types
  utilize a single bridge structure and are
  suitable for expressway provided that the loop
  movement is relatively small. In the case of
  heavy movement, an extra bridge may be
  constructed so that both turning movements are
  favored equally.

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• b) 4- way interchanges the simplest type of
  4-way interchanges is the diamond, which
  consisting of a single bridge and four way ramps.
  This type can be located within relatively narrow
  land area. It has high speed entrance and exit
  ramps on the main road and at -grade ramp
  terminals on the minor road.

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• c) The Cloverleaf It is only 4- way single
  structure interchange having no terminal left
  turns at-grade. It is the most common form of
  interchange and is required as the ultimate
  answer to intersection problems. It being very
  uncomplicated to use with

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Examples of grade separated interchanges
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Examples of grade separated interchanges (cont)
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More examples of directional interchanges
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