Intersection Design

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Intersection Design

• FEG 437

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Intersections Design Goal

• To reduce the severity of potential conflicts
  between motor vehicles, pedestrians, and
  facilities while facilitating the convenience,
  ease, and comfort of people traversing the
  intersection. AASHTO

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Design Objectives

• Give due consideration to sight distance, signs,
  grades and alignment
• Provide maximum convenience and ease of movement
• Elements of design
• human factors (habits, decision/reaction time)
• traffic considerations (design vehicle, speed,
  capacity)
• physical elements (sight distance, property)
• economic factors (energy, environment)

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General

• At-grade intersections join 2 or more roads
• Provide sufficient clearance between vehicle and
  other lanes for turning movements by design
  vehicles at less than 10 mph
• See graphics

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Operations

• Sight Distance Drivers require unobstructed view
  of intersection
• to allow drivers to perceive conflict
• Geometry needs to be simple
• Provide natural paths for movements
• Rules of road for uncontrolled intersections
  vehicle on left yields right of way to vehicle on
  right

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Sight Distance Obstruction
Hidden Vehicle
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Intersection Sight Distance

• Concept of Clear Sight Triangles (see Graphic)
• Area free of obstruction to complete maneuver
• Basic Approach driver without right-of-way can
  stop in time to yield right of way
• Stopped vehicles can enter the intersection
  safely into the path of oncoming vehicles
• Vertex is a decision point for braking to begin

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ISD Design Objectives

• Left-turn vehicle safely crosses
• Right turn vehicle can enter, accelerate without
  interfering with traffic
• Crossing maneuver

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Obstructions

• Drivers eye height assumed for design purposes
  to be 3.5 feet above raodway surface
• Approaching vehicle height assumed to be 3.5 feet
  above road

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ISD Cases

• No control vehicles adjust speed
• Stop control where traffic on minor roadway
  must stop prior to entering major roadway
• Yield control vehicles on minor roadway must
  yield to major roadway traffic
• Signal control where vehicles on all approaches
  are required to stop by either a stop sign or
  traffic signal
• All way stop
• Stopped major roadway left-turn vehicles must
  yield to oncoming traffic

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Critical speed is set to stopping distance db a
__da__
da - b
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Example
Large Tree
25 mph
72
47
45 mph
Is sufficient stopping sight distance provided?
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Example
Large Tree
25 mph
b 72
db
a 47
50 mph
da
db a __da__ da - b
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da 220 feet
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Example
Large Tree
25 mph
b 72
db
a 47
45 mph
da

• da 200 feet
• db a __da__ 47 (220) 69.9
• da b 220 72

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db 69.9 feet corresponds to 15 mph
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Example
Large Tree
25 mph
b 72
db
a 47
45 mph
da
25 mph gt 15 mph, stopping sight distance is not
sufficient for 25 mph
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Traffic Control Devices

• Manual on Uniform Traffic Control Devices (FHWA,
  1988)
• Selection requirements
• fulfill a need
• command attention
• convey clear, simple meaning
• command respect of road users
• give adequate time for response

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Categories of Traffic Signs

• Regulatory
• Warning
• Guide or informational
• Directional