Channelization and Turn Bays

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Channelization and Turn Bays
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Island Channelization

• flush, paved, and delineated with markings or
  unpaved and delineated with pavement edge and
  markings
• raised use lighting
• raised rural also delineate and slope curbs
• Size large enough to command attention
• Must delineate a path that is natural and
  convenient for the driver

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Island Channelization

• Must have sufficient approach sight distance to
  islands
• Curb radii appropriate for necessary island
  size (25 m or 75 feet at least) see exhibits 9-41
  and 9-42
• if used at series of rural intersections, keep
  geometry consistent
• see exhibit 9-35 for common use

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Consider left turn lane if advancing
volume/percent left combination exceeds these
values. For left turn volume gt 300 VPH, consider
dual left turn lane.
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Channelization Functional Objectives

• Limit Conflict Points (i.e., prohibit certain
  movements, non-overlap of opposing LTs), esp. in
  vicinity of intersection functional area (see
  figures)
• Limit Conflict Area Complexity (i.e., eliminate
  legs, add turn lanes, islands)
• Limit Conflict Frequency (i.e., add turn lanes,
  use right angles)
• Limit Conflict Severity (i.e., small angles,
  adequate turn bay length/curb radii for speed
  differential issues)

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Principles of Channelization

• 1. Undesirable/wrong-way movements should be
  discouraged
• 2. Vehicle paths clearly defined
• 3. Desirable/safe speeds encouraged
• 4. Separation of conflict points if possible
• 5. Cross at right angles, merge at flat angles
• 6. High priority traffic facilitated
• 7. Desired traffic control schemes facilitated
• 8. Remove slow, stopped, or decelerating vehicles
  from traffic stream
• 9. Provide safe refuge for peds and others

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Intersection Islands

• Definition a defined area between traffic
  lanes for control of vehicle movements or
  pedestrian refuge
• Types/functions
• Directional to control and direct movement
  and to guide motorists into the proper channel
  for their intended route

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• Divisional to divide opposing traffic flow,
  and to alert the driver to the crossroad ahead
  and regular traffic through the intersection
• Refuge to aid and protect pedestrians and
  bicyclists crossing the roadway

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Intersection Turn Bays

• New Construction Warrants (functional approach)
• all unstopped approaches on primary, high speed
  rural highway intersections with other primary
  roadways, see fig. 4-17
• Other approaches where through, left, and
  opposing volumes justify
• Where space permits, left-turn lanes should be
  considered when left-turn volumes exceed 100
  vph (Green Book)

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• Rehabilitation Warrants Typically installed
  where there is a safety issues with vehicles
  turning left
• Guides for need See figure 4-12 and Exhibit
  9-75
• Lane width 12 ft. (new) and 11 ft. or greater
  for rehab, see figure 4-5

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40mph
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50mph
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Length required
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Design
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Design
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Design
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• 1. Left Turn Bay Length
• a) PRT 10 MPH reduction occurs upstream of bay
  (more in congested, urban conditions) see fig.
  4-15
• b) accommodate deceleration from D.S. 10 mph to
  stop, see table 5-8
• c) Unsignalized, accommodate 2 minutes of demand,
  at least 2 cars (or 1 car and 1 truck if truck gt
  10), see figure 5-15 or 4-18
• d) Signalized, accommodate twice the expected
  average storage needed depends on cycle and
  phasing, see figure 5-14
• e) Right turn bay length depends on corner
  radius, see fig. 4-24

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Turn Bay Components

• (see fig 4-14)
• 1.      Approach taper
• 2.      Bay Taper
• 3.      Full lane Length
• 4.      Departure Taper
• 5. Shadow width

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Some Turn Lane Details

• 1. Transitions are often symmetrical reverse
  curves or straight line (see Exhibit 9-95)
  curves at each end are desirable - Lengths and
  radii given in the same figure
• 2. For tapers see figure 4-19
• 3. Green Book 81 to 151 tapers for high speed
  highways urban use 100 for single and 150 for
  dual left turn lane

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Striping through Intersection Difficult to See
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Curb Radius Parking Lot
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Island for No Left Turn
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Right Turn Bay Curve
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Left Turn Bay Taper
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Left Turn Separated
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Left Turn Bay (No Transition)
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Left Turn Bay Pavement Markings
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Curb Radius Curb Cut
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Turn Lane Pedestrian Refuge
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Curb Cut
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RR Crossing
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RR Crossing Raised Median
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Right Turn Island
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Island Pedestrian Refuge
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Median End Treatment
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Left Turn Bay Curve, Right Turn Bay Taper
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Left Turn Bay Circular Transition
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Median to Discourage U-Turns
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3 Leg Intersection
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Median End (Keep Right)
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Median End Delineation
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Median in front of Lowes
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Intersection Approach Lincoln Way at Elwood
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Intersection Approach 4th at Elwood Drive
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Median Elwood Dr. at Airport Rd.
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Points of Conflict

• Traffic conflicts occur where the path of traffic
  movement crosses.

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T-Intersection or Driveway
Two Lane Undivided Street
Side Street
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T-Intersection or Driveway
Two Lane Undivided Street
Side Street
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4-Way Intersection or Driveway 2 lane streets

• Note Add 4 conflicts to each for four lane
  roadways

40 conflicts with Four Lane on major street
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Limit Conflicts - Objectives

• Limit Conflict points
• Limit Conflict Area Complexity
• Limit Conflict Frequency
• Limit Conflict Severity

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T-Intersection or DrivewayAfter 1

• Two lane roadway with raised median closed, no
  left-turn

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4-Way Intersection or DrivewayAfter 1

• Added median close eliminates left-turn conflicts

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T-Intersection or DrivewayAfter 2

• Two lane roadway with raised median (left turn
  egress only from intersection or driveway)

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4-Way Intersection or DrivewayAfter 2

• Two lane roadway with raised median (left turn
  egress only from intersection or driveway)

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T-Intersection or DrivewayAfter 3

• Two lane roadway with raised median (left turn
  ingress only into driveway)

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4-Way Intersection or DrivewayAfter 3

• Two lane roadway with raised median (left turn
  ingress only into intersection or driveway)

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Two Way Left Turn Lanes at Intersection or
Driveway

• Two way left turn lanes (TWLTL) (3 lane roadway)
• Reduces rear-end accidents and allows merging
  traffic to store in TWLTL

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Pedestrian Refuge Island at Intersection for TWLTL
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Urban Intersection
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Urban Channelized Intersection-
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Raised Median Clive
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