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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 > 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 discouraged2. Vehicle paths clearly defined3. Desirable/safe speeds encouraged4. Separation of conflict points if possible5. Cross at right angles, merge at flat angles6. High priority traffic facilitated7. Desired traffic control schemes facilitated8. Remove slow, stopped, or decelerating vehicles from traffic
stream9. 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% > 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: 8:1 to 15:1 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
Side Street
Two Lane Undivided Street
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T-Intersection or Driveway
Side Street
Two Lane Undivided 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 #2Two 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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