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Channelization and Turn Bays. 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
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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
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
Consider left turn lane if advancing volume/percent left combination exceeds these values. For left turn volume > 300 VPH, consider dual left turn lane.
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)
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
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”
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”
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)
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
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
Turn Bay Components (see fig 4-14) 1. Approach taper 2. Bay Taper 3. Full lane Length 4. Departure Taper 5. Shadow width
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
