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CE 578 Highway Traffic Operations

Lecture 22: Diverge Area Special Cases, Influence Area Speeds, and Freeway Weaving Sections

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Objectives

• Diverge area special cases

• Influence area speeds

• Basic process of weaving section analysis

• Important variables

• Limitations of methodology

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Diverge Area Special Cases

• Two-lane off-ramps

• Lane drops

• Major diverge areas

• Off-ramps on ten-lane freeways

• Left-hand off-ramps

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Two-Lane Off-Ramps

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Two-Lane Off-Ramps (cont.)

• four-lanes: PFD = 1.00

• six-lanes: PFD = 0.45

• eight-lanes: PFD = 0.26

• for calculating DR, LDeff = 2LD1 + LD2

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Lane Drops

• Single lane off-ramp with lane drop: ramp-roadway capacity governs

• Two lane off-ramp with lane drop: see major diverge segment procedure

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Major Diverge Area

Analyze each leg as a basic section to determine their respective capacities and compare to demand volumes.

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Off-Ramp on Ten-Lane Freeways

Similar to the on-ramp procedure, but with different values for the volumes in lane 5 (v5).

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Left-Hand Off-Ramps

• Same as for On-Ramps except– Conversion from v12 to v23 or v34

• Six-lane freeway: 1.05• Eight-lane freeway: 1.10

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Influence Area Speeds

• Read pages 25-18 and 25-19

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Weaving Section Issues

• Diverse traffic streams

• Differences in lane utilization

• Ease of lane change

• Predicting speeds

• Predicting density

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Basic Process

• Adjust the volume• Predict the space mean • Check for constraints on lane changing• Reassess the space mean speed• Compute the average space mean speed• Compute the density• Determine the LOS• Determine the weaving capacity

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Basic Process (cont.)

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Important Weaving Section Variables

• Demand volumes

• Weaving section length

• Number of lanes

• Weaving section configuration

• Terrain type

• Free-flow speed

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Demand Volumes, Terrain Type, and Free-Flow Speed

• Adjusted volume: See basic segment methodology

• Free-flow speed: See basic segment methodology

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Weaving Section Length

• Less than 2500 ft

• Measure from entrance gore to exit gore

L

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Number of Lanes and Weaving Section Configuration

• NTotal lanes in weaving section

• Nwlanes needed for weaving equilibrium

• Nw(max)maximum lanes available for weaving vehicles

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Number of Lanes and Weaving Section Configuration

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Number of Lanes and Weaving Section Configuration

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Limitations

• Ramp metering

• Oversaturated conditions

• Effects of speed limits

• Collector distributor roadways

• Urban streets

• Known weaving volumes