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CE 578 Highway Traffic Operations

Lecture 16: Freeways Basic Sections

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Objectives

• HCM 2000 procedure process

• Significant variables

• Adjustment factors

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HCM Procedure Process

• Input data

• two parallel computations

• Two tracks to take for FFS

• Given the FFS and the adjusted volume

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HCM Procedure Input Data

• Geometric data– Number of lanes– Grade– Lane width– Lateral clearance (right shoulder width)– Interchange density

• FFS– Base free-flow speed (BFFS) or measured FFS

• Volume– Drive population– Volume (vph)– Vehicle mix– PHF

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HCM Free-Flow SpeedTwo tracks with field measured FFS or estimating FFS

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HCM Field Measured Free-Flow Speed Track

• Measured at volumes up to 1300 pc/hr/ln

• Conducted at a location that is representative of the segment

• Speeds of all passenger cars or a systematic sample

• The average of the passenger car speeds is the field measured FFS

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HCM Estimated Free-Flow Speed Track

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HCM Estimated Free-Flow Speed Track (cont.)

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HCM Estimated Free-Flow Speed Track (cont.)

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HCM Estimated Free-Flow Speed Track (cont.)

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HCM Estimated Free-Flow Speed Track (cont.)

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HCM Volume Adjustment

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HCM Volume Adjustment (cont.)

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HCM Volume Adjustment (cont.)

• Hourly volume is adjusted up to a peak 15-minute volume by V/PHF

• PHF variations are larger for rural areas and off-peak periods

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HCM Volume Adjustment (cont.)

• If the percentage of RVs is small relative to that of trucks• Buses operate similarly to trucks • Equivalents for extended freeway segments and specific

grades

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HCM Volume Adjustment for Grades

• Extended segment– No grade of 3% or more is longer than 0.25

miles or,– No grade of < 3% is longer than 0.50 miles

• Everything else should be analyzed as a specific grade

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HCM Volume Adjustment for Grades (cont.): Extended

Segments• Level:

• Rolling:

• Mountainous

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HCM Volume Adjustment for Grades (cont.): Extended

Segments

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HCM Volume Adjustment for Grades (cont.): Specific Grades

• Exhibit 23-9

• Trends of ET are as follows:– Decreases with %heavy vehicles– Increases with length of grade– Increases with % grade

• Trends for ER are similar (see Exhibit 23-10)

• Trends for ET are similar for downgrades

• No adjustment needed for RVs on downgrades

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HCM Defining Speed-Flow Curve

• Approaches that you can take:– Interpolate in the figure

given in Exhibit 23-3– Use equations below

Exhibit 23-3

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HCM Defining Speed-Flow Curve: Interpolation

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HCM Defining Speed-Flow Curve: Equations

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HCM Defining Speed-Flow Curve: Equations

• Note that the equation you use depends on which of the free-flow and adjusted volume conditions are satisfied.– If your FFS is 68 and your adjusted volume is

1200 pc/hr/ln which equation do you use?– If your FFS is 62 and your adjusted volume is

1700 pc/hr/ln which equation do you use?

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HCM Calculate the Prevailing Speed

• Interpolating from Exhibit 23-3

• Equations

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HCM Calculate the Density

• q = u * k• q = your adjusted volume (vp)• u = your calculated speed• Solve for k in units of pc/mi/ln

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HCM Determine the LOS