Post on 20-Dec-2015
transcript
2/27/06 Michael Dixon 2
Objectives
• HCM 2000 procedure process
• Significant variables
• Adjustment factors
2/27/06 Michael Dixon 3
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
2/27/06 Michael Dixon 6
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 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.): 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: 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