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 Capacity and LOS: Example Problems

 
Free-Flow Speed Adjustment

An existing six-lane freeway in an urban area has the following physical characteristics:

  • 11-ft lanes
  • 2-ft lateral clearance on outer shoulders
  • interchange density of 1 interchange per mile.

Calculate the free-flow speed for this section of freeway.

[Solution Shown Below]

 

 

 

 

 

 

 

 

 

 

Solution

When actual field measurements aren’t available, free-flow speed is estimated by the following equation, which utilizes the tables given in the Free-Flow Speed Adjustment theory and concept page:

FFS = 70 – fLW – fLC – fN - fID

where
fLW = adjustment for lane width = 2.0
fLC = adjustment for right-shoulder lateral clearance = 1.6
fN = adjustment for number of lanes = 3.0
fID = adjustment for interchange density = 2.5
FSS = estimated free-flow speed = 70 – 2.0 – 1.6 – 3.0 – 2.5 = 60.9 mph