Determine Free-Flow Speed For basic freeway segments, FFS is the mean speed of passenger cars operating in flow rates up to 1300 passenger cars per hour per lane (pc/h/ln). If FFS is to be estimated rather than measured, the following equation can be used. It accounts for the roadway characteristics of lane width, right-shoulder lateral clearance, and ramp density.
FFS = 75.4 – fLW – fRLC – 3.22TRD0.84 (6.6)
FFS = estimated free-flow speed in mi/h, fLW = adjustment for lane width in mi/h,
fRLC = adjustment for lateral clearance in mi/h, 3.22TRD0.84 = adjustment for total ramp density in mi/h (with TRD in ramps/mi).
The constant value of 75.4 in is considered to be the base free-flow speed (BFFS) and applies to freeways in urban and rural areas. The following sections describe the procedures for estimating the adjustment factor values.
Lane Width Adjustment When lane widths are narrower than the base 12 ft, the adjustment factor fLW is used to reflect the impact on FFS. Such an adjustment is needed because narrow lanes cause traffic to slow as a result of reduced psychological comfort and limits on driver maneuvering and accident avoidance options. Thus, FFS under these conditions is less than the value that would be observed if base lane widths were provided.
Lane width (ft) Reduction in free-flow speed, fLW (mi/h)
Lateral Clearance Adjustment When obstructions are closer than 6 ft (at the right shoulder) from the traveled pavement, the adjustment factor fRLC is used to reflect the impact on FFS. Again, these conditions lead to reduced psychological comfort for the driver and consequently reduced speeds. An obstruction is a right-side object that can either be continuous (such as a retaining wall or barrier) or periodic (such as light posts or utility poles).