From Goswami:
"Operating speed is the speed at which drivers are observed operating their vehicles under free flow conditions (unconstrained by control delay). The 85th percentile of the distribution of observed speeds is most frequently used as the operating speed."
Be aware that there are different categories for travel speeds:
Average running speed (in motion only)
Average travel speed (includes delays) = space mean speed
Time mean speed = average spot speed = average speeds of vehicles passing a point on a highway > space mean speed
Free flow speed = Desired speed without delays
Operating speed (defined above)
Design speed = maximum safe speed based on geometry conditions
