Talk:Fundamentals of Transportation/Traffic Flow

Under "Traditional Model (Parabolic)", the last bullit says:

"The slope of the flow density curve gives speed. Rise/Run = Flow/Density = Vehicles per hour/ Vehicles per km = km / hour."

Sure, the unit is speed. But the flow density curve has two areas (C and D) with negative slope. Whose speed is being measured? Because there is generally no vehicle whose speed is negative.