This research reveals statistical characteristics of speed dispersion and its relationships with fundamental traffic flow parameters in northern California. Nearly a quarter million vehicle observations of a five-lane urban freeway are examined individually by lane and aggregately for a total of seven categories. Speed dispersion is measured by coefficient of variation of speed (CVS) and standard deviation of speed (SDS). CVS displays an exponential form of occupancy or space mean speed, and is two-phase linear to flow. Variation of CVS is stable and similar across lanes during light traffic, and afterward increases and diverges into three groups. SDS in contrast does not present any simple equation of the fundamental parameters. Both CVS and SDS of the all lane mix are greater than those of other categories given fixed occupancy or mean speed.
The Transportation Research Board (TRB) 89th Annual Meeting