Impact of Traffic Flow on Travel Time Variability of Freeway Corridors

Travel time variability is determined by variations in demand and capacity. Knowledge about these demand and supply factors can help improve the reliability of travel time and hence the quality of traveling. The precise nature of the relationship between, for example, variations in inflows and travel time variation is still largely unknown. This paper uses empirical traffic data from both Regiolab-Delft in the Netherlands and the Beijing Olympic area to analyze the variability of travel times depending on inflow conditions. Preliminary analysis shows that travel time variability is a function of inflow characterized by two so-called critical inflows (critical transition inflow λ1 and critical capacity inflow λ2, which are both lower than capacity). These critical inflow levels subdivide traffic into a fluent traffic region, a transition traffic region, and a capacity traffic region. Variation of inflow has little or no effect on travel time variation below λ1. But both demand and capacity variations have a positive correlation with travel time variability in between λ1 and λ2. When volumes are above λ2, the inflow slightly affects the travel time variability. Under all inflow levels, the variation in capacity appears to have more impact on travel time variability than does the variation of traffic flow.