Analysis of the Long-Term Variations in Traffic Capacity at Freeway Bottleneck

Abstract

In this paper, the long-term variation in traffic capacity at nine typical bottlenecks on intercity expressways in Japan is analyzed. The evaluation indices of traffic capacity are defined based on three factors: the ease of phase transition from free flow to congested flow is represented by the fifth-percentile traffic volume of breakdown probability; its reliability is defined by the gap between the 50th- and fifth-percentile traffic volume of breakdown probability; and the smoothness of traffic flow after the capacity drop is evaluated as the mean of the queue discharge flow rate. The findings show that: (i) the fifth-percentile traffic volume of traffic breakdown probability shows a long-term decreasing tendency, and the values in August and December are lower than the yearly average; and (ii) the gap between the 50th- and fifth -percentile traffic volume of traffic breakdown probability and discharge flow rate tend to decrease, but the trends are not significant. Finally, we suggest that the decreasing trends might be attributed to slight changes in driving skills, driving characteristics, or vehicle types.