Characteristics of Breakdown Phenomenon in Merging Sections of Urban Expressways in Japan

Abstract

This paper aims to investigate the breakdown phenomenon at on-ramp merging sections of urban expressways in Japan. The breakdown events are treated here as a probabilistic phenomenon. Six merging sections at which breakdowns frequently occur are selected to be different in their geometry in relation to acceleration lane length and ramp entrance side. Three different aspects of merging capacity are recognized: the maximum outflow rate before the breakdown, the outflow rate when the breakdown starts to occur, and the outflow rates during the queue discharge period. These outflow aspects are analyzed after they are converted into an equivalent passenger car unit to eliminate the effect of heavy vehicles. Then the probability of breakdown at these sections is investigated as a function of outflow rates. The observed breakdown probabilities follow the Weibull distribution function at all sections, with close values of shape parameter and wide range values of scale parameter. The values of the scale parameter are used to investigate the effects of the acceleration lane length and entrance ramp side. Moreover, the breakdown probability is investigated as a function of the occupancy observed immediately downstream of the merging section. Last, the effects of ramp flow ratio and interval length of aggregated data are discussed.