Abstract

1. Introduction & Objectives Traffic congestion has been an important social issue in all over the world. It will result in great economic loss due to time delay and environment deterioration. In Japan, traffic congestion on intercity expressways on weekends and holidays is not an unusual scene. Looking at the percentage of congestion occurring on Japanese intercity motorways by bottleneck type, nearly 80% occurs in uninterrupted flow sections of sags or uphill slopes (60%) and tunnel entrances (20%). Congestion at merge areas accounts for approximately 13%. At a high level of traffic flow before occurrence of congestion, there exists a difference of travel speed between outer lane and inner lane, eventually causing an increase in use of the inner lane. The resultant inequality of lane utilization results in a breakdown of traffic flow in the inner lane of multilane motorway while flow of the outer lane remains below its capacity, thus decreasing the directional capacity. Once congestion occurs at a bottleneck, capacity would drop from pre-queue breakdown flow rate to lower queue discharge flow rate during congestion. As a measure against congestion, queue discharge flow from a bottleneck could be increased by providing drivers with information on the location of the head of the queue using roadside VMS. It is also possible to correct the overuse of the median lane by adding an auxiliary lane around the bottleneck at a sag. Besides, correction of unbalanced lane use could also be achieved by the provision of traveller information, such as “Please keep left” or “Please use left hand lane” to drivers through the VMS before occurrence of the congestion, (care should be taken here that vehicles drive on the left in Japan). However, there is no guideline on effective addition of an auxiliary lane in sag section for mitigation of motorway traffic congestion. This paper uses a microscopic traffic simulation model to study the effective way of adding an auxiliary lane in sag section of a dual 2-lane motorway for mitigation of traffic congestion. 2. Results The study considers 2 types of auxiliary lanes in sag section, i.e. one diverging and merging from outside, and one diverging from inside and merging from outside. The length of auxiliary lane varies every 500m from 500m to 2, 000m. The auxiliary lane is added at various locations ending at or starting from the bottom of a sag, or starting from the start of a vertical curve, or starting from around the midpoint of upstream half of a vertical curve. Altogether 15 cases are considered for the simulation study. The microscopic traffic simulation model is used in the study. Prior to the simulation runs, some parameters are calibrated to check if the 5-min average speed and median lane utilization agree with the observed data. The spatial distribution of lane utilization before, inside and after an auxiliary lane is also examined. The output of the simulation runs in the study is breakdown flow rate for each case of auxiliary lane. It is seen from the simulation results that for an auxiliary lane diverging and merging from outside, the optimum length should be 1, 000m – 1, 500m and be added from around the start of vertical curve to several hundred meters beyond the end of vertical curve. This is in accordance with the observation that shock waves take place in the length of auxiliary lane of a sag section. The auxiliary lane longer than the optimum length does not seem to result in higher breakdown flow rate. With the same length of auxiliary lane, the one diverging from inside and merging from outside would yield higher breakdown flow rate than the one diverging and merging from outside from the viewpoint of increasing the breakdown flow rate.