EMPIRICAL ANALYSIS OF CONGESTED TRAFFIC FLOW CHARACTERISTICS AND FREE SPEED AFFECTED BY GEOMETRIC FACTORS ON AN INTERCITY EXPRESSWAY

In Japan, much lower traffic capacities occur at S-curves, sag sections, and the entrances of long tunnels on intercity expressways. The traffic capacities at such sections and points were estimated at about 60 to 70% that of the basic expressway segments. Oscillating characteristics of traffic flows under congested conditions on urban expressways are already known. However, the traffic flow characteristics on the urban expressways have yet to be analyzed. The traffic flow characteristics under congested conditions, especially the accordion-like actions, the maximum flow rate at bottlenecks, and the relationship between the speed characteristics and geometric factors are analyzed. The data used were collected from 106 detector stations set up in each lane on a 220-km section of the Tomei Expressway. Forty-eight stations were in the upstream direction and 58 stations were in the downstream direction. Three severe bottlenecks on the Tomei Expressway are described. Propagating speeds of the density boundaries between the free and congested-flow regions were estimated. The characteristics of the accordion-like actions caused during the congested conditions on the intercity expressway were almost the same as those of urban expressways. The free speeds in each lane at the detector stations were estimated in the Q-V sub S plane. Free speeds were affected by the geometric factors at the detector stations. Multivariate analysis was introduced to quantify the effects of the geometric factors on the free speeds.