Movement Nature of Speed-Flow Relationship on Congested Expressway

Abstract

The nature of congested traffic flow has been not fully understood until now. An isolated bottleneck on the expressway generally influences a limited segment length, which means attenuation effects of the bottleneck on traffic stream by a distance within the influence area. However, such effects of bottleneck on speed-flow relationship have been inadequately emphasized. This paper investigates the location effects of a bottleneck on the speed-flow relationship. With consecutive 5 weekday median lane data collected by dual loop detectors on a congested expressway in Shanghai, integral speed-flow relationships including upper limb, lower limb, and capacity area are obtained from six sites. Further analyzing those sites on the same basic expressway segment unveils that it is the movement of the upper limb and lower limb that produces the integral curve. The right end of the upper limb gets an increase speed in with the site farther to its upstream bottleneck, whereas the lower limb gets a drop in speed with the site closer to its downstream bottleneck. Such a movement feature is in agreement with general driving behaviors of discharging or approaching a bottleneck. The factors intensifying or weakening the movement tendency are also identified.