Modeling Traffic Flow At Unsignalized Roundabouts Based On Continuum Flow Models

In recent years, with the rapid growth of social economy, the advancement of urban modernization, the continuous expansion of urban population and scale, and the rapid improvement of motorization level, traffic congestion has become one of the bottleneck of city development. To relieve traffic congestion, a series of measures and methods have been issued successively, including building new roads, widen and modification old roads, using the advanced traffic management systems, and so on. Long practice indicates that intersection is the key point of the road efficiency. Analyzing the traffic flow characteristics at intersection has important significance for enhancing road efficiency. This paper is modeling and analyzing the traffic flow characteristics at a special intersection with self-organized scheme, i.e., unsignalized roundabout.By observation, there are only merging, diverging and weaving conflicts at the unsignalized roundabout, which is analogical to freeways. Additionally, the approach link and departure link are corresponding to the on-ramp and off-ramp, respectively. If we do not compare the speeds and distances between adjacent approaches or exits on the two types of road facilities, an unsignalized roundabout can be considered as a short ring "freeway" Therefore, this paper treats the unsignalized roundabout as a short ring "freeway" and this treatment enables us to make the best of those models and algorithms already developed for freeway traffic analysis. At first, we group the traffic flows inside a roundabout into different classes according to the approach links they entered and the departure links they left to model the traffic flow characteristics by the LWR model and then we unified the model to describe this complicated dynamic system. Following, we obtain the difference scheme of the model through a circular homogeneous symmetrical unsignalized roundabout with four approach and departure links which own the same number of lanes and then we verify the effectiveness of the model and robust of the difference scheme by analyzing one numerical experiment. At last, we further explore the influence factors of unsignalized roundabout traffic flow by another series of numerical experiments. Simulation results show that the configuration of lane numbers between circulatory roadway and approach links, the free-flow speed of circulatory roadway and U-turning vehicles have great effects on roundabout traffic flow; in the roundabout which contains a larger traffic flow, the lane numbers of circulatory roadway should not be less than the approach links; the larger free-flow speed of circulatory roadway or the less U-turning vehicles, the more efficient of roundabout. These results indicate that our model can be applied to signal-free roundabout well, and has positive practical guidance significance for the reasonable design and organizational planning roundabout and even the whole city road network.