A Study of Traffic Oscillations at the Bottlenecks on Urban Expressways

Traffic oscillations at the typical bottlenecks (on-ramp,off-ramp)on the expressways of Shanghai are in-vestigated.A method of cumulative movement sequence,which is based on comparisons between cumulative values for a long term and mean values for a short term,is used.The features of the traffic oscillations including propagating speeds, amplitudes and the relationships among amplitudes,moving step size and background speeds are analyzed.It is found that the oscillations arise in the downstream part of the queue at a bottleneck and propagate to the upstream with a nearly con-stant speed of about 20~25 km/h.Under different moving step size,the correlation coefficient between the amplitude of an oscillation and the moving step size at 2 types of bottlenecks are 0.964 2~0.999 8 and 0.982 2~0.999 4,which shows a high positive correlation.No significant correlation is found between the amplitude of an oscillation and the background speed.Finally,study results also indicate that the layout of road infrastructure plays an important role in amplifying or impairing the amplitude:the amplitude is amplified when the oscillation crossed a diverging ramp,but the opposite result is shown on a merging ramp.The study results of this paper could provide a basis for designing on-ramp control algorithm for expressway bottleneck areas.