Off-ramp Blockage On Freeways

With the fast economic development in China,traffic congestions in urban freeways are getting more and more serious,and heavily affect the sustainable development if urban cities.Closely related to traffic congestion,freeway off-ramp blockage is both a result of and one reason of serious lasting congestion.Freeway congestion in many cases may spill back for several kilometers,blocking a number of on/off-ramps upstream of the congestion origin.As a result,the total off-ramp flow may substantially reduce,thus leading to off-ramp blockage.Due to the reduction of off-ramp flow,vehicles bound for the blocked off-ramps are trapped in the mainstream congestion,causing the continuing spillback of congestion that blocks more off-ramps at the further upstream with similar effects and so forth.The off-ramp blockage effect is a specific example how the performance of a freeway network decreases with the reduction of the total off-ramp exit flow under congestion,but there is a lack of analytical results to provide more insights.In this paper,we study the mechanism and impact of off-ramp blockage via analytic analysis and traffic studies.First,some flow conditions on bottleneck activation are first established,and the mechanism of the off-ramp blockage is theoretically explored.Second,we conduct macroscopic and microscopic simulations to demonstrate the analytical results obtained using the macroscopic traffic simulation system METANET,microscopic traffic simulation system VISSIM,and an self-developed macroscopic 2nd-order traffic simulation tool.The input-output characteristics of freeways under congestion,the sensitivity of off-ramp blockage to the turning rates and the OD ratios are examined in simulation.The features of macroscopic fundamental diagram of a congested freeway are also studied.Third,we studied how to apply ramp metering to attenuate freeway traffic congestion and prevent the serious impact of off-ramp blockage.This work can shed some light on an aspect of traffic network dynamics that was not given enough attention before.