Mitigating freeway off-ramp congestion: A surface streets coordinated approach

Congested conditions downstream from off-ramps often propagate upstream blocking all freeway lanes and affecting traffic that does not cause the bottleneck. Most efforts to understand this phenomenon have focused on how the queues form and propagate in the freeway, while scant attention has been paid to the causes of the formation of this type of queue. Exiting traffic often compete with traffic from surface streets for a limited capacity so that if part of this competing traffic is detoured to other streets the off-ramp will see its capacity increased. This paper studies this alternative by modeling what we consider fairly common conditions for the surrounding network. We propose a methodology to determine the flow of competing vehicles to be detoured to underutilized roads in the local network in order to improve the system's capacity and reduce total delays under stationary conditions. We also study the conditions under which this strategy may be beneficial during a rush hour period. The methodology aims at keeping the off-ramp flowing uncongested, eliminating the queue in the freeway and leaving the remaining capacity for the competing traffic. An experiment to test the mechanism was conducted on an urban freeway in Santiago, Chile confirming the opportunity for improvements in the system as a whole. It also showed that when the off-ramp ends in a weaving section, the capacity of this section drops significantly when both approaches reach congestion, emphasizing the importance of preventing these queues from appearing. (C) 2011 Elsevier Ltd. All rights reserved.