Traffic Flow Control in Congested Motorway Networks Using Buffers

Around large urban areas, daily recurrent congestion in the form of queueing at bottlenecks is nowadays a normal pattern. Apart from the congestion that directly follows from the capacity deficit at bottlenecks, more and more motorways are affected by a secondary congestion problem due to blocking of motorway exits and entries by long queues that build up upstream of bottlenecks, causing delay to travelers who are not going through the bottleneck. A solution to avoid congestion-induced blockage is the construction of so-called buffer facilities. These are local widenings of the motorway by adding one or more lanes just before a bottleneck. Buffers can shorten queues by an amount that is more than proportional to the number of added lanes. In this way congestion-induced blockage can be avoided and total delay is reduced significantly. Buffers are a cost-effective way to fight motorway congestion. The principles and functioning of buffer facilities are explained. Design elements and criteria as well as calculation of buffer dimensions are considered. Attention is given to the control of traffic flow at the entrance and exit of buffers. This is demonstrated with a case study from the Netherlands.