Title: EFFECT OF HIGHWAY GEOMETRY ON FREEWAY QUEUING AT MERGE SECTIONS
Abstract: To facilitate the pre-analysis of dynamic traffic management solutions for freeways (ramp metering, buffers, etc.), a quick assessment is needed of the effectiveness of such solutions. In such a pre-analysis an estimate is needed of congestion characteristics such as mean queue speed, queue growth and dissipation speed, travel time loss and queue length. Freeway merge sections with lane drops are particularly critical in such analyses, as they often contain a bottleneck. One of the key issues surrounding congested merge sections is how congestion will affect the different roadways approaching the merge. Based on the fundamental diagram of traffic flow and Lighthill and Whitman's shock wave theory, an approximation method has been developed for estimating the queuing characteristic. The paper shows that, for a given road section, the ratio of the total number of exiting traffic lanes to the total number of incoming traffic lanes, called the Geometric Factor, is a good determinant of queuing characteristics if the Geometric Factor is smaller than one. For geometric lane drop bottlenecks the Geometric Factor is smaller than one by definition, and congestion starts if the demand-flow exceeds the supply-flow. Any merge configuration with a Geometric Factor smaller than one will function as a geometric bottleneck. During complete congestion, with queue growth on both the merge ramp and the freeway upstream of the ramp, all lanes upstream of the bottleneck have almost the same queue speed and flow. For this condition, supply-flow in pcphpl on both approaching roadways is a function of the Geometric Factor. Likewise the Geometric Factor determines the practical capacity of an on-ramp. During partial congestion, with queue growth only on the mainline or the ramp, the mean queue speed is only dependent on the flow rate of the uncongested roadway. A first evaluation of the method with data from three merge locations on freeways near Amsterdam and Rotterdam give good results. With a capacity speed of 90 kmph a Geometric Factor of 0.33 results during complete congestion in a queue-speed between 6 and 8.6 kmph; a Geometric Factor of 0.8 in a queue-speed between 33 and 41.6 kmph, varying with the start wave speed between 15 and 24 kmph.
Publication Year: 1998
Publication Date: 1998-06-01
Language: en
Type: article
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Cited By Count: 2
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