CONVERSION OF A LARGE ROUNDABOUT IN BAGHDAD CITY TO A SIGNALIZED INTERSECTION

Al-Neami, Ali H. K.; Al-Shekhlly, Khalaf Taha · 2003 · Crossref

DOI: 10.31026/j.eng.2003.01.04

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Summary

This study addresses the traffic congestion and safety hazards caused by the insufficient capacity of a large unsignalized roundabout, Huriya Square, in Baghdad. Motivated by a rapid annual increase in vehicle volume (approximately 12.5%) and the resulting economic losses from traffic delays, the research aimed to reduce vehicle delay and queue length by converting the roundabout into a signalized intersection. The authors evaluated whether signaling could improve performance at a site characterized by significant differences in road widths and heavy left-turn traffic, conditions that often limit the effectiveness of such conversions. The research was conducted in two stages. First, the authors validated two traffic analysis software packages, SIDRA and HCS, against field-observed data from a trial site (Eastern Baghdad Station intersection). Statistical regression analysis revealed that SIDRA’s predicted vehicle delays were more consistent with observed data ($R^2 = 0.905$) compared to HCS ($R^2 = 0.821$). Consequently, SIDRA was selected for the subsequent design phase. Second, the authors collected two years of traffic flow, origin-destination, and saturation flow data for Huriya Square. They used SIDRA to evaluate five signaling strategies and four geometric layouts. These included modifying the roundabout’s geometry (reducing central island diameter, channelizing traffic) and converting it to a standard signalized intersection based on a previous proposal by Scott Wilson and Partners, which was then modified by the authors. The analysis compared the proposals based on degree of saturation, mean vehicle delay, queue length, and reserved capacity. The initial roundabout modifications yielded low reserved capacities (2% and 18%) and moderate delays. The standard signalized intersection proposal resulted in a reserved capacity of 11% and higher delays. The authors’ modified fifth proposal, which optimized the geometric layout, phasing, and timing, achieved the best performance metrics: a degree of saturation of 0.65, a mean delay of 21.3 seconds per vehicle, a maximum queue length of 7 vehicles per lane, and a reserved capacity of 38%. The local authority, Amanat Baghdad, adopted the fifth proposal, and the roundabout was converted to a signalized intersection. Post-conversion observations confirmed a considerable improvement in traffic movement performance. The study demonstrates that careful optimization of geometric layout and signal timing is critical when converting large roundabouts, particularly in environments with complex traffic patterns, to achieve significant reductions in delay and increases in intersection capacity.

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• intersection design