Determining u-turn adjustment factor for signalized intersections in Doha, Qatar

Hamad, Khaled; Abu-Hamda, Hassan · 2017 · OpenAlex-citations

DOI: 10.1051/matecconf/201712007008

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Summary

This study addresses the lack of a standardized U-turn adjustment factor in the Highway Capacity Manual (HCM) 2010, which fails to account for the specific impact of U-turn traffic on the saturation flow rate and capacity of signalized intersections. Because U-turns require lower speeds and smaller turning radii than left turns, they increase discharge headways and reduce intersection capacity. The authors aimed to develop a locally calibrated adjustment factor for Doha, Qatar, where U-turn traffic is significant and driver behavior differs from North American norms, which are the basis for existing HCM guidelines. The researchers collected field data from three signalized intersections in Doha during peak weekday hours between February and May 2013. Data were gathered via video recordings to measure queue discharge times and vehicle counts. The study analyzed 198 queues, comprising 2,327 U-turn vehicles and 1,564 left-turn vehicles. To isolate the effect of U-turns, intersections were selected based on strict criteria, including uniform lane widths, level grades, exclusive left-turn lanes with protected phases, and the absence of disturbances from pedestrians, parking, or heavy vehicles. A passenger-car equivalent factor of 2.0 was applied to account for heavy vehicles. The methodology involved calculating average discharge headways and developing regression models to relate these headways to the percentage of U-turn traffic. The analysis tested linear, logarithmic, quadratic, and exponential models, with the quadratic model providing the best fit ($R^2 = 0.591$). The results confirmed that average headway increases as the proportion of U-turn traffic rises, thereby reducing intersection capacity. The derived U-turn adjustment factor ranges from 0.95 when U-turns constitute 10% of the traffic stream to 0.76 when the stream is 100% U-turns. The study found that the negative impact of U-turns is more pronounced at lower percentages compared to previous studies conducted in North Carolina, Florida, and Taiwan. While other studies suggested a linear decrease in capacity, this research demonstrated a quadratic decrease, concaving upward. The significance of this work lies in providing traffic engineers in Qatar and similar regions with a localized tool to more accurately estimate intersection capacity. By incorporating this adjustment factor into saturation flow calculations, engineers can design more efficient signal timing plans, potentially reducing vehicle delays. The authors conclude that local traffic conditions and driver behaviors significantly influence intersection performance, reinforcing the HCM’s recommendation to adapt capacity models to local contexts. They note that future studies should expand the sample size and investigate other variables, such as turning radius and vehicle size, to further refine these estimates.

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