STRAIGHT LANE SATURATION FLOW AND ITS RATE IN SERBIAN CITIES

Stanić, Branimir; Tubić, Vladan; Čelar, Nikola · 2011 · OpenAlex-citations

DOI: 10.3846/16484142.2011.623762

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Summary

This paper synthesizes three decades of research on straight lane saturation flow rates at signalized intersections in Serbia, addressing the variability of this critical traffic performance measure. Saturation flow, defined as the maximum vehicle throughput during a green phase, is fundamental to intersection design and signal optimization. The study was motivated by observed discrepancies between historical baseline values established by Webster and Cobbe (1963) and contemporary local measurements, suggesting that saturation flow is not a constant but varies significantly based on local conditions, driver behavior, and traffic composition. The authors review historical data from various international sources and Serbian studies conducted between the 1970s and 2007. Early Serbian surveys utilized the Road Research Laboratory methodology, while more recent studies (Stanić 1991; Čelar 2007) employed direct headway measurement techniques during queue discharge. Čelar’s 2007 study specifically analyzed eight intersections in Belgrade, measuring individual vehicle headways, amber light utilization, and start-up lost times under varying saturation degrees. The research categorizes saturation flow into ideal, operative (base), and actually realized types, examining how factors such as signal plan structure, lane width, city size, and driver discipline impact these values. The findings reveal a significant upward trend in saturation flow rates over time. While Webster’s original baseline was approximately 1850 pcu/hr, Serbian studies recorded values ranging from 1600 to 2120 pcu/hr in the 1990s, increasing to 1765–2150 pcu/hr by 2007. This represents an approximate 15% increase compared to early standards. The study identifies that straight lane saturation flow is heavily influenced by the signal plan type, with conflicting movements reducing flow rates. Furthermore, analysis of driver behavior showed that in highly saturated conditions, drivers frequently utilize the amber light phase, effectively extending green time by up to 90% of its duration. Start-up lost times were found to average between 0.3 and 1.2 seconds, with no significant correlation to the achieved saturation flow rate. The significance of this work lies in its demonstration that saturation flow is a variable parameter dependent on local context rather than a universal constant. The authors conclude that generic models are insufficient for accurate traffic engineering in specific locales. Instead, they advocate for local-level determination of saturation flow values, accounting for unique factors such as fleet characteristics, driver aggressiveness, and intersection geometry. The study validates the use of direct headway measurement as an effective, low-cost technique for capturing these dynamics, emphasizing that accurate local data is essential for optimizing signal operations and intersection capacity.

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• traffic density