Estimating Base Saturation Flow Rate for Selected Signalized Intersections in Al-Najaf City

Al-Mosawi Ulla G.; Al-jameel Hamid A. · 2023 · DOAJ

DOI: 10.1051/e3sconf/202342703044

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Summary

This study addresses the critical need for localized traffic data in Al-Najaf City, Iraq, to improve the accuracy of traffic signal timing and intersection capacity analysis. Practitioners often rely on default values from international software, such as the 1,900 passenger cars per hour per lane (pc/h/ln) recommended by the Highway Capacity Manual (HCM). However, saturation flow rates are heavily influenced by local driving habits, traffic conditions, and climate. Using generic values can lead to erroneous decisions regarding delay calculations and levels of service. The research aims to estimate the specific base saturation flow rate for signalized intersections in Al-Najaf to provide engineers with more accurate calibration parameters for traffic modeling software like LinSig, SIDRA, Vissim, and Aimsun. The methodology involved field observations at the Al-Salam intersection, selected for its flat gradient, standard 3.7-meter lane widths, and sufficient queue lengths. Data collection occurred during weekday peak hours in April and May 2023 using video surveillance to record vehicle movements. The study analyzed 187 lineups comprising a total of 9,931 through-moving vehicles. The researchers calculated the saturation headway—the average time gap between vehicles after the initial start-up phase—excluding outliers and measurements that did not meet specific criteria. The analysis considered variables such as turning movements, gradient, number of through lanes, and speed limits (60 and 80 km/h). The saturation flow rate was derived using the formula $s = 3600/h$, where $h$ is the saturation headway in seconds. The results indicate a mean saturation headway of 1.55 seconds, yielding a base saturation flow rate of 2,323 pc/h/ln. This figure is substantially higher than the 1,900 pc/h/ln standard proposed by the HCM. The study notes that this higher rate is comparable to findings from other nations with similar traffic conditions and aggressive driving behaviors, such as those observed in South Africa. The degree of saturation for the Al-Salam intersection was calculated across different approaches, with values ranging from 3.55 to 3.66. The findings demonstrate significant variations in saturation flow rates depending on specific movement conditions and intersection characteristics. The significance of this research lies in its provision of locally calibrated data for regional engineers, enabling more precise assessments of intersection capacity. By utilizing the determined base saturation flow rate of 2,323 pc/h/ln, traffic models can better reflect actual local conditions, potentially reducing wait times and improving traffic flow efficiency for road users. However, the authors caution that the data was collected from a single intersection. They recommend that these findings be validated with a larger sample size across multiple intersections before being generalized for broader application in Al-Najaf City.

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• traffic density