Determinations of critical gap and follow-up time at roundabouts in Jordan Wyznaczanie bezpiecznych odległości pomiędzy pojazdami i czasów dojazdu na rondach w Jordanii

Hazim, Nabil; Bazlamit, Subhi M.; Salem, Zaydoun Abu; Alghazawi, Omar; Odeh, Issam · 2019 · Crossref

DOI: 10.7409/rabdim.019.015

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Summary

This study addresses the determination of critical gap and follow-up times at roundabouts in Jordan, parameters essential for assessing capacity and level of service at unsignalized intersections. The research was motivated by the extensive use of roundabouts in Jordan, particularly in the capital city of Amman, where traffic congestion and excessive delays are prevalent. Existing design practices often lack localized data for these parameters, and previous models from other regions may not account for the specific aggressive driving behavior and mountainous terrain characteristic of Jordan. The study aims to provide highway designers with accurate, locally relevant data to improve the design and performance evaluation of roundabouts. The methodology involved field observations at seven roundabouts in Amman, selected to represent various geometric properties and approach slopes ranging from -8% to 6%. Data collection utilized video recordings from cameras mounted on adjacent buildings during weekday peak periods (10 am to 6 pm). The researchers extracted gap observations from the footage, adhering to the terminology of the NCHRP 3-65 project. Follow-up times were measured as the time intervals between successive vehicles entering the roundabout from the same approach. Critical gaps were determined graphically using accepted and rejected gap distribution curves. The study also accounted for external factors, such as police-directed traffic, by excluding observations influenced by such interventions to ensure data reflected natural traffic flow. The results indicate that Jordanian drivers exhibit aggressive behavior, often using rolling stops rather than complete stops, which leads to shorter follow-up times. The mean follow-up time ($t_{x0}$) was found to be 1.06 seconds, significantly lower than the 2.6–3.1 seconds recommended by the Highway Capacity Manual (HCM). A significant relationship was established between the approach slope and the critical time gap. The study derived an exponential regression equation: Critical Time Gap = $4.2568 \times e^{0.0441 \times \text{Approach Slope}}$, with an $R^2$ of 0.9439. Critical gaps ranged from 3.4 seconds at a -6% slope to 6.5 seconds at an 8% slope. These values were compared with literature from Germany, Spain, Portugal, and the HCM, showing comparability despite the unique local driving conditions. The significance of this research lies in its provision of a validated model for estimating critical gaps based on approach slope, which is particularly relevant for hilly terrains like Amman. The findings highlight the impact of aggressive driver behavior on gap acceptance, suggesting that standard models may overestimate gaps in similar contexts. The derived relationship can be incorporated into capacity analysis and traffic simulation tools, aiding engineers in designing more efficient roundabouts and accurately predicting their operational performance. This study contributes to the broader field of traffic engineering by emphasizing the need for localized calibration of traffic parameters to account for regional driving cultures and topographical constraints.

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Topics

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• gap acceptance
• following distance