Performance Evaluation of a Roundabout Interchange Using a Coordinated Signal Control System

The geometry of a roundabout interchange frequently appears in freeway networks and can provide a system interchange of two freeways or a service interchange of a freeway with an arterial. This research explores the application of ramp metering as an adaptation of a method to a specific geometry of a roundabout interchange, particularly during varying traffic conditions ranging from light to heavy. The current ramp metering algorithms neglect the complexities associated with this geometry; thus, the Heuristic Ramp-metering Coordination (HERO) was accordingly modified in this study. The primary objective is to mitigate congestion and minimize delays by effectively controlling vehicle flow at the on-ramps. This study examines a roundabout interchange featuring four on-ramps and four off-ramps, implementing four distinct control strategies: (1) No control, (2) Asservissement Linéaire d’Entrée Autoroutière (ALINEA) local control, (3) HERO, and (4) Modified HERO (MHERO). The results show that managing traffic at the on-ramps can prevent or delay blockages in the interchange. Each strategy is tested to assess the impact of ramp metering on traffic dynamics at this type of interchange. The coordinated strategies (HERO and MHERO) perform better than local control (ALINEA) and no control. Strategy 4 (MHERO) is more effective than Strategy 3 (HERO) in reducing delays and increasing average speed. In Strategy 2 (ALINEA), the average travel time is reduced to 23% compared to the baseline strategy without any control. This ratio further decreases to 15% in Strategy 3 (HERO) and to 38% in Strategy 4 (MHERO), demonstrating progressive improvement relative to the no-control strategy. Overall, ramp metering in general and specifically MHERO is proven effective in managing traffic flow at a roundabout interchange during heavy traffic.