Signalization options to improve capacity and delay at roundabouts through microsimulation approach: A case study on arterial roadways in Ghana

Abstract

Roundabouts are very useful when flows are balanced but can have operational challenges when flows increase. Most roundabouts on arterial roadways in Ghana experience recurring congestion during peak traffic. The four roundabouts between Ejisu Municipality and Asokwa Municipality on the N6 highway are typical of the situation found on arterials countrywide. They experience recurring long queues and delays during the peak hours and over-representation of truck rollover accidents. In Ghana, historically, most roundabouts with capacity problems have been developed into interchanges at very high cost without consideration of other effective at-grade configurations. When roundabouts are signalized, various configurations can be explored to enhance capacity while preserving the safety benefits. Using simulation techniques, various configurations and design options were tested for their effectiveness at two of the roundabouts, located at KNUST police station and Ejisu, with similar demand and high left turning volumes. The objectives of this study were to a) evaluate the performance of the selected roundabouts on the corridor; b) explore alternative intersection configurations to improve capacity and delay. Using video data collection techniques, 2-h time stamped footages of AM and PM peak traffic flows were captured for two days. Geometric design parameters of the existing roundabouts, queue lengths, travel times, and approach flows were also captured. VISSIM models of the selected roundabouts were developed and calibrated for the baseline conditions. Signalized unconventional roundabout configuration, with a phase for major road through traffic were simulated and assessed for capacity, delay and queue length. The results showed that signalized roundabout intersections effectively increased capacity by as much as 50% in some cases, reduced delay and queue length significantly, and maintained the benefits of roundabouts including safety. The performance could be further enhanced by introducing separate left turn phases to reduce conflicts or through a right turn U-turn (RTUT) concept.

Keywords

• Transportation
• Signal strategy
• Roundabout
• Road safety
• VISSIM microsimulation
• Congestion