Improving Urban Street Network Efficiency by Prohibiting Conflicting Left Turns at Signalized Intersections

The accommodation of left turns at signalized intersections plays a significant role in the overall operational efficiency of urban traffic networks. Allowing left turns in traditional ways (such as implementing protected, permitted, and protected-permitted phasing schemes) reduces available intersection capacity but provides vehicles the most direct routing options to reach their destinations. Prohibiting left turns allows simpler phasing plans but requires vehicles to travel longer distances. This paper uses macroscopic performance measures to quantify and compare the efficiency of urban grid networks when various left-turn treatments, including left-turn prohibitions, are implemented. The findings reveal that prohibiting left turns generally increases a network's maximum trip completion rate, especially when trips are not very short; however, left-turn prohibitions reduce efficiency in very light or heavy traffic conditions. A dynamic strategy is proposed: left turns are allowed only during light traffic or heavy congestion and are prohibited for moderate congestion levels near capacity. This strategy allows the network to operate at the highest efficiency possible for any traffic accumulation. This strategy is identified and tested with analytical and simulation investigations of idealized grid networks. The results provide insight into how more realistic networks might be managed to maximize efficiency by changing only how left turns are treated at signalized intersections.