Drive Longer, but Travel Faster: Alternative Left-Turn Strategies at Signalized Intersections

BY guangchuan yang

Conventional signalized intersections that serve left-turn, through, and right-turn traffic have been in use for many decades. However, as urban and suburban areas grew and traffic volumes increased, these conventional intersections began to show their limitations in terms of traffic congestion, safety issues, and environmental impacts, especially during peak hours.

Among the various factors that lead to these issues at signalized intersections, left-turn movements have long been considered by transportation professionals as a key contributing factor. From the perspective of a transportation engineer, left-turn movements pose a series of operational challenges and safety concerns. These issues often involve conflicts between left-turning vehicles and opposing traffic, pedestrians, and other road users. By default, left-turning vehicles must yield to oncoming traffic, leading to potential conflicts and rear-end collisions. Consequently, left-turning vehicles may experience long delays, resulting in traffic congestion and driver frustration. To address the safety issues posed by left-turn traffic, typically, protected left-turn phases (i.e., oncoming traffic is stopped to allow left-turning vehicles to proceed) are used at signalized intersections to reduce delays and the risk of crashes for left-turning traffic.

While protected left-turn phases can improve safety and traffic flow, they are not free of trouble. Implementing protected left-turn phases will increase delays to through traffic, decrease intersection capacity, and deteriorate the progression of the signal coordination. This is mainly because the addition of left-turn signal phases takes green time away from through movements and adds to the inter-phase lost time, which takes away from usable cycle time and reduces the efficiency of the intersection. As traffic volumes are continuously increasing and overloading the design capacity of many intersections during peak periods, the impacts of protected left-turn signals on intersection operations tend to be more significant.

To address these issues, transportation engineers and researchers have been continuously exploring alternative intersection designs that can accommodate left-turn traffic and meanwhile without harming intersection overall operational efficiency. Instead of implementing protected left-turn phases, an alternative way is to re-direct the left-turn traffic from the main intersection to sub-intersection(s) located downstream or upstream of the main intersection. One such design is the Quadrant Roadway Intersection (QRI), which includes a main intersection and secondary intersections that are linked by a connector road in one or more quadrants of the main intersection. Technically, rerouting left-turn traffic from the main intersection, while introducing out-of-direction travel distance for rerouted left-turn traffic, allows for the implementation of a two-phase signal control scheme at the main intersection, which enables using a shorter cycle length. The reduced number of signal phases also means less lost time occurs during phase transition. This results in higher capacities and lower delays for through and right-turn traffic at the main intersection, thus a higher level-of-service (LOS) is expected. Moreover, rerouting left-turn traffic reduces and spreads out the number of conflict points, which reduces the crash risk at the main intersection. Nevertheless, reducing congestion at the main intersection does not necessarily reduce overall travel time in the system relative to the conventional intersection when secondary intersections are involved. Moreover, each QRI design has a unique traffic flow pattern (i.e., direct left-turn, loop left-turn, jughandle left-turn, reverse jughandle left-turn), which may redirect a specific number of left-turn vehicles to re-enter the main intersection and thus increase through traffic volumes at the main intersection. Therefore, when deploying a QRI, a major question that needs to be explored is whether the delay savings at the main intersection will offset the out-of-direction travel time and additional control delay at the secondary intersections.

To answer this question, this research, based on the geometry and traffic flow data collected at a real-world conventional intersection, employed microsimulation modeling to compare the average time to travel through a half-mile distance via five QRI designs (one with a single quadrant, two with a pair of diagonal quadrants, and two with full quadrants) with the counterpart conventional intersection under various traffic demand scenarios. Simulation results show that all QRI designs outperform conventional intersection design under all demand scenarios in terms of weighted average travel time. The reductions in travel time range from approximately 8% to 28%, depending on the QRI geometry features and left-turn re-directing strategies. Travel time reductions tend to be more significant when adding 20% more traffic to the roadway system (ranging from approximately 11% to 56%). Generally, QRIs with a direct left-turn design have a smaller average travel time compared to those with a loop left-turn design.

Today, alternative intersections are considered valuable tools in transportation planning and engineering to address the challenges of growing traffic volumes and urban development while promoting safer and more efficient travel. These intersections continue to evolve and adapt to meet the demands of increasingly complex traffic situations, representing a significant shift in transportation design and management. It is essential that drivers understand the rules for navigating alternative intersections and always follow the traffic signs and signals to prevent wrong-way driving.