Dynamic Traffic Control: The Treatment of Left Turns in an Urban Grid Network

Open Access
Deprator, Anthony Joseph
Graduate Program:
Civil Engineering
Master of Engineering
Document Type:
Master Thesis
Date of Defense:
June 06, 2016
Committee Members:
  • Vikash Varun Gayah, Thesis Advisor
  • Traffic
  • Traffic Flow Theory
  • Traffic Operations
  • Traffic Control
  • Dynamic
  • Dynamic Traffic Control
  • Urban Grid Networks
  • Operations
  • Transportation
Urban traffic congestion is a significant issue that costs hundreds of billions of dollars annually in the United States alone from delay, fuel consumption, and environmental impacts (Schrank et al., 2105). Transportation engineers have tried to combat this congestion in a variety of ways, one of the most inexpensive methods being traffic control. Various attempts have been made to quantify how traffic control impacts the performance of urban transportation networks. A recent study examined the impacts of street directionality—specifically, the use of one-way vs. two-way streets—on traffic network performance (Gayah and Daganzo, 2012) and found that the capacity of two-way street networks could always be increased by simply restricting left turn movements at signalized intersections. The additional travel distance imposed by doing so is more than made up for by the additional capacity that would be available at signalized intersections, and this results in shorter vehicle travel times when the network is operating at capacity. However, because this study focused only on capacity conditions (i.e., the maximum throughput allowed by the networks), it failed to consider the full range of all anticipated traffic conditions, including both uncongested and congested states. Furthermore, this study only focused on static strategies in which one control scheme (either allowing or banning left turns) was applied at all times. This is a limitation as traffic networks are dynamic, and traffic control should also be dynamic to reflect the existing traffic conditions. In light of this, the current research examines the impact of accommodating or restricting left turn movements on traffic network performance across the full range of traffic states that can be expected. Grid networks of different sizes and with different link lengths were created and simulated in the Aimsun micro-simulation software to examine their performance under various left turn control strategies. The performance measures considered include: fuel consumption, vehicle emissions, and measures of traffic performance—specifically, using the Network Exit Function (NEF) and the Macroscopic Fundamental Diagram (MFD). These performance measures are calculated and presented as a function of the current traffic state, measured by the accumulation or number of vehicles using the traffic network. This can be used to easily identify accumulations for which left turns should be allowed or restricted. Additionally, this study also examines the benefits of allowing or restricting left turns dynamically based on the current traffic state. Overall, the results find that restricting left turns can be very beneficial as a network starts to become congested; however, restricting left turns actually reduces performance when the network is relatively empty. The optimal performance is thus achieved by allowing left turns when few vehicles are in the network and then restricting left turns when the network becomes more crowded. This dynamic strategy maximizes the traffic performance of the network at all times and results in the lowest vehicle travel times and reduced delays.