Environmental Constraints in Urban Traffic Management: Traffic Impacts and an Optimal Control Framework

Environmental problems caused by traffic have been one of the critical issues in urban areas for decades. These problems can be addressed (at least partially) by corresponding traffic management strategies. One common environmental concern in urban areas is to keep the environmental damage below a certain level, called environmental constraints. As a result of the complexity of the traffic system however, traffic management targeted at respecting environmental constraints will not only influence environmental aspects but also other aspects, such as traffic performance. It is necessary and important to understand the relationship between environmental constraints and traffic performance, to set proper environmental constraints and formulate corresponding control measures. The central target of this research is to develop insights into how environmental constraints influence the traffic system. Such insights may then improve the decision-making process of imposing the environmental constraints in traffic management. Therefore, the research begins with the roles of environmental constraints in the traffic system, with theoretical analysis and case studies. Three arguments are made: firstly, respecting the environmental constraints are traffic management goals which need to be achieved; Secondly, the environmental constraints will not be respected spontaneously and hence need to be enforced by appropriate control measures; And finally, merely using these control measures to impose the environmental constraints underutilizes the capabilities of control measures for other targets. Based on these arguments, two important questions that are raised during the decision process of imposing the environmental constraints can be solved: how to assess the impact of imposing the environmental constraints and how to seek the best control measures for the imposed environmental constraints. For the first question, the unique, non-negative and metric assessment criterion Loss of Optimality (LO) is proposed. Loss of Optimality reflects the reduced potential to reach (unconstrained) optimal traffic performance as a result of imposing different environmental constraints. It is determined by the environmental constraints, available control measures and the traffic network design. Numerical case studies show that Loss of Optimality can correctly capture the impacts and increases our understanding of the trade-off between the damage from traffic to the environment and vice versa. For the second question, this work proposes a framework for optimizing the system performance with environmental constraints (OSP-EC). This framework mathematically formulates the selection of proper control measures for environmental constraints. The authorities should select the optimal control measures from those measures satisfying the environmental constraints, to achieve the so-called "second-best" optimal system performance. Two practical cases related to the environmental constraints are analyzed under the proposed LO assessment criteria and OSP-EC framework. The first case theoretically explores the parallels and differences between tolls and additional delays (e.g. speed limit) as alternative control measures for achieving the same environmental constraint. The monotonicity of emission factors with speed plays a crucial role in the selection of the type of control variables. If both tolls and delays are available, tolls work better than additional delays in most cases. The second case investigates low-emission zones (LEZ) and the impacts of different environmental constraint sets under the same available control measures. We revisit the concept of the low-emission zone and illustrate the advantage of applying OSP-EC concepts while designing low-emission zones. Furthermore, assuming that link tolls are available to control the LEZ, we analyze the impacts of different LEZ environmental constraint sets, including the sizes, constraint values and the locations. Theoretical proof is given that the Cooperative tolls combining the marginal external congestion cost and unit link shadow price (CO-toll) is an optimal toll set satisfying the environmental constraints; CO-toll on a link is determined by the traffic flow on that link and the low-emission zones that the link belongs to. The theoretical results from the two cases can contribute to the development of efficient algorithms for solving the OSP-EC problem. Additionally, authorities can use those theoretical results to find better policies for the practical problems related to environmental constraints.status: publishe