Optimizing Traffic System Performance with Environmental Constraints: Tolls and/or Additional Delays

Environmental concerns are increasingly important targets in urban traffic management. The way traffic spreads over routes in a network can affect substantially the environments well as the level of congestion. This paper explores the selection and location of control measures that influence route choice aimed at optimizing traffic system performance subject to environmental constraints (OSP-EC problems). We address two groups of link-based control instruments commonly considered in urban traffic management: tolls and additional delays. Tolls penalize drivers monetarily (e.g. congestion charging) while additional delays represent those measures directly increasing travel time (e.g. speed limits) on the controlled links. We first identify the parallels and differences of how tolls and delays reallocate traffic flows over routes and links in a network, how they affect total system cost and total emission, and finally how they produce different solutions for the OSP-EC problem. When only tolls can be used, one intuitive solution to the OSP-EC problem is a set of tolls that superimposes environmental shadow prices for actively constrained links onto the marginal system cost pricing that is to be charged on all links. When only delays are considered, the selection of the best locations to implement control variables is more complex. We show that the monotonicity of emission factors with speed plays a crucial role for 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 theoretical results can contribute to the development of efficient algorithms for solving the OSP-EC problem, and help authorities to solve this policy problem.