Model predictive control approaches for urban traffic networks: A comparison between optimization algorithms

Abstract

In this thesis, the effect of traffic light control on urban networks is investigated, with the main focus on the optimization algorithms used to solve the optimization problems resulting from a model predictive control approach. This will be done by using a macroscopic traffic flow model (the S-Model) to simulate the real-time traffic flows inside a network. A microscopic emission model (the VT-micro emission model) is further added to also include impact of the vehicle behaviour (accelerating, decelerating, driving at a constant speed, etc.) on the total amount of emission gasses that are exhausted by the vehicles inside the traffic network.

In this thesis the optimization algorithms will be investigated with respect to the reduction of the cost function as well as the computation time needed compute the (sub)optimal solution. The cost function can consist of the Total Time Spent (TTS) by the vehicles inside the network, the Total Emissions (TE) the vehicles exhaust while inside the network, or a combination of both.

We make use of Matlab for implementation of the models and the optimization algorithms and SUMO as a traffic simulator, which will be used to simulate a real-time traffic network.

The traffic flow model and the emission model are both non-smooth and non-convex, which in general would require the use of a global optimization algorithm together with multiple starting points. We also use a smoothening function on the models to work with a local optimization algorithm that works with derivatives of the cost function and both models. By using multiple starting points for this method, we hope to obtain similar results. The optimization algorithms that are implemented and investigated in this thesis are: the Genetic Algorithm (GA), the Simulated Annealing (SA) algorithm, the Pattern Search (PS) algorithm, and the Resilient backPROPagation (RPROP) algorithm.

To compare the results of the four optimization algorithms, four different scenarios are considered. The final results show that all control methods perform better than the FT controller. Overall the GA algorithm performs best without using a multi-start approach, with PS and SA having similar results. The RPROP method is either close to the other methods (0-2%) or quite far off (10-15%), depending on the scenario. When looking at computation time, PS is the fastest. It is twice as fast compared to the GA in most scenarios. SA however takes around fifteen times the amount of computation time compared to the GA. RPROP has varying results again compared to the GA algorithm. A better cost function for the TTS as well as a more optimized algorithm for the RPROP method can resolve both of these issues but future work might need to prove this.