Variable Speed Limits (VSL) is a well-established method of reducing congestion especially on highways and motorways. Often, VSL is used manually though in literature there exists some control mechanisms to implement automatic VSL. This work proposes a method of implementing VSL by measuring the traffic conditions of the road and using a Proportional Integral (PI) controller to produce the optimal VSL based on existing conditions and the desired outcome. The aim of this control system is to reduce the amount of vehicles entering a congested area, allowing the congestion to resolve more quickly. To examine the effectiveness of the strategy, a section of the M50 was simulated using a model to describe traffic flow called the Cell Transmission Model (CTM). The simulation model considered observed traffic conditions from loop detector data supplied by Transport Infrastructure Ireland (TII) with and without the controller incorporated. The results showed a reduction in travel time of up to 27% with the controller applied. Sensitivity analysis was performed to investigate the robustness of the model to certain input parameters associated with both the controller and the implementation of the VSL in the CTM.