There is clearly a need for optimising traffic systems in order to reduce congestion and improve network reliability. A system optimal assignment is a traffic flow pattern that minimises total network costs. In reality, travellers are not under any centralised control, but instead choose routes in order to minimise their own individual travel costs, which in general does not lead to a system optimal assignment. Travellers may be induced to choose routes that are closer to yielding a system optimal assignment through the use of tolls and signal control. The paper considers both approaches within a static traffic model (where flows and costs stay constant over time) and within a dynamic traffic model (where flows and costs vary over time).