In this paper a freeway traffic control problem is considered. Control is exerted by means of the variable speed signs of the Dutch Motorway Control and Signalling System. After determining an important effect of the advisory speed signals on driver behaviour, a model for traffic in one section of a freeway is presented and its stability properties are investigated. Based on this model a hysteresis type control policy is proposed that optimizes the throughput of the freeway section and succeeds in postponing congestion. The latter is illustrated by means of a simulation.