This chapter investigates this phenomenon and the level of variability of demand which is necessary for a route guidance system based upon instantaneous knowledge to outperform a system where drivers pick their routes wisely based upon knowledge of the network on previous days. The STEER (Signals/Traffic Emulation with Event-based Responsiveness) traffic simulator is part of the road network simulator suite of software which is under development at the University of York. The route selection procedure used for vehicles with no guidance system is based upon routing vehicles in an attempt to equilibrate the network. The system used in the simulation to simulate route-guidance was an extremely simple one. At each junction a guided driver selects a shortest path to the destination using Dijkstra's algorithm with the estimated travel times used for each link being the average of the travel times of the vehicles to exit the link in question.