1. A method of controlling the output of information to a driver, the method comprising: monitoring at least one operating parameter of a vehicle; estimating a driver workload based on said at least one operating parameter; and controlling the output of information to said driver based on the estimated driver workload; wherein the driver workload is estimated with reference to a plurality of thresholds which define a plurality of driver workload ratings, the method comprising periodically cycling through some or all of said defined driver workload ratings having a lower rating than a current estimated workload rating.