The measurement of drivers' performance at the limit of capability is difficult due to methodological problems, moment to moment variability of drivers, differences between drivers, and their interactions with the characteristics of the vehicle, road, and environment.Aspects of longitudinal and lateral vehicle control are discussed by reference to results of braking and steering tests, with emphasis on the variations between the performance of drivers.The effectiveness of drivers in vehicle braking is shown to be a function of the brake system deceleration/pedal force gain. Overall braking performance could also be improved by increasing the abilities of drivers who are poor in this task, by training in brake modulation on dry and wet pavements. The best drivers are as effective as an antilocking brake system, except on the equivalent of ice covered pavement. In steering control drivers increase their response frequency bandwidth as task difficulty increases. Comparisons are shown between an inexperienced and an experienced driver in curve negotiation up to the limit of control.BOTH PEOPLE AND MACHINES have limits. In the case of machines the limits can be predetermined, by and large, before they are built. By comparison, humans come preassembled, packaged, and largely at random. A person can be trained to use a device, but the basic performance limits imposed by his physiology and psychology remain unchanged.