The effectiveness of four modern anti-lock braking systems has been assessed in an extensive programme of track tests. The performance of the systems in three different manoeuvres, on a range of road surfaces, has been compared to that attainable by the vehicles with the systems wholly or partially disabted. It is shown that while the deceleration performance of an anti-lock system is closely related to its degree of complexity, the improvement in controllability that is provided by any system ensures that the accident avoidance potential of all of the vehicles tested is greater than that of the non anti-lock vehicles. This potential is shown to be higher for the complex, and expensive, electronic systems but still significant for a cheap and simple mechanical system.