This chapter presents and compares the concepts of human and technical reliability, and discusses some consequences for the automation of driving tasks. 'Success probability', 'failure probability', 'unavailability', error probabilities and transition probabilities are defined. Some reasons are given for questioning the prevailing opinion that the human element is the most unreliable part of the road traffic system. Individual accident risk is low on a trip, although there is considerable risk in the whole population, due to the large number of vehicles and journeys. Various models and procedures have been proposed for analysing and assessing human reliability. Human error is basically understood as a human output, outside the tolerances established by the system requirements within which a person operates. Explanations of human error in road accidents are still very incomplete, though much can be gained by observing and analysing traffic conflicts. The author argues that it is remarkably difficult for the reliability of technical equipment to exceed human reliability. This strongly suggests that it is better, not to replace the driver, but to use intelligent technical methods to assist his driving task. Such assistance includes improved driver training, as well as in-vehicle and infrastructure measures. The author concludes that a human-centred automation, cooperating with the human element, will optimise overall transport system performance.
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