Automation and the nature of driving - The effect of adaptive cruise control on drivers' tactical driving decisions

Advanced driving assistance systems that offer support by operating for example longitudinal control will have an effect on the transport system. Previous studies have also shown that drivers may be slower to respond in hazardous situations with systems like adaptive cruise control (ACC) engaged, if the system suddenly fails. Little understanding has, so far, emerged to explain why. In this thesis, a situated approach to cognition was used to explain how the drivers’ goals and priorities might change with the opportunity of delegating control to a system. Additionally, drivers’ tactical driving decisions were studied to determine how a system such as ACC is integrated into driving. Such changes could, possibly, impact measures like response times as well. The research focused on how drivers handle the addition of ACC to their drive, especially when managing traffic conflicts. Traffic conflicts are not rare events, but form part of everyday driving. Three methods were used: A questionnaire addressing drivers’ understanding of system limitations, a simulator study on handling traffic conflicts when using ACC, and a database study of a field operational test comparing driver responses with and without ACC. The studies show that drivers do take system behaviour into account when handling traffic conflicts, sometimes allowing the system to act, sometimes by resuming manual control themselves. Previous experience with ACC was also found to affect not only drivers’ knowledge of system limitations, but also their response time to unwanted system behaviour. Drivers were, as previously found, slower to respond with automation than without in simulated driving. In contrast, when studying driver response times in the field test, drivers were faster to respond to a cut-in situation with ACC active than without. The freedom of drivers to use the system as they wish may cause it to be used under different circumstances than in simulator studies, thus explaining the inconsistency with previous results. The results of this thesis indicate that tactical driving behaviour in common traffic situations is an important factor when discussing the effects of ACC and other advanced driver assistance systems. Thus, the cases in which driving is affected by the system are extended from system failures to a wide range of different situations. Merely using operational measures will miss this aspect, thus risking a depiction of driving with automation as more risky than it is. Rather, driving with automation needs to be studied from a tactical perspective, determining first how the systems are being used. Only then can the relevant operational measures be studied