Intelligent Speed Adaptation (ISA) is one of the most promising new technologies for reducing the prevalence and severity of speed-related accidents. Such a system could be implemented in a number of ways, representing various "levels of control" over the driver. An ISA system could be purely advisory or could actually control the maximum speed of a vehicle. A compromise would be to introduce a system that allows a driver to choose when to engage ISA, thus creating a “voluntary” system. Whilst these voluntary systems are considered more acceptable by drivers, they will not offer safety benefits if they are not used by the driver. Two studies were carried out that examined the relationship between drivers’ reported and actual speeding behaviour, their propensity to engage a voluntary ISA system and their attitudes towards such a system. These studies were carried out in a driving simulator and in an instrumented vehicle. In both the studies, drivers’ propensity to exceed the speed limit was lowered when ISA was available but this effect was confined to the lower speed limits. In general, drivers engaged ISA for approximately half of their driving time, depending on the speed limit of the road and indeed, on the nature of the road and the surrounding traffic. This was particularly true in the field study where drivers were more inclined to “keep up with” the surrounding traffic. The results from the on-road study indicated that those drivers who considered ISA to be both a useful and pleasant system, were overall more likely to engage it. However, those drivers who confessed to enjoying exceeding the speed limit were less likely to use ISA. This is an important finding when considering the mechanisms for implementing ISA: those drivers who would benefit most would be less likely to use a voluntary system
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