speech warnings. However, it is apparent that speech can cause irritation, and therefore it would be prudent to provide speech warnings as a default option and make nonspeech auditory warnings available as a user option. Of course, as in all cases where auditory displays are used, system designers must ensure that it is possible to turn the displays off and that false alarms are kept to a minimum.
Regarding future work on CAS displays, based on the assertion that "the research platform that can deliver a realistic and ecologically valid traffic negotiation environment is a vehicle operating in real traffic" ( Zaidel, 1991), the previous recommendations require validation in actual road trials. As previously stated, the optimum CAS design will feature a combined visual, auditory, and tactile display. Given that work on the "smart" gas pedal has indicated particular advantages of the haptic channel, there is a need for further research on integrated modality displays.
The results of the second study suggest that a CAS warning criterion should not be based solely on a TTC measure but one augmented by an additional distance factor. The distance/speed plot produced by braking scores obtained in the study was approximately equal to the line produced by plotting TTC 3 sec plus 1 ft for every mile per hour of following speed, but it is not possible to recommend a specific algorithm. Further research is needed, in actual driving scenarios, to examine drivers braking judgments for moving targets over a wide range of speed conditions--including those situations where the target vehicle is decelerating. An observed speed effect was in the opposite direction to that hypothesized: That is, subjects tended to brake later at faster rather than slow approach speeds. The faster relative speeds involved approaches to stationary target vehicles, and moving targets in the slower approaches. It was argued that subjects were using different mechanisms to judge braking in the two situations, and that calculation of a moving target is more complex, takes longer to process, and leads to later braking decisions.
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