was directed mostly to the road (80%), and minimally (4%) inside the vehicle. Interestingly, the proportion of attention allocated to the in-vehicle display was very similar across the three conditions of the first experiment, probably due to the presence of the paper display of instructions.
An address searching task in an unfamiliar city environment, regardless of the type of navigation aid used, was indeed experienced as a more difficult task than just following a route. Under conditions of minimal information provided by the Guidance List, the QOD was degraded relative to the Familiar Route condition. However, under the more supportive Ideal Navigator condition, the task was made easier, resulting in fewer navigation errors and better quality of driving compared with both the Guidance List and the search-free Familiar Route conditions. Most drivers expressed a preference for the Ideal Navigator as a navigation assistance device.
The second experiment demonstrated that a Voice guidance aid can lead to better driving performance, less visual attention directed to the in-vehicle displays, and more attention directed to the forward scene than a Screen guidance system displaying the same verbal information. In fact, the Voice-Automatic condition produced quality of driving that was equivalent to the Ideal Navigator. However, drivers using the Voice modality reported higher task load than drivers using the Screen modality. This may be due to between-group differences in TLX ratings and/or the need to keep instructions in short-term memory under the Voice modality.
Figure 17.1 shows that an Automatic display resulted in consistently better performance than an Interactive system. The Interactive Screen condition produced the poorest overall quality of driving, not much different than navigating with the basic paper list. It should be noted that although all mean QOD values fell within the range of 5.0 to 5.6, the differences between conditions were significant at the .05 level of confidence.
For the Voice condition, driving with an Automatic system was also subjectively less demanding than driving with an Interactive system perhaps because the latter imposed additional demands associated with deciding when to request information and the need to depress the repeat key.
It was noted that drivers' personal preferences were consistent with the system's relative advantage as indicated in these figures. For both display modalities, the Automatic mode of control was preferred over the Interactive mode of control, though the preference was more pronounced in the Voice modality.
The results of these experiments reaffirm the sensitivity and reliability of the QOD technique for evaluating the safety of TICS. Further efforts should aim to refine and validate the QOD methodology.