results could then be combined with those here to determine appropriate timings for any possible type or combination of maneuver.
As well as providing timing advice for single turns, the results could be used in decisions regarding when to "stack" instructions (i.e., give instructions for two subsequent maneuvers in a single message). According to the rule: When the distance between two subsequent maneuvers is less than the minimum preferred distance for that speed, the instructions should be "stacked."
Because the results of this study focused mostly on turns from a single carriageway into a side road, there was no influence of road sign information. In the case of maneuvers where a road direction sign is present, the driver must be given the guidance information in good time prior to the sign. This enables the driver to match the information from the system to that in the external environment. The exact timing will be dependent on the length of the in-vehicle guidance message and the complexity of the external information. Within this study the onset of the instruction was defined as the start of the verbal instruction and the instruction given was quite short (e.g., "Take the next left turn"). Long verbal messages are inadvisable due to their temporal nature but, should a long message be necessary in an exceptional case, the timing of the instruction should take this into account.
HARDIE has identified the need for a single source of guidelines covering information presentation by route guidance and navigation systems. Such literature did not exist in a form that was accessible and easily applied by designers. Research conducted within the project produced a set of guidelines based on current human factors knowledge. The empirical study reported has made an important step in the area of timing of route guidance instructions. Previous research in the area has been minimal and the detrimental effects of this can be seen in current prototypes of such systems. This study did not have the resources available to study all the maneuvers of interest, nor all the variables that could affect timing. However, it did result in a concrete guideline for a particular type of maneuver and has highlighted points that should be taken into consideration in future studies.
The guidelines produced are by no means complete, nor are they unchangeable. New, as yet unforeseen, technologies may require adaptation of the guidelines, although it is intended that the guidelines be as technology independent as possible. One of the main advantages of presenting such guidelines is that they provide a basis for discussion among experts in the area, thus enabling addition to, or refinement of, their content.