Once the modality of the alert has been selected, the next question concerns how the visual warning information should be presented to the driver. If only one-stage of warning is implemented, this question becomes a simple matter of determining the most appropriate icon. An icon such as the one that the CAMP FCW program used would probably be appropriate (see Figure 9.4). However, if multiple stages of warning are used, the question becomes multifaceted. The designer must not only determine the optimal number of stages, but also how the display will transition between those stages. The designer must determine what dynamic visual properties to employ for communicating an increasing level of threat. The stimuli must be selected to balance the attention-getting and informational requirements constraints of driver acceptance.
Dingus, McGehee, Manakkal, Jahns, Carney, and Hankey (1997) developed and tested several time-headway displays. Unlike the CAMP experiments that used a single stage display, Dingus et al. used time-headway to drive their multiple-stage displays. Dingus et al. evaluated the three displays that are presented in Figure 9.5 and are described as follows:
  
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