## 5 Factors That Influence the Effectiveness of In-Vehicle Messaging
### Characteristics of In-Vehicle Displays  
Substantial human factors research has investigated the interaction between drivers and vehicle information displays. Increasing vehicle automation leads to new challenges in display design. Several display characteristics, influential in human use, have been identified including the type (e.g., head-up vs. head-down), modality of information presentation, information format, and information load. As noted, Head-Up Displays (HUDs) and Head-Down Displays (HDDs) each come with some advantages but also pose unique challenges for presenting road sign information during automated driving. HUDs may provide the benefit of reduced re-fixation time; however, they can absorb driver attention with the amount of visual information, particularly during a takeover request. In contrast, HDDs may reduce visual load but could lead to longer response times, as visual attention must first be directed to the display and then re-directed to the roadway, if needed. Therefore, HUDs could be more suitable for supporting driver perception of road environment cues, while HDDs may be more beneficial for reduced information load when a longer response time is available (e.g., a notification in advance of a takeover). It is also important to ensure consistency in interface design for vehicle systems that present the same type of information. Such displays should always occur at the same location, either on a HUD or HDD. HDDs require greater use of visual cues to direct driver attention to displays, as a result of the display not being co-located with the roadway in the same way as a HUD. As of the time of this review, there has been no comparison of HUDs and HDDs for presenting roadway signage information and the utility of either type of display remains an open research question. Regarding modalities of display information attention, the combination of visual, auditory and even haptic cues could be effective in guiding spatial attention of a driver to an in-vehicle display. For non-safety related messaging, the use of these cues can improve driver's processing of upcoming exits that may be of interest, such as food or lodging locations. Modalities of perceptual cueing and their combinations should be examined for information presentation using HUDs and HDDs, as specific pairings may vary in effectiveness during automated driving. Regarding the amount of information presented through an in-vehicle display, the format of content needs to be optimized to support driver information processing and to allow enough time for decision-making on takeover requests. Research has shown that pictorial information paired with text tends to be more effective than just text-based information and should be applied for display design [20]. This approach can also help to reduce display clutter and increase readability. Effective organization of visual information on the in-vehicle display can also improve driver preference and driving performance while using the in-vehicle displays. In a recent study, [PERSON] and colleagues [121] found that as compared to a visual interface with six panels of information, drivers reported higher preference toward a design with three panels, and also showed better vehicle control when using a design with four panels. This finding suggests the importance of exploring the visual organization of in-vehicle displays. In addition to optimizing visual communication, auditory cues can also aid in reducing driver visual attention load [122]. Therefore, auditory menu cues along with visual presentation of other displays should be utilized to reduce driver workload. Research has also demonstrated that use of auditory cues for menu navigation is beneficial in orienting driver spatial attention [123]. The required processing time for each modality of information presentation, or combination of modalities, should determine the use of modalities for specific situations, such as planned takeovers, emergency takeovers, and ambient communication of trip-related information.