## 5 Evaluation Results and Discussion
### System Overview and Limitations  
The evaluation of the proposed system offered encouraging results and an indicative appraisal of users' acceptance of the prototype AR-HUD system. The aforementioned results suggest that the combination of AR HUD and gesture recognition offered a viable alternative to current in-vehicle interfaces for infotainment systems. The selection of the HUD as a conduit of the provision of visual information to the driver and its positioning confirmed previous and concurrent research studies in the field [2; 24; 26; 27; 28; 58]. The study's results both on user performance and on system acceptability, as presented in previous sections, are concordant with published findings. The collisions occurrences results, in particular, resembled closely previous experiments that utilised HUD interfaces and contrasted them against typical HDD devices [2; 58; 60; 61]. Coupling the latter with a gesture recognition interface design, following the same design mantra, enabled the users to perform the selected and specific tasks without complicated multilayered interfaces. As per previous studies, uncomplicated gesture recognition systems could be utilised instead of typical manual or voice-controlled interfaces that tend to distract the driver by taking the eyes off the road [29; 32; 33; 34; 62; 63; 64; 65]. Yet, several limitations hindered the research process as presented below. The position and size of the current HUD-UI is fixed and allows no modifications. This limits the usability of the system as the drivers had to modify the position of their seat as to see clearly and operate the AR HUD. The addition of the customisable size and position of the HUD would likely enhance the proposed system's usability and driving experience of the driver. This limitation could have potentially affected some of the simulation results. As presented in the \"Simulation Requirements\" section, the development of a full-scale VR driving simulator carries a considerable cost. The reduction of the cost wherever possible dictated the utilisation of off-the-shelf equipment which had to be installed retrospectively in the vehicle's interior. Hardware devices employed for the simulation such as the Leap Motion are commercially available computing equipment/peripherals and they are not designed for immediate use in other domains. This was evident during the simulation, as some users with larger hands found it difficult to pin-point and select the different UI options, as revealed in the post-evaluation questionnaire. Furthermore, the aforementioned device's usability has not been tested in a real-life vehicle. In its current form, it is unclear if it is suitable for a vehicle in motion. Another measuring element that intentionally has not been employed was the eye-tracking of the users, typically utilised to identify their gaze position and time during the simulations. Although eye-tracking is available in the VRDS laboratory, it was deemed unusable for two reasons. Figure 17: User experience results regarding the ease of use for the HUD interface. The first one stems from our previous attempts, which presented major issues with the users' acceptability and the performance of the tracking equipment. In particular, as the driver gazes between the road in the VR environment (i.e., layer 1) and the AR HUD interface (i.e., layer 2), the eye-tracking system was not able to distinguish between the two layers. The reason is that the proposed HUD interface is projected at 2.5 m ahead of the windshield to blend with the external environment and avoid double accommodation of the driver's gaze as mentioned previously [2; 24]. Eye-tracking could have been used in the cases of typical small-size HUD interfaces that they tend to appear at the same level or in close proximity behind the windscreen depending on the projection system, HUD device, and glass combiner used [2; 24; 58; 60; 65].