## 3 Proposed AR HUD System
### Visual Interface  
The visual elements of the interface were designed to provide simple and non-distractive indications of mobile phone calls, text messaging, and navigation information which are deemed as the most attention-seeking systems [39; 40]. Following previous work, various combinations of symbols and their activities have been tested and gradually distilled to only three main symbols, namely (a) Home, (b) Text Message, and (c) Map/Navigation as depicted in Figure 3[4; 41; 42; 14].  
The Home symbol follows the standard approach of transferring the user to the main menu instantly from other parts of the interface. In this case, the Home symbol is re-setting any interaction that the driver had with the other two symbols. The second symbol, the Text Message symbol, had a dual functionality. It predominantly provides the driver with a gentle/understated warning related to the incoming messages. The number of messages received is highlighted in a superscript circle positioned in the right top corner of the Text symbol as depicted in Figure 4. The secondary functionality of the latter symbol is to present the number of missed calls and provide in a brief text format, the metadata of the missed calls which include callers' name and the time of the call. The activation and interaction with the symbols are prioritised by interrupt strategies which distil the incoming information and release them in a timely and safe manner, preventing the user from browsing the infotainment data.  
Figure 2: Screenshot of the fully functional AR HUD and conventional HDD interfaces in the VR driving simulator.  
The interrupt strategy is activated while driving in high speeds typically encountered in a motorway environment which has the highest probability of fatal collisions. Hence, driver's concentration should remain solely in the main task of driving. Similarly, the same communication interruption and archiving of incoming information are activated in the dense urban environment. Weather conditions are also taken into consideration and increase or reduce the time of the withheld infotainment data depending on the adversity of the weather phenomena. An additional set of sub-symbols were developed to read and navigate in-between the received messages and the navigation data, as presented in Figure 5. The design of these symbols maintained a simple form of arrows (left and right) and a circular indication showing the number of the received/unread messages as  
Figure 4: Screenshot of the simulation presenting the sub-symbol of the received Text Message as a superscript red circle with the number of received and stored messages.  
Figure 3: Three main interface symbols as appear in the VR driving simulator: (**a**) Home, (**b**) Navigation/Map, and (**c**) Text Message symbols.  
mentioned above. During the user-interface (UI) design it was considered essential to provide self-explanatory and straight-forward symbols that operate promptly in contrast to the vast majority of current automotive industry interfaces which are tuned towards the \"family style\" required by each brand. The latter intends to maintain a brand's identity from the exterior to the interior of the vehicle design and between the different vehicle models provided by the same brand.  
Yet, this could also be a negative influence in the interface design, which is forced to follow particular visual elements that might not help the driver or in some cases hinder the driving process. Such interfaces are further burdened with the incorporation of multiple other infotainment systems that in many cases need to be also redesigned to follow the above branding mantra, resulting in complex interfaces and various layers of selections.