## 5 Evaluation Results and Discussion
### Indicative Driving Patterns-Discussion
(**a**) Vehicle speed (Km/h) over time (s), (**b**) Distance (m) from the leading vehicle over time (s). Notably, the lead vehicle is braking abruptly behind immobile traffic. The user is challenged to respond quickly and avoid the collision. However, during the overall simulation, the driver is receiving four messages at different intervals, related to navigation and mobile phone texts activity. Two of the four messages are related to navigation updates and in particular, the first two messages, related to navigation, alert the driver for potential dense traffic ahead and delays appearing in the following sections of the motorway. The last two are mobile-phone text messages related to a social event. The context presents small variations between the two simulations. The sequence of the messages changes randomly between the simulations. This scenario is repeated randomly for each system (i.e., HUD and HDD). The user illustrated in Figure 15(a),b drove the simulation with the HUD first and in turn with the HDD. The two grey sections in both graphs indicate the last event in which the driver receives the fourth message, and at the same time, the lead vehicle performs a sudden brake to avoid collision with the traffic jam ahead. In Figure 15(a), the user drives close to the motorway upper-speed limit (70 mph/112 Km/h) with reasonable fluctuations in speed. When the user receives some information through the HUD, he is braking lightly to interact with the HUD, as can be seen by the two small peaks in the graph just before 5 s and marginally after 9 s.  
Yet, the user does not change the speed abruptly or swerve between lanes as he can see at the same time the external environment and the braking vehicle ahead. Notably, the user through the HUD managed to read the previous brief navigation warnings regarding the traffic jam ahead. Anticipating such issue, the user slows down from the top legal speed gradually and reaches on a full stop behind the traffic on 11.5 s. He starts driving again as the traffic is resolved. In contrast, when the user is trying to access the information from the HDD, is struggling to read some of the messages and the driving speed pattern becomes erratic, presenting abrupt deceleration and acceleration as illustrated by the red line of the same graph in Figure 14(b). In addition, being unable to read the navigation warnings, the user maintains a fast pace of driving. On the final message, it can be observed that the driver is trying to access the information looking and interacting with the HDD device. The following 2 s the driver realises that he is in a direct course to collision and attempts a full brake unsuccessfully, resulting in a rear collision at 72 Km/h with a stationary lead vehicle. Such collisions are severe and frequently fatal or resulting in debilitating injuries of both vehicles' occupants. Interestingly, the driver with the use of the HDD maintained higher speed and reached the event area in a shorter period of time and unable to access the navigation warnings safely, met the traffic 7 s earlier that with the use of the HUD. Further driving patterns that appeared are presented in Figure 15(b), illustrating the distance (m) from the lead vehicle in correlation to the time (s) driven prior to the collision event. The last 200 m separating the driver's vehicle from the lead vehicle is reduced rapidly, in less than 5 s while the driver is preoccupied with the HDD interaction, resulting in a major collision. In contrast, while the driver is using the AR projected information of the proposed HUD, the vehicle's distance from the stopped lead vehicle is plotted as a gradual curve, reaching a full stop approximately 40 m behind the immobile traffic. The aforementioned findings relate to less stressful driving with the use of HUD, as advocated by 97% participants, and are in accordance with the findings of previous experiments that utilised prototype HUD systems to alleviate the driver's cognitive load and reduce driver's distraction [2; 14; 47].