Contemporary automotive, navigation and infotainment requirements have evolved the traditional dashboard into a complex device that can often distract the driver. Head-Up Displays (HUDs) have recently attracted the attention in the field of automotive research, promoting the reduction of driver's reaction time and to improve spatial awareness. The aptitude of the proposed HUD interface lies within the driver's focusing ability to the HUD interface and the actual traffic. This paper analyses the performance behaviour through user-tests using different focal levels for the projection of a full-windshield HUD interface. For this purpose, a VR driving simulator has been developed to test the different depths of field configurations of a HUD while driving in various weather and traffic conditions with and without the HUD. Our simulation results reveal the users' preferences regarding the focal point of the superimposed interface and present a comparative evaluation of the different focal levels and their impact on drivers' behaviour and performance.