Head-Up-Displays (HUDs) have long been used in airplanes;​ however, it has not yet been verified how the difference in environment would affect their legibility and/or driver visibility if they were to be used in automobiles.

Experiments were conducted based on the Double Task Method in which subjects were made to identify both the display and objects in the forward field of vision in a short time period of 0.3 to 0.5 seconds. The results show that the display recognition error rate is reduced when the display is positioned on a plane from horizontal to 10° below the forward line of vision and at a distance of 0.8 or more from the eyes. They also show that improved legibility obtained by HUDs reduces the recognition error rate in the forward view. Measuring the time required to recognize the display using an Eye-Mark-Camera, it was shown that recognition time with HUD is 0.2 seconds less than with conventional digital displays. Another experiment involving tinted Combiner on which the HUD is projected was carried out, in which subjects were tested on their perception of objects appearing for 0.27 seconds in the forward field of vision. The result shows that the recognition error rate does not increase due to tinting of the Combiner providing that transmittance is 40% or more.

These results both substantiate the HUDs' high legibility while, at the same time, proving its effectiveness in assuring safe driving from the standpoint of enhanced driver attention to the front view.