Effects of luminance contrast and font size on dual-plane head-up display legibility ("The Double 007 Rule for HUDs")

Head-up displays (HUDs) can provide important driving information such as speed and navigation directly to a driver within their immediate field of view. Specifically, a dual-plane AR-HUD can provide drivers with larger images with richer information, more intuitive information presentation, and enhanced navigation features. To ensure driver comfort, proper presentation format of the text on the dual-plane HUD needs to be investigated. A human factors experiment was conducted to explore proper luminance contrast and font size of the text on a HUD. Twenty older and nine mid-age drivers were instructed to recognize letters in 14 levels of font size under each of eight levels of luminance contrast (ranging from 1.2:1, to 8:1) on each plane. The best test performance was observed at a luminance contrast of 7:1. We report the minimum and upper bound of 95% confidence interval of legible vertical visual angle for 99.5% of older drivers at each luminance contrast level. Although the typical legibility rule of thumb for font height versus distance on standard displays is 0.007 (known as the Bond rule), our experiment suggests that double that size (0.014) was needed to cover 99.5% of older drivers at a contrast ratio of 1.2:1 on a HUD.