Three laboratory studies addressing the magnitude of age-related differences in visual performance and their effect on delineation recognition and sign word-message legibility were conducted by using a repeated-measures experimental design. A method of limits procedure using a Landolt-C detection task defined contrast sensitivity decrements among drivers aged 65 to 80 relative to drivers aged 18 to 49; the average threshold elevation factor for all older drivers tested was in the 2 to 2.5 Range, and was as high as 20 for the poorest performers in the older driver test sample. Also, a self-selected sample of older drivers with unrepresentatively good visual performance capabilities was indicated through comparison of multiple older driver groups in this research. Significant age effects were observed in quantifying the required brightness (contrast) of pavement striping to discriminate a left- from a right-bearing curve at varying distances downstream on a two-lane roadway, as well as the required character size to read single words and complete (novel) messages on regulatory, warning, and guide sign stimuli. Correlations between measured contrast sensitivity for test subjects and their performance on the two subsequent tasks were calculated; maximum variance-accounted-for by this visual performance index in the delineation recognition task was under 11% and reached 27% for the legibility task. It was concluded that cognitive factors play a significant role in driving tasks previously hypothesised to rely principally on sensory capabilities, with implications for the design of traffic control element countermeasures to accommodate the older driver population.
Abstract