Determining Legibility Distance for Highway Signs: Is the within Subject Variability Being Overlooked?

Laboratory studies of warning symbol signs have been shown to underestimate legibility distances by up to a factor of two when compared with field studies. However, this research suggests it is more than simply experimental setting contributing to disparity in research findings. Using a group of old and young drivers, six symbol signs were investigated in both settings. With six trials per sign, legibility distances, defined as the distance at which the sign is correctly identified from a menu, were collected. Large within subject variability was discovered in both age groups. This variability led to alternative ways of defining the dependent variable equivalent to designs of past studies examining legibility distances of the same signs. Different results arose out of the subsets created. The consideration is not just should a field-based versus laboratory-based methodology be used. An argument is posed that recommended distances at which signs are placed must be determined from a “worst-case” scenario. This premise requires a reexamination of our research methodologies for determining placement of highway signs.