The ergonomics of road signs: explicit and embedded speed limits

The European traffic sign system encodes the meaning of a sign by use of varying forms, colours and symbols. However, signs differ greatly in how explicit their message is. Thus, the meaning of most direction signs is self-evident, while that of some others is less obvious. The European built-up area sign limits driving speed to 50 km/h, like speed limit signs, but its rectangular form and symbol not associated with speed limit distinguish it clearly from standard speed limit signs. Also, the function of the built-up area sign is more complex than the one of speed limit signs: the built-up area sign both limits speed and informs about entering a built-up area. The purpose of this study was to investigate whether the built-up area sign is noticed and obeyed as well as speed limit signs. In three field experiments the 50 km/h, 80 km/h signs and built-up area signs were alternated and speed changes were measured. Results indicated that the built-up area signs elicit only negligible speed responses compared to the speed limit signs while supervision (visible police car) results in a similar effect for both signs. The same results were also obtained in the laboratory where slides about traffic situations were shown to subjects who were asked if it was necessary to decelerate. These findings suggested that, in contrast to the built-up area sign, drivers have developed a mental model which facilitates more effective noticing of speed limit signs. The built-up sign does not share these facilitating effects, because of its dual function and symbol and form which are not associated with speed limits.