Visibility on the Road

Abstract

Existing methods of measuring the visibility of an object or of assessing the visibility in an installation are discussed. The ideas underlying the concept of visibility are then analysed with the intention of making clear the nature of some of the difficulties which have given rise to controversy.

One must distinguish between the visibility of an object and the visibility in an installation. The latter IS best regarded as an assessment of all actual or potential visibilities in the installation. There has been controversy as to the correct scale for the visibility of an object and about the meaning, if any, of the numerical values given by such scales. It is concluded that all scales which put objects in the same order of visibility are equally valid, but some are more convenient than others because they are more simply related to the visual phenomena vital to the task of the observer: in this case safe driving.

The attempt to represent the visibility in an installation by a single number is an attempt to put in order of merit the totalities of actual and potential visibilities in different installations. It is concluded that no useful meaning can be attributed to a comparison of these totalities in terms of visibilities alone. Any such comparison breaks down in certain cases and these are likely to be just the cases in which guidance is most needed. Installations may be compared, however, in terms of safety. The visual safety of an installation, as distinct from the visibility in an installation. can therefore be represented by a single number. The practical difficulties in the way of obtaining the visual safety are discussed. Progress seems to depend on gaining an understanding in clear numerical terms of the part played by visibility in the causation of accidents. Present knowledge is extremely meagre.