In order to serve their purpose properly, traffic signs must have an adequate visibility and legibility. During night-time driving, drivers are more or less dependent on the vehicle's headlights for detecting and reading signs. In order to increase the visibility as much as possible, signs are made retro-reflective. This means that the face material of the sign is optically designed so that when illuminated by the headlights, a large part of the light is directed back in the direction of the vehicle. During daytime the daylight level is generally high enough for the road users to receive the sign message without extra light sources. The retro-reflective properties are therefore not necessary in this situation. Traffic engineers generally distinguish between vertical signing, meaning the well known signs having a vertical plate displaying a message to the road users, and horizontal signing, which comprises messages designed as markings on the road surface (i.e. the "road markings"). This paper is mostly devoted to point out the conclusions of the most recent research in the field of vertical signing and, more specifically, those investigations dealing with daytime visual performance of traffic signs. (A)
Abstract