The main visual problem for traffic driving through a long tunnel is presented by driving into the tunnel by daylight. When the tunnel entrance is not illuminated sufficiently, it is observed as a "black hole". In practice the main problem is the "luminance veil" which is extended over the field of vision. This veil decreases the contrast of all sub- jects which are in the tunnel entrance. The relationship between the contrast as can be measured in the laboratory and the contrast in the field is called the field factor. This factor represents the fact that the perceiver is at the same time a road user: besides the perception of eventual objects the vehicle must be handled and the traffic must be watched. This report has the aim to determine the field factor in practice.
Abstract