A Method of Evaluating Seeing Distances on a Curved Road and its Application to Headlight Beams in Current Use

Abstract

A method is described by which direct seeing distances can be evaluated when two vehicles meet on a curved road at night. The method is a development of that already used to determine seeing distances from vehicles when they meet on a straight road. It has been applied to three headlighting systems in current use, the modern British beam, the symmetrical European beam, and the single-lamp meeting beam representing earlier British practice.

Considering meetings on both straight and curved roads, it is shown that in some circumstances the modern British system is better than the symmetrical European system, and that in others the order is reversed. The modern British system is almost always better than the European in revealing the important nearside object, and seeing for all object positions is considerably better during the early stages of a meeting on a curved road, when the observer is on the inside of the bend. This result is important, because the conditions closely resemble those of open-road driving with the meeting beam, a frequent occurrence on roads when oncoming vehicles are anticipated. Taking into consideration also the fact that the modern British system is the less susceptible to vertical misaim, it is concluded that it is the better suited to actual road conditions.

A comparison of the modern double-lamp British system with its predecessor, the single-lamp system, shows that on narrow roads, both straight and curved, the differences between the systems are generally small for all object positions. During the later stages of the meeting, however, recovery from the minimum seeing condition is consistently more rapid with the single lamps, for objects in the driyer's own traffic lane.