A Method of Evaluating Seeing Distances on a Straight Road for Vehicle Meeting Beams

The paper describes a method of calculating the distances at which objects may be seen in the light of a driver's own headlamps when one vehicle meets another on a straight road. It is based on experimentally determined seeing distances for beams of uniform intensity, obtained when the observer's vehicle was moving at 30 miles per hour and the glare lamp was stationary. The data, which were obtained for a given road width and position of the test object, are extended to other widths and object positions by means of the Stiles-Holladay law that the effect of a glaring light varies inversely as the square of the angular separation between the glare source and the line of sight.

The method has been used to evaluate seeing distances for four object positions across the road for a meeting beam typical of current British and American practice in which the beams are deflected to the nearside. The results show that with this system of lighting the minimum seeing distance for an object in the nearside lane occurs when the object is somewhat nearer the driver than the opposing lamps, and that the value of the minimum direct seeing distance varies considerably with the position of the object across the road.

Comparisons between calculation and measured seeing distances in trials in which both vehicles were moving are in good agreement for objects along the nearside of the road. At other object positions comparison is not possible at present because the experimental evidence available is insufficient.