MEASUREMENTS OF VISIBILITY FROM THE DRIVING SEAT OF MOTOR VEHICLES

Abstract

Even in daylight in clear conditions the driver's visibility, both forward and rearwurd, is restricted by the structure of the vehicle. Important sections of the view can be obscured ; thus a windscreen pillar can obscure a pedestrian Or cyclist until it is too late to avoid an accident. A given vehicle has certain visibility characteristics. The work described in this paper is the measurement of these characteristics for a range of vehicles.

The method of measurement is based on an American technique in which two lamps are positioned to correspond with the eyes of an average driver. The visibility is measured by moans of the shadows cast by body pillars, bonnet, etc., and by the light reflected rearwards by the mirror. Both forward and rearward visible areas can be interpreted as plans of visible areas on the ground, In the choice of eye position, consideration was given to the position of the eyes relative to the teat and to the position of the seat in its travel, The seat is positioned with reference to the accelerator pedal at a distance determined from measurements of a small sample of drivers in different cars and of owner-drivers and their cars. For private ears the ‘ eyes ’ are placed on a vertical tangent to the seat squab at a fixed height above the undeflected seat, this position being based on measurements made by the General Motors Corporation of America. For commerical vehicles the same eye height is used, but a fore and aft correction is applied for variations of the angle of the seat-back to the vertical, which can be large in this class of vehicle. This correction factor was obtained from the averages of measurements On -3 drivers, using a seat in which the angle of the back could be varied- This specification of aye position is a very simple one, and was adopted to avoid the use of dummies and because it was considered to be adequate for the comparisons needed. In use it has Occasionally given misleading results, e.g. when large variations Of scat softness have bean encountered.

A wide range of cars and commercial vehicles has been examined. A typical result from one of these is given. The chief points of comparisons between vehicles are the extent of the forward obscuration by the windscreen pillars and bonnet and the vertical and horizontal angles of rearward vision. In cars these factors are affected by styling. In particular, with low roof-lines, the eye level tends to come high in the car, where the pillar thickens to meet the roof, resulting in large obscurations. The high eye level also restricts the positioning of internal mirrors in that it is difficult to obtain good rearward visibility without causing undue forward obscuration.