NIGHT DRIVING - EFFECTS OF GLARE FROM VEHICLE HEADLIGHTS ON MOTION PERCEPTION

Elderly drivers often experience disability glare at night from the he adlights of oncoming vehicles. To assess the effect of glare from vehi cle headlights on visual performance for seeing moving targets, experi ments were performed at night on a dimly lit road with observers seate d in a stationary motor car viewing a computer-generated stimulus disp lay at a distance of 23 m (the stopping distance for 50 kph). The disp lay was set 2 m to the side of a second stationary car whose position on the road was that of an oncoming vehicle with respect to the observ er, The headlights of the observer's car were on low-beam while those of the opposing car were switched off (control condition), on low-beam or on high-beam. Experiments were performed using mean display lumina nces of 50 cd/m(2) and 0.5 cd/m(2). Spatial contrast sensitivity funct ions for the directional discrimination of drifting (8 Hz) sinusoidal gratings were measured using three different viewing conditions: norma l vision (binocular visual acuity (BVA) = 6/6); blurred vision (BVA = 6/9-); and simulated intraocular lens opacities (BVA = 6/6-). The data were fitted with an exponential function, which was extrapolated to 1 00% contrast to estimate dynamic visual acuity. The results show that simulated lens opacities, which have little or no effect on standard d ay time measures of visual acuity, have a marked effect on night-time measures of contrast sensitivity for moving targents. Taking into acco unt the average luminance of objects lit by road lighting, we estimate that high-beam glare reduces maximum contrast sensitivity by an order of magnitude in persons affected by mild lens opacities, giving a dyn amic acuity of 1.0 c/deg (6/180 Snellen equivalent) or less. From this and other studies we argue that there is now a strong case for the in troduction of vehicle-licensing sight re-testing at regular intervals in the UK. In addition, we suggest that vehicle-licensing authorities consider the feasibility of introducing sight tests under night-time d riving conditions.