INDIRECT FACTORS AFFECTING REFLECTIVE SIGNS BRIGHTNESS (ABRIDGEMENT)

The design is outlined of an experiment which simulated volumes of 300, 600, and 1,500 vehicles/lane/h on a test road. All vehicles in all tests employed low-beam head lamps since common use of lower beams is well documented and is the rule with high volumes. The study found that for unlighted overhead signs, the single vehicle with low beams produced luminances of 3.4 to 6.9 cd/m sq (1 to 2 ft-L). With 3 vehicles spaced at 152-m (500 ft) increments, sign luminance for the test vehicle increased from 3.8 to 9.6 cd/m sq (111 to 2.8 ft-L). With 6 vehicles spaced at 91-m (300-ft) increments, sign luminances for the test vehicle increased from 9.3 to 14.0 cd/m sq (2.7 to 4.1 ft-L). For 15 vehicles spaced at 5-m (50 ft) increments, corresponding to near capacity for an average facility, sign luminances for the test vehicle increased from 26.7 to 28.4 cd/m sq (7.8 to 8.3 ft-L). The improvement noted at longer distances is due to the close angular proximity of the adjacent headlights. For shoulder mounted signs, lower beam sign luminances were 13.7 to 54 cd/m sq (4 to 16 ft-L) with reflective sheeting material for a signle car. During the experiment with 15 cars, it was found that for all materials and distances, an average of 19 percent of the sign luminance comes from the driver's head lamps. The measurements made in rainfall conditions displayed higher luminances.