The value of high mast lighting has been highly controversial since its introduction in the early 1960s. Proponents suggested that high mast lighting offered considerable enhancements to visibility. Opponents, on the other hand, argued that high mast lighting was expensive to build, offered little improvement to visibility, and often resulted in light trespass and light pollution. By the early 1980s, new data became available which suggested the superiority of high mast light over conventional systems. The reasons cited were
• An improved visual field negating the “tunnel effect” caused by a limited lateral dimension when using conventional mounting heights. The tunnel effect prevents the driver’s eyes from reaching a reasonable level of retinal stability—a failure believed by some to be the cause of a substantial number of accidents at night [12].
• Improved luminance uniformity within the principal visual field. The uniformity eliminates the need for the eye to adapt to a wide range of luminances, which adversely affects visual acuity. Many experienced engineers are of the opinion that luminance can be reduced when using high mast lighting because of the compensating factors of improved uniformity and reduced veiling glare.
• Disability veiling brightness negatively affects a driver’s visual performance. In practice, luminaires spaced at long distances require large light sources with high beam intensity in the upper angles of the vertical plane. Light emitted from a luminaire above 75° can be considered a contributor to glare. High mast luminaires confine their distribution within the limits of 60° to 65° and practically eliminate the disability veiling brightness. Brightness from high mast systems is also reduced through geometric arrangement. Increased mounting height and greater offset remove the luminaires from the driver’s active viewing area [12].
• The location of the high mast pole contributes to a clear roadside and results in a reduction in the number of vehicular collisions with fixed objects [12].
• Studies done using the older mercury vapor light sources indicated that on both diamond and cloverleaf type interchanges, high mast lighting systems utilized fewer luminaires and less energy than conventional lighting [12].
• There is a growing emphasis across the nation to eliminate or control “obtrusive light.” Keeping this in mind, a designer might want to consider using some of the more newly designed, full-cutoff, high mast luminaires that have been found acceptable and to be aware of keeping the mounting heights as low as feasible for a given situation.
The growth in vehicular traffic combined with the continuous search by transportation authorities for safer and more cost-effective roadway design has resulted in a shift toward multilane roadways. The new freeway designs cannot be effectively illuminated by conventional methods using low mounting heights and light sources of limited lumen output. Because of these requirements, high mast systems offer a distinct advantage over alternative systems. High mast systems also offer advantages in cases where future road widening is expected. The poles can be located 50 ft (15 m) or more from the traffic lanes, enabling future road widening without the need for changes in the lighting system [12]. Figure 7.66 shows a typical lighting design.
