Roadway lighting has long been used on city streets where it serves a number of functions in addition to providing for safety and efficiency of traffic operation. Only in recent years have highway departments and municipalities realized the benefits that could be had by lighting heavily travelled high-speed arterials such as freeways and major streets in outlying areas. This new concept in roadway lighting embraces a single-purpose function, namely, to light the roadway for the express purpose of improving the safety and efficiency of traffic operation. There is no doubt that illumination of principal roadways can be justified on the basis of safety. This is well documented in the Congressional Record (1), where the Honourable Charles Farnsley, Representative from Kentucky, reported on the benefits of roadway lighting and presented documentary evidence. As substantial mileage of the Interstate System was completed in Texas, the state highway department recognized the need for lighting these facilities, especially in the urban areas. However, officials were greatly concerned that the design criteria for roadway lighting had not changed to meet the challenge of the new generation of modern highways. Because of this concern, a research project in roadway lighting was initiated in 1964 by the Texas Transportation Institute and the Texas Highway Department in cooperation with the U. S. Bureau of Public Roads. The objective of the first phase of this research was to determine the optimum geometric configuration of lighting systems that would provide functional and economical roadway lighting for freeway-type facilities. Prior to this research, the Texas Highway Department specifications for roadway lighting called for luminaries to be mounted at a maximum height of 30 feet and a spacing generally not to exceed 160 feet. These specifications were typical of many throughout the country. Lighting systems designed on the basis of these early specifications left much to be desired for traffic operations on high-speed roadways. A "puddle of light" was spilled onto the roadway immediately under the luminaire. This bright puddle of light completely overshadowed the effectiveness of a lower intensity of illumination that could be directed to the fringe of the lighted area. The environment resulting from this system of illumination was that of extreme variations. The driver, passing from the puddle of light into the area of relative darkness was continually experiencing changes in his environment and rapidly becoming a victim of fatigue without realizing it. The human eye adapts from light to dark and dark to light conditions quite rapidly and thus, with this system, the driver's eyes are in a continuous state of adaptation, causing fatigue during prolonged exposures. Therefore, one of the major objectives of the research was to improve the uniformity of illumination systems. The purpose of this study was to determine the optimum height and spacing of luminaries in lighting systems for access-controlled facilities. Considerations in determining the optimum geometric configuration were (a) obtaining a more functional system; (b) reducing glare; (c) reducing the overall cost of installation, operation, and maintenance, and (d) reducing the number of poles to reduce fixed-object hazards.
Abstract