Roadway lighting offers necessary visibility for vehicle and pedestrian at night, which improves transportation condition and eases driver's fatigue. At the same time, LED roadway lighting also can enhance road capacity, ensure traffic safety, and beautify our urban image.
1. Lighting Basics
Good visibility under day or night conditions is one of the fundamental requirements enabling motorists to move on roadway in safe manner. Properly designed and maintained street lighting will provide comfort and safety during conditions for both vehicular and pedestrian traffic Visibility is the state of being perceived by the eye.
The purpose of roadway lighting is to attain a level of visibility which enables the motorists and pedestrian to see quickly, distinctly, and with certainty all significant roadway details, such as the alignment of the road and any obstacles on or about to enter the roadway. Nearly all aspects of traffic safety involve visibility. Some factors that directly influence visibility are brightness of an object on or near the roadway, general brightness of roadway background-ambient light, size of object and identifying detail, contrast between an object and its surroundings, contrast between pavement and its surroundings as seen by the observer, time available for seeing the object, glare, driver vision, and condition of windshield.
Good visibility on roadways at night results from lighting (both fixed and vehicular), which provides adequate pavement illumination with good uniformity and appropriate illumination of adjacent areas, together with reasonable freedom from glare.
2. Types of Lighting System Configurations
Various lighting system configurations are defined and discussed in this section.
(1). Continuous Freeway Lighting
Continuous freeway lighting places continuous lighting that encompasses the roadway and area immediately adjacent to the roadway over a substantial distance along the freeway.
(2). Partial Interchange Lighting
Partial freeway lighting is the illumination of only the parts of the interchange that are most critical to the night driver, which are the merge-diverge areas of the ramp connections, intersections, and other critical roadway features.
(3). Complete Interchange Lighting
Complete interchange lighting is applying lighting to the interchange to achieve illumination of all roadways in the interchange.
(4). Underpass Lighting
Where AASHTO's Roadway Lighting Design Guide indicates that underpass lighting is desirable, the lights are mounted on the abutment of the bridge or on a pier for each direction of travel on the roadway. If such mounting would place a luminaire more than approximately 10 feet from the edge of the traveled roadway, the luminaire is typically mounted on the bottom of the diaphragm.
Generally, for continuously lit freeways, underpass lighting should be installed for structures greater than 50 feet in length. For underpasses that are longer than 200 feet, underpasses should be lit all day.