When it comes to tunnel and roadway lighting, all technologies are not created equal. In fact, researchers have discovered two emerging trends to illustrate this point.
First, more emphasis has been placed on color temperature due to its critical impact on safe nighttime driving. Second, controlling light distribution has become important for saving energy and reducing obtrusive light.
Color makes a difference
The eye is sensitive to different color temperatures and levels of light, which affect the true lumen output of a lamp. It is well known that the eye’s response to color depends upon the amount of light available. Research has also shown that a lamp’s color output has a profound impact on nighttime visibility.
Under low light levels, our eyes are most sensitive to the blue spectrum that is plentiful in metal halide lighting. Under high light levels, like daylight, our eyes are more receptive to the yellow spectrum of light abundant in high-pressure sodium (HPS). As the day passes and light levels diminish, the effectiveness of HPS lighting decreases while it increases for metal halide. HPS lighting systems would require much higher luminance levels than a lower-wattage metal halide system to have the same effectiveness under low light conditions.
Consider two stretches of nighttime road lit at the same light level (as measured by footcandles). One is illuminated with metal halide and the other with HPS. Although the levels of light are identical, the space lit with metal halide lighting will appear much brighter to us than the area lit with HPS. This is because the human eye perceives white light better under low light conditions.
Recent findings from leading scholastic institutes, including University of Michigan, Ferris State College, and Rensselaer Polytechnic Institute, indicate that lighting can play a key role in reducing nighttime vehicular accidents. For example, a dark roadway illuminated with HPS may result in a driver taking nearly 50 percent longer to react to a potential road hazard than under metal halide lighting. Under certain conditions, metal halide can be up to four times more effective in aiding drivers in detecting pedestrians.
Focused light distribution—Reduces light pollution,
In recent years, concern has grown over the rise in obtrusive light, the general term referring to light trespass, glare, and light pollution, which cause sky glow. In fact, a substantial portion of all U.S. outdoor lighting, including parking lots, billboards, and roadways is being misdirected upwards. For example, according to the International Dark Sky Association (IDA), nearly $1.5 billion in electricity is wasted annually, generated from the burning of six million tons of coal.
Poor design and installation of nighttime lighting can be costly in terms of light trespass. Glare is especially dangerous for drivers at night—it occurs when light shines directly in people’s eyes and impairs nighttime vision, a common result of high wattage floodlights used along roadways. Light trespass is light that spills over from one property to the next and invades the privacy or peacefulness of neighboring dwellings. When a large quantity of outdoor lighting is inappropriately focused upwards, it creates a glow above cities and towns that can be seen for miles and that blocks the ability to stargaze.
A large quantity of high-intensity discharge light is not the answer to roadway lighting. A properly designed fixture helps control light pollution by focusing light only on the intended area or object to be lit. This fixture should be fully shielded so that no light is emitted upwards. When correctly mounted, the fixture will help control light output and light only its designated area.
HPS and metal halide are more effective under different light levels. Metal halide is more effective under low light conditions, and it plays an important role in crime prevention by improving obstacle detection and reaction times, reducing crashes per mile, and providing increased sense of safety in parking garages, roadways, and other public areas.
Properly designed fixtures help control light output and keep the light in its designated area. These fixtures should be used with energy-efficient lamps, which produce high-quality white light. More light is not always better light; it is the light’s spectrum, quality, and direction that are critical for nighttime vision.