Lighting Research Center Lighting Research Center
 

The Long and Lighted Road: Lighting and Driving

Traffic Signals

Traffic and Pedestrian Signals, Signs and Markers
Traffic signals provide information that is key to safe and smooth traffic flow.

For nearly 70 years, illuminated signals have guided motorists and pedestrians at potentially dangerous roadway intersections to prevent collisions among vehicles traveling in different directions. These red, yellow and green signals have become ubiquitous along roadways throughout the world. Their properties are carefully standardized by such bodies as the Institute of Transportation Engineers, who writes the standards on which traffic signal regulations in North America are based. These standards describe minimum luminous intensities, and distributions of intensity that are required by these signals to be visible in a range of viewing conditions.

For example, standards in North America require green signals to have twice the luminous intensity of red signals, and yellow signals must have nearly five times the intensity of red signals. *

For most of the time traffic signals have existed, their design and construction has remained quite constant. A signal head (a single circular indication) consists of an incandescent filament lamp, surrounded by a reflector and fitted with a colored glass lens. The shape of the reflector and optical properties of the lens determine the distribution of intensity.

Evolution of Traffic Signals
Top left: Incandescent traffic signals have been in use for many decades. Top right: Early LED signals used hundreds of tightly packed LEDs. Center left: With improved optics, fewer LEDs could be used. Center right: High output LEDs result in signals that appear similar to incandescent signals. Bottom left: With further improvements in LEDs, perhaps three-color signals will be developed. Bottom right: Head-on view of traffic signals, starting top left and going counter-clockwise.

With the development of light emitting diode (LED) technologies having the capacity for high light output in the 1990s (see "LEDs: Indicators to Illumination?"), manufacturers have begun to develop signals using LEDs; in fact, about 10% of all red traffic signals in the United States used LEDs in 2000, according to Strategies Unlimited, a market research organization that conducts an annual market update of LED lighting products. In early signals, more than 600 individual LEDs might be mounted together in a disk formation to produce the circular indication, but these signals did not meet the requirements for distribution of luminous intensity. The addition of lenses in front of the LEDs and increases in LED light output brought their number from 600 to around 200. More recently, very high output LEDs with broader intensity distributions have been developed, and in many ways, the design of LED traffic signals has begun to mirror conventional incandescent signals, with a clustered LED light source set back into a chamber and optical elements diverting the light from this source in the proper distribution.

Traffic Signal Apparatus
The LRC has tested intensity requirements for traffic signals using LEDs.

Red LED traffic signals have been in use longer than other colors. The aluminum gallium indium phosphide (AlGaInP) LEDs that produce red signal colors have been around longer than the indium gallium nitride (InGaN) LEDs used for green signals, and green LED signals meeting the specifications of the ITE are newer. Although yellow LED signals also use the AlGaInP technology that red LEDs use, the much higher luminous intensity requirements for yellow traffic signals have made yellow LED signals technically impractical, and as of October 2000, no commercially available yellow LED signal can be purchased that meets ITE specifications. Research through the National Cooperative Highway Research Program (NCHRP) will be completed next year that will address whether any changes in luminous intensity requirements for traffic signals are appropriate; other organizations (including the Lighting Research Center: see www.lrc.rpi.edu/ltgtrans/led) have also studied requirements for traffic signal visibility. The LRC has found, for example, that luminous intensities of green and yellow signals do indeed need to be higher than of red signals to give the same response as to a red signal.

Missed Signal Results
LRC research shows visibility differences among red, yellow and green signals having the same luminous intensity.

Because LED traffic signals use less energy than their incandescent counterparts, a number of programs exist to help spur their use. The U.S. Environmental Protection Agency has recently published an ENERGY STAR specification for traffic signals using LEDs (www.energystar.gov. ), and several other government organizations and electric utilities have started programs to increase market penetration of LED traffic signals. The Consortium for Energy Efficiency has gathered information about such efforts on its web site at http://www.cee1.org/.

New Pedestrian Signals
An ongoing study for the New Jersey Department of Transportation, led by John Van Derlofske and Peter Boyce of the Lighting Research Center, is examining the effectiveness of in-pavement crosswalk marking signals. Such devices can use either incandescent or LED light sources, and can be programmed to flash when someone enters a crosswalk, signaling to oncoming drivers the presence of a pedestrian in the roadway. Van Derlofske and Boyce are finding that these systems reduced the number of conflicts between automobiles and pedestrian, and that they reduced the speed of oncoming drivers relative to an unmodified intersection. These systems are less expensive to install than a completely signalized intersection, and can be used where traffic flow patterns do not warrant a traffic signal installation.
Traffic Signs and Markers
Not all visual information provided to drivers incorporates lamps and lighting technologies. A growing awareness and optimization of the materials used in roadway signs and markers has occurred. Highway signs that are perfectly visible during the daytime might be invisible to drivers at night unless these signs reflect light from headlamps back toward the driver's line of sight. For this reason, such signs and markers use retroreflective materials that have been developed to efficiently and uniformly reflect headlight illumination without causing glare. Such materials must also be durable and able to survive many seasons of inclement weather, vehicular emissions and potential vandals.

*Editor's note: The standard for light-emitting diode (LED) traffic signals has changed since this article first appeared. The 2005 standard for LED traffic signals from the Institute of Transportation Engineers requires the luminous intensity of green and yellow signals to be 1.3 and 2.5 times higher than that of red signals, respectively.




Rensselaer Polytechnic Institute
LRC Intranet Web mail Lighting Research Center