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3. Codes and Specifications


The Institute for Transportation Engineers (ITE) is the primary organization in the U.S. that oversees specifications for devices such as traffic signals. Currently, these specifications are based on old technologies - primarily, incandescent lamps. This is because incandescent lamps have been the main light source used in traffic signals for more than 70 years. For example, the minimum intensities required of yellow signal heads are higher than those required of red signal heads, because a yellow incandescent lamp filter transmits more light than a red filter. Because of this artifact, yellow traffic signal heads that meet current ITE specifications must contain a large number of LEDs, resulting in very high first cost.

The ITE is currently revising its specifications for traffic signals, based on visibility research. This activity may result in lower intensity standards. However, such specifications do not carry the force of law; they are merely guidelines that may be used by various jurisdictions. In fact, several jurisdictions, anticipating the increased use of LED traffic signals, have developed their own specifications (Wyand, 1996; Suozzo, 1998), including Caltrans, Oregon Department of Transportation, and the cities of Philadelphia, PA and Davis, CA (Click here to learn about some state and local specifications). In general, there appears to be widespread agreement among the specification community that LED traffic signals will replace conventional incandescent signals.

To date, the most successful LED traffic signal application has been the red signal head. This success is due to several reasons. In the typical traffic signal, the red signal will be switched on approximately 50% of the time, compared to lower on-times for the green and especially the yellow signal heads. In addition, the red LEDs are currently the most cost-effective for use in traffic signal heads because of their relatively high luminous output, requiring fewer LEDs per signal, and the lower manufacturing costs of red AlInGaP LEDs, which result in lower wholesale costs (see Cost) than for green LEDs, for example.

Currently, yellow traffic signals require a significantly higher number of LEDs in order to meet the ITE specification standards. There is general agreement among transportation officials and among LED traffic signal manufacturers that the high intensities required of the yellow traffic signal head may be an artifact of the incandescent lamp technology used in traffic signals for the past 70 years. An incandescent lamp filter that results in yellow light will be able to have a higher transmittance than a corresponding red filter. Traffic engineers, wanting to specify standards for safe traffic signals, were thus apparently able to specify higher intensities for the yellow signal heads. The ITE is currently in the process of revising its specifications for traffic signals using LEDs, based on visibility research, that may result in lower intensity standards than currently specified.

Due to uncertainties about the precise recommendations that will come from the performance specifications being drafted by ITE, a number of government agencies have developed or are in the process of developing their own specifications for LED traffic signals, including Caltrans, Oregon Department of Transportation, and the cities of Davis, CA and Philadelphia, PA (Wyand, 1996; Suozzo, 1998). Such specifications, especially those of Caltrans (due to the large number of traffic intersections under Caltrans' jurisdiction), have already had a significant impact on the products that have thus far been developed by manufacturers (Wyand, 1996).

Potential consequences

The proliferation of traffic signal specifications throughout the U.S. may have a number of side effects. Since they are prepared by state and local jurisdictions, they are likely to conform to any local or regional requirements, such as weather conditions or electricity costs. In addition, the demand built by these specifications have already had a significant impact on new product development (Wyand, 1996).

At the same time, the number of different specifications may fragment the market and increase prices of LED traffic signals, if products that meet each jurisdiction's specifications are customized. Furthermore, it is possible that the enthusiasm with which LED traffic signals are being greeted is without supportive evidence of good product performance.

Options and recommendations

Among the approaches toward codes and specifications that could be taken is a comparison of the various specifications that exist or are in development. Identifying those performance issues that are found to be important to all jurisdictions can help to minimize a fragmented market. At the same time, the performance of individual products in meeting these specifications could be evaluated in order to equip specifiers with information for selecting appropriate equipment.

The ITE standard-setting process is one avenue for ensuring that future LED traffic signals will meet minimum criteria for visibility and reliability. A further step to include these factors as well as life, maintenance cost, energy use and other factors could also be taken by developing a performance-based specification for LED traffic signals. Such a step would require the development of methods for objectively and fairly comparing different technologies.


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