This article considers decreased nighttime traffic signal lamp brightness in terms of the effects on safety and efficiency of the highway system. The emphasis is on the reception and use of information from round traffic signals and the effects of reduced brightness on visibility. It is critical to the safety and efficiency of approach locations where signals are warranted, that drivers receive information and respond to traffic signals correctly and rapidly. Accident statistics have shown that driving at night is more hazardous than driving during daylight hours. This results from impaired driving performance due to lack of visibility. The superior night visibility of signals is desirable for several reasons: The possibility of extraneous glare sources; the lack of environmental cues; the potentially adverse effects of rain, snow and fog; and, the probability of fatigue and other factors impairing driver performance. The following recommendations are made: Eight inch (200 mm) signals should not be dimmed; twelve-inch (300 mm) signals may be dimmed at night when conditions warrant, dimming should be implemented on an intersection-by-intersection or signal face-by-signal face basis and based on empirically derived visibility criteria and engineering judgement; empirical research should be undertaken to establish night and day visibility criteria for U.S. traffic signal indications, including arrows and pedestrian signals.

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Filing Info

  • Accession Number: 00173894
  • Record Type: Publication
  • Files: TRIS, USDOT
  • Created Date: May 31 1978 12:00AM