Alternative Traffic Signal Illumination: Human Factors

Problem

New technologies have produced traffic signals with significant advantages over standard incandescent signals, including greater efficiency and lower costs. But are they as visible to the human observer as the older signals?

Method

We developed what we call a Usability Factor (UF), which compares the visibility of a test lamp to a standard lamp, as perceived by a human observer, when the luminance of the two lamps matches, as measured photometrically. If the UF is greater than one, then less actual measured illumination would be required to render it as visible as a standard light (implying greater energy efficiency); if less than one, more illumination would be needed. If the UF is unity, then photometric measures alone can assess its visibility.

Findings

We found that for a 12-inch red LED lamp, there was no significant difference from a standard lamp. And orange LED pedestrian head was also close; an orange neon pedestrian head had a high UF but the actual illumination is low enough that such a lamp is impractical. An orange fiber-optic pedestrian head had a low UF (so more light is needed to make it visible), a red LED arrow's UF was slightly higher than unity, and a red fiber-optic arrow had a very low UF.

We also conducted extra tests on the 12" red LED, finding that a sun-phantom (produced when the sun is low behind the observer and shining into the lamp) had less effect on the lamp's visibility than on a standard lamp, and that glare and fog seemed to have little effect as well.

The Usability Factor is a useful quality index which can be used in combination with the standard 44-point test or other appropriate photometry-based tests to evaluate the visibility of a lamp.