Monochromatic Light Sources

Traffic lights give up about 30% efficiency compared to regular incandescents to withstand traffic vibrations and the high frequency of on/off switching.

But an even greater gain comes from the fact that only one color is needed. The red/yellow/green lens in traditional lights block up to 70% of the photons.

Additionally, LED bulbs can be tailored to focus light where it is needed, so less light is needed.

A 12 watt LED array can do the same job as a traditional 116 watt bulb. Projected power savings is $600/year/intersection (8 sets of signals/intersection), but initial costs of installation and LED arrays is over 50x that of incandescent (ROI ~7 years). Although manufacturers claim lifetimes of 10,000-100,000 hours (1.5-10 years continuous), real world performace is uncertain.

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LEDs are also well suited for applications that only require monochrome light. LEDs can provide a more efficient and lower maintence solution for such applications as traffic lights. If you look around, many of the traffic signals throughout Palo Alto have been replaced with LEDs: not just the Red/Yellow/Green traffic lights, but also other lights such as Walk/Don't Walk signals.