Efficacy is the ratio of light output from a lamp to the electric power it consumes and is measured in lumens per watt (LPW). Figure 6.5.1 shows the efficacy of numerous types of light bulbs.
Figure 6.5.1. Efficacy of light bulbs
LED lighting - A typical signalized four-way intersection may have three lanes per approach (two through lanes and a left turn lane), plus pedestrian crossings.
Conventional incandescent lamps in a single four-way traffic light, shown in Figure 6.5.2, consume roughly 85 kWh of electricity per day and cost about $1,600 per year to operate. LED lights use just 10 percent of the electricity that incandescent lamps use, so the opportunity for savings is enormous.
Figure 6.5.2. Traffic light