It's hard to say specifically, as savings vary widely depending on the size of the city, the electric rates in that city, the number of traffic lights, and whether or not all of the traffic lights have been replaced with LED lights.  Further more, the savings vary depending on the color of the light as well.  Still, one yellow light replaced with a LED light will save between $31 and $62 over its lifetime, which can be projected as three to four years.  Now, multiply that by the number of lights replaced in the city, and you'll have an estimated savings!