Yep xmas is right. A lot of LED street lights, especially in cities or DOT-owned installations, have wireless "nodes" instead of traditional photocells. These nodes are synced to a computer network and will turn the lights on based on the program. For instance, RIDOT has their freeway lights set to turn on 30 minutes after sunset and shut off 30 minutes before sunrise. This time schedule works fine when the skies are clear but on cloudy days with inclimate weather, the freeways are dark. Providence on the other hand has their network lights set to come on and off at sunset and sunrise, respectively.
Cranston use normal photocells when they went LED so the turn-on and turn-off times vary day-to-day. Some photocells are more sensitive than others, even among those with the same footcandle rating. The standard footcandle turn-on level is 1.5ftc. "Energy Saver" photocontrols are 1.0ftc. 2.6ftc photocells are also made, which turn on a little earlier and good for more urban areas with more stray light at night and higher traffic conflict (pedestrians, other vehicles).
ANSI recommends higher-lumen outdoor lighting to turn on sooner than lower-lumen lighting. That way, as the sun goes down, the lights turn on just as the ambient light level is dropping below what the light puts out. Basically, when the light turns on, you should only slightly be able to tell by looking at the ground. If the ground gets more than slightly brighter when the light turns on, the light is coming on too late. If you can't tell a difference at all by looking at the ground, the light is coming on too early. That's one of the reasons photocontrols are supposed to face North (for accurate turn-on/off times). The other reason being to protect the photoeye from direct radiation from the sun, though that only applies to the older Cadmium eye PCs, not the new silicon eye PCs.
