At low focal ratios, a point of light will blur in the shape of your camera lens aperture blades. E.g. if you have a 6-blade aperture, then the opening would be in the shape of a hexagon. Some camera lenses have enough aperture blades and round the edges enough that they create a near circular shape.
An out-of-focus traffic signal would likely look like a green, yellow, or red hexagon (depending on which light was illuminated at the time ... since traffic lights hopefully don't have all lights on at the same time or chaos would ensue.)
A lot of people really take advantage of this with strings of holiday lights. A focused subject in the foreground with something that has holiday lights in the background, will typically result in this effect.
There are even people who go so far as to create "designer" aperture shapes. E.g. take a black card, cut a small shape out of it (say... a heart-shape) and put that in front of your lens. When you take a photo, the out-of-focus points of light will blur into that heart-shape.
Tim Campbell
5D III, 5D IV, 60Da
