Well, you don’t actually say which renderer you are using here, so let me just speak in general terms and in reference to Blender Internal (BI).
You do not want to approach this problem by trying to “emulate reality.” Instead, you want to find the computationally cheapest way possible to produce a plausible-enough result. In this case, if what you really need is a cone of light hitting the street, stick a spotlight in there, pointing straight down. Set the color and intensity of the lamp appropriately. If you also want the lamp-housing to appear to glow, consider simply changing the material so that it’s an appropriate bluish-white color and that it is set to “emit” so that it will produce its color whether it’s being hit by another lamp or not. If you want the glowing lamp to appear to light up the lamp-stand itself, how about adding another weak lamp in the housing, an omnidirectional one this time, and make it layer-specific so that it doesn’t “see” the glass lamp-housing. (Or, so that it only “sees” the thing that you want it to light-up, if you need more control.) Adjust the light levels way down, as well as the fall-off of the lamp.
Remember that, in BI at least, you can never actually see a CG “lamp.” Even if it’s pointing straight at you. You only see whatever it “illuminates.”
Did I actually have light passing through glass? Umm, no. But I’ve tried to bring-together all of the most-critical “behaviors of light” to tell your eye that this is a glowing street-lamp.
Now, you say that it’s a foggy night and that you want a “cone of light?” Well, first consider whether the shot is actually improved by that sort of thing, then look for another “trick.” Namely, compositing. Like another RenderLayer containing … a cone … and maybe you use the alpha-channel of that cone to modulate a color (which will be “the light”) with an animated noise texture (the “moving fog”) and which also attenuates the hue-and-saturation . . . but, I digress.
The main idea that I want to plant here is that you should “look at the light.” Not how the image in front of you happens in reality, but rather, what the light is doing, there in front of your eyes. Then, go out there and look for the computationally cheapest way to “fake” that. Good enough to persuade the viewer’s already-experienced eye that this is a street lamp. And, to achieve a good, well-balanced exposure.