Here are a few things I could detect and couple of rules that seem to make sense:
Telephone PolesSince a telephone pole is round the sound bounces off of it in all directions equally. The sound reflection pattern can be described as a "swell". As you approach the pole you can hear reflections gradually fade in and peak as you pass it and fade out evenly. This would be the same for any round object that you are passing along the direction of its curvature. The object, and this goes for all objects as far as I can tell, will have a greater magnitude of sound returning to your ear the closer you are to it. Sound degrades (or disperses) over distance so it makes sense that the closer you are the "more" sound waves you will be receiving.
MailboxesI was struck when I noticed the difference between telephone poles and mailboxes. The response curve of a mailbox is generally slightly smaller due to it's size (of course they are usually closer to the road than poles) but the interesting thing was that since they have a flat
face (the door) the response curve is more of a square wave than a sinusoid (like the telephone pole). In other words, the amplitude of the sound reflection increases quickly when you are
exactly perpendicular to the flat face, and then drops off quickly once you pass. So they have a much more brief presence.
Parked CarsOf course moving cars are easy, they emit their own sound! But parked cars are unique in that you can recognize the metallic material they are made of because of the higher frequencies they reflect. For some reason it seems that wooden fences, tree clusters, and trash cans don't reflect these higher frequencies. This is probably due to there more absorbative properties. When frequencies start getting absorbed, usually the higher once are first to go since low frequencies, by nature, travel greater distances.
FencingFlat fencing is generally fairly distinct, switching "on" quickly when you approach and "off" quickly when it ends, and generally remains very constant when you are passing it. Wooden fencing doesn't generally reflect the higher tones.
Metal fencing, in particular, the "3"-shaped metal extrusion gaurd rails that you see everywhere, is different in that it seems to reflect the lower frequencies and the higher frequencies (probably due to the non-absorbative material) but there doesn't seem to be a lot of
mid-range tones being reflected. I don't have an explanation for that, and I may be wrong about it. Maybe I'll record some of these and do some analysis.
I plan to continue my findings here as I discover them. If anyone has anything to add to help me out I would much appreciate it if you left comments!