Parallel & Perpendicular - What's that got to do with a Guide Dog

My O&M instructor has been working with me on listening to the parallel and perpendicular sounds of traffic. What does that mean? If a person is legally or totally blind, usualy the only way to know when it's safe to cross a street is to listen to the pattern of the traffic. If I want to cross a street the traffic traveling from left to right and vice/versa is my perpendicular and the traffic on my left or right is my parallel. Even with a Guide Dog it is important to learn the sounds when the traffic starts up at a light or stop sign letting you know that it's safe to cross. A Guide Dog cannot read the street lights or signs so I have to know so I can tell the dog when it's safe to cross. Fortunately, the dog's instinct for survival may kick in if I give the wrong signal to start when it really is unsafe and may not continue to cross the street. That's still safer than me just taking a chance on my own and getting flattened.