Firstly, I have made and attached a crude diagram of the situation that I believe is being described in your question. My advise for anyone reading is to look at the diagram, because a picture speaks a thousand words and it will clear things up a bit.
In this scene, we have car B rolling up to the intersection. Meanwhile we have car A speeding along intending to continue straight through the intersection (and off of the right edge of the diagram). Cars A and B cannot see each other because of the blind corner created by some type of obstruction like a building. The approximate position and angle of a traffic mirror that would be employed in this type of situation is drawn in the diagram. While the diagram is crude and the mirror would likely be in a slightly different position (raised and not in the middle of the road), and a different size (not bigger than the cars), the angle of this mirror is what matters in a 2D representation of the situation. The angle of the mirror in the scene relative to the cars is also the only thing needed to solve the problem. In this type of situation, where one needs to see around a 90 degree corner, the mirror needs to be 45 degrees to the incoming (incident) light rays coming from around the corner. This is the same angle used with the mirror in a simple periscope, or when someone uses a reflective object to look around a corner in an action movie. By simple geometry, if the mirror surface needs to be 45 degrees from the incident light rays coming from around the corner, then the angle of incidence is 45 degrees and the angle of reflection is also 45 degrees (Angle of Incidence = 90 degrees - Angle of Reflection).