A clear triangle concerns corner lots. The reason that it is there is because of safety. The idea being that when you are diving and are approaching a corner, you want to be able to see what is around that corner before you come to it.
In order to determine the clear triangle, you first start out in the street at the interest of the sidewalks. Then you measure back 40 feet from this point. From this point, you need to be able to see the end of the other 40 foot line.