Improper user interface design could lead to hazard, but hazardous user interface design is not limited to computers; it can happen in real life. In this case, we're talking about a traffic intersection.
One may argue that a good driver will take all road signs into account. However, a driver under stress, for example, one who is honked by a car behind, or a driver who is distracted by cellphones, could very well pick the wrong visual cue. The inconsistent traffic light is a hazard, and a flaw in the user interface design.
A proper redesign would replace the non-directional green light to left and right arrows, like one in the middle of the intersection. It could also keep the red light lit at all times just to be abundantly clear.