This is clear to designers and engineers. The obvious answer is the human confirmation / verification compared to what the cars system thinks it sees. 1000s and 1000s of verification test are happening daily on 100s of styles and types of stoplights complication intersections. Not just simple 2-way "+".
Andrej Karpathy was in a recent video that showed stop signs being hand held (up or down), on vehicles (buses, etc), partially blocked (trees, other cars (bus), etc).
http://www.youtube.com/watch?v=hx7BXih7zx8