Google’s self-driving car most likely uses mapping of traffic signs using google street view images for roadway inventory management. If traffic signs are not in its database, it can still “see” and detect moving objects which can be distinguished from the presence of certain stationary objects, like traffic lights. So its software can classify objects based on the size, shape and movement patterns. Therefore it is highly unlikely that a person would be mistaken for a traffic sign. See: How does Google's self-driving car identify pedestrians?
Image: Technology Review
To support such a claim, Illah Nourbakhsh, a professor of robotics at Carnegie Mellon University, gave an interview to the New York Times magazine cover story on autonomous driving cars, and includes this hypothetical scenario, saying:
If they’re outside walking, and the sun is at just the right glare level, and there’s a mirrored truck stopped next to you, and the sun bounces off that truck and hits the guy so that you can’t see his face anymore — well, now your car just sees a stop sign. The chances of all that happening are diminishingly small — it’s very, very unlikely — but the problem is we will have millions of these cars. The very unlikely will happen all the time.
Even so, the risk would be minimal, since the car is always looking out for traffic, pedestrians and other obstacles.