badger, I had that reaction a couple days ago when an SUV ran a stop sign. She slowed down but didn't look and went right on through! I was already in the intersection when she approached it. (Not that it is such a huge intersection--I was moving slowly because of road conditions.) So I couldn't have done anything if it had occurred to me that she might not stop.
It just took me by surprise. Even though once or twice I've had similar experiences, they are rare enough that I forget them in the interim and I just expect people to stop at stop signs, because they always do.
It could have happened just as easily if I'd been in a car instead of on a bike. I suppose the same is true of her--only her SUV is so much more deadly than a bike!!
2009 Trek 7.2FX WSD, brooks Champion Flyer S, commuter bike