If the speed limit was 15 mph, and the Waymo vehicle was traveling at 17 mph before braking, why do you believe the Waymo vehicle would honor a 12 mph speed limit? It didn't honor the 15 mph limit.

Ignored by some, not all humans. I absolutely drive extra slowly and cautiously when driving past an elementary school during drop off and pick up precisely because kids do dumb stuff like this. Others do too, though not everyone of course, incredibly.