There's several factors they could use to bring a level of uncertainty to the algorithm. Compare against the previously clocked speed limit, compare against map

There's several factors they could use to bring a level of uncertainty to the algorithm. Compare against the previously clocked speed limit, compare against map data, compare against the speed of other vehicles in a proximity, judge based on density of crosswalks, parked cars, traffic, etc. This isn't a failure of autonomy imo. Speed limits are arbitrary placeholders that are more for supplementing the slow reaction speed of the human brain. If we started focusing more on making the algorithm implement actual analysis of the environment instead of "the rules of the road", we might see better results.