At what point would the driver have been compliant with the law with regards to speed?
The posted speed limit (40 mph in this case) is the speed limit under "ideal" conditions. The law requires a person to not to exceed a much lower limit under poor weather conditions, and obviously much much lower if the conditions are icy.
From the Massachusetts Highway Manual: "“No person operating a motor vehicle will run it at a rate of speed that is greater than reasonable or proper." It is clear evidence that the person was exceeding what was "reasonable and proper" if their speed contributed to the accident. Additionally, quoting again: "it is the responsibility of each motorist to reduce his/her speed for unfavorable weather conditions, poor visibility, or higher traffic volume. Posted speed limits are also a guide and serve as a guide to enforcement officers to what is a reasonable maximum speed for ideal conditions.
and that posted speed limit, like all other posted speed limits (except in some western states) is set artificially low to encourage ticket revenue. if you went to that road on a normal day, you would likely find 80% of people doing 50-55 in that 40 zone.
imagine if we gave government the power to use black boxes to enforce a vague standard like "reasonable and proper". the goal here is simply to potentially criminalize any motor vehicle incident that involves injury. the government wants that power, and the black box helps give it to them.