4 if it's good weather. 8 times in bad. Your reaction time in a bigger vehicle is different because of the weight. So both your guesses apply.

Maintaining distance does help, however it's not a perfect matrix. People cut in front of big vehicles constantly expecting the same distance and it just doesn't help. The added view helps. An entire industry is based on it and backed by all sorts of reasons.

What they haul plays a role as well. Liquids and gases slosh funny and higher speeds.