The 55mph drivers might be county&state workers-shirkers. They have leeway on getting to work on time. Those who labor in the dreaded private sector can not afford to be so relaxed.
Never drive behind vans of any kind. The drivers are guaranteed to be slow and dangerous and this includes van drivers working in the dreaded private sector (tradesman) and soccer moms (of course)
By and large immigrant drivers are awful... get ahead of them too. Chinese being the worst, I once got rear ended by one. Where? In China Town!!!! Even worse if the immigrant is female.