I thought "granny spinners" because they leave old ladies spinning like tops in their wake.
Actually it has a cycling meaning, which is to describe someone who is creeping slowly up a hill using their lowest gear (the "granny" gear), but I figure they've intruded on our territory, so why not steal their term?
But I expect someone will some up with something much better as time goes on.
What shocks me is how I would be ashamed to ride on the pavement to avoid the traffic, but they just don't feel that shame at all.