School run driver's

Why is it that those people that are part of the school run brigade drive like total idiots. Maybe their driving is bad all the time, just made worse by the thought of having to get the kid(s) to school, or by the sheer volume of similar idiots on the road. Don't know what it is but they either drive really slowly, or really fast - nothing in-between! And the road/weather conditions make no difference to the speed! Add to this the rudeness and lack of consideration for any other road user's, with the way they drive, pull out of junctions / roundabout's and then park by the school. Why?