Comment by bluGill

Not a slow driver in most cases, many slow drivers - all who want to go faster but cannot. There is just so much traffic that you can't go faster, and neither can the person in front of you.

I used to drive 20 under all the time (I achieved 57mpg once doing that) - but since this was an empty rural highway the few cars that were around saw me well in advance and moved over and passed without a problem.