Being able to gauge your speed, whether it be a mounted speedometer or a smart phone app...can save a person a lot of money.
Places like going south on Morse (downhill), where it is easy to quickly
and inadvertently exceed 20mph (driving electric or gas carts) and get caught by the cops that often hide at the postal station on the west side of the road...can lead to an expensive lesson.
I know two people that found this out...the hard way.