A police officer pulls over a car full of nuns...

A police officer pulls over a car full of nuns and asks
"Why were you driving so slowly?"
The nun driver replies
"The speed limit is 20 miles an hour, officer."
"No, Sister, the speed limit is 70."
"Then what are all the signs that say 20 on them?"
"This is highway 20. They just mark what highway you are on."
"Oh... That explains why the other sisters were screaming back when we were on Highway 166."

This joke makes fun of nuns. It relies on the reader to come to the conclusion that the nun was driving on a highway at 166 miles an hour because she didn't understand what a speed limit sign looks like.

The fact that the joke starts out with a fairly normal sounding situation (nuns in a car getting pulled over for driving slowly seems like something that I would expect) makes the punchline seem pretty absurd. No-one that could get a drivers license should mistake a speed limit sign for a sign that marks what highway you are on.