The driver is a little old lady, and there are two old ladies in the back seat.
The cop asks, "Why were you driving just 20 miles per hour?"
The old lady responds, "I was just going the posted speed limit!" and points to a sign up ahead.
The cop smiles and says, "That's not the speed limit sign, that's the sign for this highway — Route 20!"
One of the old ladies in the back gasps out, "We tried to tell you, Eugenia!"
The cop takes another look at the old women in the back and sees that they are wide-eyed and disheveled. One of them is tightly gripping the door handle.
"What's the matter?" the cop asks.
She responds, "We just came off of Interstate 190."