(Note for those unfamiliar with the British road network: 'A' roads are main, or arterial, roads and they are all identified by 'A' and a number.)
An elderly man is driving his wife on a road trip around the south east of England. Shortly after getting on to the A2, a police car signals them to pull over, and the old man obliges. The police officer walks up to the driver's side and the old man winds down the window.
The policeman asks him, "Is there something wrong with your car, sir?"
"No. I don't think so."
"Then why were you driving so slowly?"
"Do you have any idea what speed you were doing?"
"Yes. Exactly 2 mph."
"The speed limit along here is 70 mph. You can't drive that slowly. It's very dangerous."
"Oh. I thought the speed limit was 2 mph, as it's the A2."
"It is the A2, but the speed limit along here is 70 mph. I will let you off for now, but you need to speed up to around that speed."
The old man agrees to do so. As the police officer is about to return to his car, he looks over at the old lady in the passenger seat and sees that she has been staring dead ahead for the whole time and is covered in sweat, shaking. He asks what's wrong with her.
"Oh her? She's been like that since the A259."