A traffic cop is sitting at a patrol point on the M18 near Doncaster one afternoon, hoping to catch people speeding, when he sees a 15 year old Honda Civic go past really slowly, about 20mph. He realises that going very slowly is as bad as speeding so he goes after the car and stops it, making it pull over onto the hard shoulder.
When he reaches the drivers window and looks in he sees the vehicle is occupied by 5 little old ladies. The 4 which are passengers are sat there white as sheets, with their nails dug into the upholstery from gripping so tightly, and the lady driving says to the officer "What seems to be the problem? I was sticking exactly to the signposted speed limit, 18mph!"
"Oh Madam," says the policeman, "That's not the speed limit, its the number of this motorway, the M18"
A bit embarrassed, the woman grinned and thanked the officer for pointing out her error.
"But before I let you go, Madam, I have to ask... Is everyone in this car OK? These women seem awfully shaken and they haven't muttered a single peep this whole time," the officer asks.
"Oh, they'll be all right in a minute officer. They're always like this for a while after we've turned off the M180"