In Philadelphia, Gram and Gramps were stopped by the police because Gramps?

From the text:

Last year when Gram and Gramps drove to Philadelphia, they were stopped by the police, for irresponsible driving. 'You were driving on the
shoulder,' a police- man told Gramps Gramps said, 'Shoulder? I thought it was an extra lane. Look how smooth it is you don't get nice smooth shoulders like this back in Kentucky. 'That's a mighty fine shoulder.